arX

iv:1

706.

0265

9v1

[ph

ysic

s.ch

em-p

h] 5

Jun

201

7

Two-color Thermosensors based on[Y1−XDyX(acetylacetonate)3(1,10-phenanthroline) Molecular Crystals

Benjamin R. Anderson∗,1, Ray Gunawidjaja1, and Hergen Eilers1

1Applied Sciences Laboratory, Institute for Shock Physics,

Washington State University, Spokane, WA 99210-1495∗

(Dated: October 2, 2018)

We develop a two-color thermometry (TCT) phosphor based on [Y1−xDyx(acetylacetonate)3(1,10-phenanthroline)] ([Y1−xDyx(acac)3(phen)]) molecular crystals for use in heterogeneous materials.We characterize the optical properties of [Y1−xDyx(acac)3(phen)] crystals at different temperaturesand Dy concentrations and find that the emission is strongly quenched by increasing temperatureand concentration. We also observe a broad background emission (due to the ligands) and findthat [Y1−xDyx(acac)3(phen)] photodegrades under 355 nm illumination with the photodegradationresulting in decreased luminescence intensity. However, while decreasing the overall emission inten-sity, photodegradation is not found to influence the integrated intensity ratio of the 4I15/2 → 6H15/2

and 4F9/2 → 6H15/2 transitions. This ratio allows us to compute the temperature of the complexBased on the temperature dependence of these ratios we calculate that [Y1−xDyx(acac)3(phen)]has a maximum sensitivity of 1.5 % K−1 and our TCT system has a minimum temperature resolu-tion of 1.8 K. Finally, we demonstrate the use of [Y1−xDyx(acac)3(phen)] as a TCT phosphor bydetermining a dynamic temperature profile using the emission from [Y1−xDyx(acac)3(phen)].

I. INTRODUCTION

Plastic bonded explosives (PBXs) are hetero-geneous materials consisting of energetic organicmolecular crystals (e.g. RDX, PETN, and TNT)embedded in a polymer matrix, with possibleadditional additives being: plasticizers, antiox-idants, taggants/markers, and friction-generatinggrit. These energetic organic molecular crystals canbe initiated by a variety of stimuli, including ther-mal, mechanical or electrical. In the case of non-thermal initiation, it is believed that these stimulifirst heat the energetic material, which then causesthermally-induced chemical decomposition to occur.For mechanical shock initiation this heating is due

to the shock induced formation of hot-spots withinthe material. Much research has been devoted tounderstanding shock induced hotspot formation [1–14] with the main mechanisms being: pore collapse[14], plastic deformation [3, 12, 13], micro-fractures,binder/crystal interface friction, and crystal/crystalinterface friction. While these mechanisms havebeen identified to form hot-spots in shocked hetero-geneous materials, it is currently unknown which ofthese mechanisms are responsible for chemical en-ergy release for a given material composition andloading condition [12].Determining this dependence requires perform-

ing simultaneous time-resolved microstructural andthermal imaging of heterogeneous sample undershock compression. Performance of these measure-

∗ branderson@wsu.edu

ments is difficult, as shock-compression events typi-cally occur on the ns-µs time scales and microstruc-tural imaging typically utilizes synchrotron X-Rayphase contrast imaging [15–19]. One approach toperforming in situ microstructural imaging is the re-cently inaugurated NNSA sponsored Dynamic Com-pression Sector (DCS) at the Advanced PhotonSource (Argonne National Laboratory) [20, 21]. TheDCS provides the opportunity to perform time re-solved X-ray phase contrast imaging of shock com-pressed heterogeneous materials, which provides thefirst of the two required measurements.

The second required measurement is rapid ther-mal imaging with ns-µs resolution. To this end weare developing temperature sensors based on two-color thermometry (TCT) phosphors, which can beused in heterogeneous materials to provide spatiallyresolved thermal imaging. Previously, TCT phos-phors based on inorganic crystals have been usedto perform temperature imaging in a wide varietyof contexts [22–34], but for our application – inertPBX analogues – we desire to have organic TCTphosphors that have similar mechanical propertiesto the energetic materials.

The first step, towards this goal, is to de-velop appropriate organic molecular crystals thatcan be used for two-color thermometry. Toachieve this goal we choose to use Dy3+-doped yttrium ternary complexes that can begrown into molecular crystals with the first suchmaterial being [Y1−xDyx(acetylacetonate)3(1,10-phenanthroline)] ([Y1−xDyx(acac)3(phen)]). In thisstudy we report on the optical properties of[Y1−xDyx(acac)3(phen)] at different temperaturesand concentrations.

2

II. BACKGROUND

A. Two-Color Thermometry

Dy3+ is a well known lanthanide ion used for awide variety of optical applications [22–30, 34–42],with a common application being two-color ther-mometry (TCT) [22–30, 34]. TCT using Dy relieson two closely spaced Dy energy levels (4I15/2 and4F9/2) to act as a probe of temperature. When a Dydoped material is optically pumped (with an appro-priate wavelength) the ions are excited into the 4F9/2

energy level, from which they can also populate the4I15/2 energy level due to thermal effects. The ratio

of the populations in the 4I15/2 energy level n2, and

the population in the 4F9/2 energy level n1, is de-termined by the Boltzmann distribution, such thattheir ratio is:

n2

n1= e−∆E/kBT (1)

where ∆E is the energy difference between the 4I15/2and 4F9/2 energy levels (approximately 0.115 eV foraqua ions [43–46]), kB is Boltzmann’s constant, andT is the temperature.This ratio between the 4I15/2 and 4F9/2 en-

ergy levels can be determined using emission spec-troscopy; as the ratio of the 4I15/2 → 6H15/2 and4F9/2 → 6H15/2 transition is proportional to the ra-tio of populations:

I2I1

∝n2

n1

= Ae−∆E/kBT , (2)

where A is a proportionality constant.

B. Ligand Enhanced Luminescence

While Dy3+ has advantageous optical propertiesfor TCT, on its own Dy3+ (and the other lan-thanide ions) has a small molar absorption coeffi-cient, making it an inefficient phosphor. To over-come this limitation much work has been done onembedding different lanthanide ions in ligands suchthat the ligands (which have large absorption co-efficients) absorb the pump light and subsequentlytransfer the absorbed energy to the lanthanide ion[45, 47–61]. This energy transfer is typically radi-ationless and can occur via several possible mecha-nisms: electron exchange [48, 53], dipole-dipole in-teractions (both allowed [62–65] and unallowed tran-sitions [48, 66]), dipole-quadrapole interactions [48],and excitons [67, 68].

FIG. 1: Energy level schematic of[Dy(acac)3(phen)] displaying energy transfer fromthe ligands’ triplet state to Dy3+’s lowest excited

state.

This energy transfer has been empirically found tobe most efficient if the ligand’s lowest triplet energylevel is between 0.23 eV and 0.62 eV greater than thelanthanide’s primary luminescent state, as energyback-transfer is more likely to occur for energy differ-ences of < 0.23 eV [51, 69]. One such material, sat-isfying this criterion is [Dy(acetylacetonate)3(1,10-phenanthroline)] ([Dy(acac)3(phen)]) [52, 70–73],which is the subject material of this study. Todemonstrate the energy level spacing of the ligandsand Dy ion we provide a simplified energy level dia-gram in Figure 1 where the energy levels are takenfrom the literature [45, 47, 52, 74, 75]. Note thatin general there are many more energy levels for theligands and Dy than displayed in Figure 1, but wesimplify the diagram to highlight the relevant tran-sitions.From a review of the literature [45, 47, 52, 74, 75]

we find that the acac ligand has an S0 → S1 tran-sition corresponding to ≈ 3.596 eV (345 nm), phenhas an S0 → S1 transition corresponding to ≈ 3.263eV (380 nm), and Dy3+’s primary luminescent state(4F9/2) is at ≈ 2.583 eV (480 nm). In our study weuse a frequency tripled Nd:YAG laser with a wave-length of 355 nm for our excitation source, whichplaces the excitation wavelength nearly resonantwith acac’s S0 → S1 transition. This means thatduring excitation the primary absorption is due tothe acac ligand, which can then nonradiatively tran-sition to the lowest triplet state T1 (energy ≈ 3.134eV) via intersystem crossing. Once in the tripletstate the energy can be transferred from the ligandto the Dy3+ ion with the approximate energy differ-ence between acac’s T1 state and the 4F9/2 state of

Dy3+ being 0.554 eV. Since this energy difference is

3

greater than 0.23 eV, backwards energy transfer willbe minimal.

III. METHOD

A. Materials

The primary component of our two-colorthermosensors is [Dy(acetylacetonate)3(1,10-phenanthroline)] ([Dy(acac)3(phen)]).[Dy(acac)3(phen)] has previously been investi-gated for use as a single molecule magnet [70, 71],a phosphor for white OLEDs [72, 73], and a NIRoptical phosphor for use in telecommunications [52].In these previous studies the authors prepared thematerial with a Dy concentration of 100 mol%,while in our study we dilute [Dy(acac)3(phen)] with[Y(acac)3(phen)]. We perform this dilution in orderto characterize the influence of Dy concentration onthe material’s optical properties. This combinationof [Dy(acac)3(phen)] and [Y(acac)3(phen)] resultsin a hybrid molecular crystal, which we denote[Y1−xDyx(acac)3(phen)].To prepare [Y1−xDyx(acac)3(phen)] we use the

following procedure [71]: First we prepare a solutionof acetylacetone (0.06 M) and 1,10-phenanthroline(0.02 M) in a DMF/methanol mixture (1:1 vol./vol.ratio). To this solution we add an aqueous so-lution of potassium tert-butoxide (KOtBu) (0.06M) at an equal volume ratio. Once mixed weadd another equal volume aqueous solution con-taining Dy(NO3)3.5H2O and Y(NO3)3.6H2O in dif-ferent Dy/Y mole ratios such that the total con-centration is 0.02 M. For this study we make[Y1−xDyx(acac)3(phen)] with Dy concentrations of1 mol%, 5 mol%, 10 mol% and 15 mol%. Gradu-ally white precipitates appear and the suspension isallowed to age under stirring at room temperaturefor 4-6 hours. The precipitates are isolated via cen-trifugation (6000 rpm for 3 min) and finally driedin vacuum at 80 ◦C for ≈ 12 hours. The resultingpowder consists of [Y1−xDyx(acac)3(phen)] molecu-lar crystals (MCs).To characterize the structure of the

[Y1−xDyx(acac)3(phen)] MCs we use X-raydiffraction (XRD) with a Cu-Kα radiation source(λ=1.5418 A) operated at 40 kV and 40 mA. TheX-ray beam is monochromated using an X’Celeratormonochromator (PANalytical B.V.) and collimatedusing a fixed divergence slit with 0.04 radian Sollerslits, 0.5◦ divergence slit and 10 mm mask. Thediffracted X-rays are detected using a PIXcel3Ddetector (PANalytical B.V.) with the resultingdiffraction patterns being analyzed using X’PertHighScore Plus, Version 3.0 (PANalytical B.V.).

FIG. 2: XRD pattern for [Dy(acac)3(phen)] withmultiple sharp peaks visible. Inset: Optical

microscope image of crystals.

TABLE I: Average crystal parameters for[Y1−xDyx(acac)3(phen)] determined from XRD.

These parameters are found to be invariant with Dyconcentration (within experimental uncertainty).

Parameter ValueCrystal System MonoclinicSpace Group P21/c

a (A) 16.1b (A) 21c (A) 9.4α 90◦

β 116◦

γ 90◦

V (A3) 2841.82

Additional characterization is performed by opticalmicroscopy of the MC powder.

Figure 2 shows a representative XRD patternfor [Dy(acac)3(phen)], with the pattern displayingsharp peaks consistent with a polycrystalline struc-ture. This structure is also confirmed by optical mi-croscopy, with the inset of Figure 2 being an op-tical image displaying multiple crystals of varyingshapes and sizes. To determine the crystal parame-ters (structure, space group, lattice size, etc.) of thematerial we analyze the XRD pattern for each Dyconcentration and find that the crystal parametersare within uncertainty of each other for all Dy con-centrations. We therefore average the parametersand tabulate their average value in Table I. Theseparameters are found to be consistent with previousXRD measurements of [Dy(acac)3(phen)] MCs [71].

4

B. Spectroscopy

To measure the luminescence spectra of[Y1−xDyx(acac)3(phen)] at different tempera-tures we use a custom fluorescence spectroscopysystem and powder heater. The spectroscopysystem consists of a frequency-tripled Nd:YAGlaser (Continuum Powerlite II 8000, 355 nm, 8ns), focusing and collection optics, and two dif-ferent spectrometers (one for temporally gatedspectral measurements and the other for ungatedmeasurements). The spectrometer for ungatedmeasurements consists of an Acton SpectraPro2750i monochromator, attached current PMT, anda Spectrahub interface, while the spectrometerfor gated measurements consists of a PrincetonInstruments PI-Max 3 ICCD connected to anActon SpectraPro 2500i spectrometer. In additionto spectral measurements the ungated system isalso used to measure florescence lifetimes with thePMT output connected to a Tektronix DPO 4104oscilloscope. Figure 3 shows a schematic of thespectroscopy system.In order to evenly heat our

[Y1−xDyx(acac)3(phen)] molecular crystal powderwe use a custom powder heater. The heaterconsists of a block of Aluminum (2” D × 1” H)containing a 1 cm diameter indentation in whichthe powder is placed. This configuration allowsfor heating the powder from both the bottom andsides. For heating we use a 120 W canister heaterwhich is controlled by an Omega CN32PT-220 PIDcontroller with a K-type thermocouple providingtemperature feedback.

IV. RESULTS AND DISCUSSION

A. Ungated Spectra

We begin our study of [Dy(acac)3(phen)] by con-sidering its ungated emission spectra at different Dyconcentrations as shown in Figure 4. Note that theintensity in Figure 4 is scaled such that the inten-sity at 520 nm is normalized. From Figure 4 wefind that the emission spectra consist of four broadpeaks corresponding to emission from Dy with thepeak centers being 453 nm (4I15/2 → 6H15/2), 482

nm (4F9/2 → 6H15/2), 577 nm (4F9/2 → 6H13/2),

and 660 nm (4F9/2 → 6H11/2 ).In addition to the four Dy peaks in Figure 4, we

also observe a broad background emission centeredat 520 nm (which is found to originate from the or-ganic ligand) and a peak at 532 nm (correspond-ing to residual second harmonic in the pump). Theligand emission is isolated from the Dy emission by

measuring a pure [Y(acac)3(phen)] sample, as shownin Figure 5.

B. Gated Spectra

Given the proposed application of[Y1−xDyx(acac)3(phen)] as a TCT phosphor itis necessary to remove the ligand’s emission fromthe measured spectrum. This removal allows foran accurate probe of the Dy populations in the4I15/2 and 4F9/2 energy levels. While the ligand’semission can easily be subtracted using multi-peakfitting when performing spectral measurements, thistechnique is not an option when performing time-resolved TCT imaging. Instead, for time-resolvedTCT imaging, we rely on the different luminescencelifetimes of the Dy3+ ions and the ligand.To demonstrate the different lifetimes we plot the

normalized emission intensity (for a 1 mol% sample)at 482 nm (primarily due to Dy) and 520 nm (dueto ligand) as a function of time in Figure 6. FromFigure 6 we find that the 520 nm intensity followsa single exponential – with lifetime 0.6 µs – whilethe 482 nm intensity is bi-exponential – with life-times of 16 µs and 0.6 µs. Given the value of theshort lifetime (for the 482 nm intensity) and the ob-servation that [Y(acac)3(phen)] has emission at 482nm, we conclude that the fast component of the bi-exponential is due to the ligand, while the long life-time component is due Dy’s 4F9/2 → 6H15/2 transi-tion. The order of magnitude difference in emissionlifetime between the ligands and the Dy ion meansthat we can remove the ligands’ emission by usinggated measurements with a delay greater than theemission lifetime of the ligands.Using a gate delay of 1 µs we re-perform

the spectral measurements on the different[Y1−xDyx(acac)3(phen)] samples (scaled spec-tra are shown in Figure 7) and obtain improvedcontrast between the Dy and ligand emission as theligand’s emission is mostly removed. From Figure 7we find that the contrast between the emission at520 nm and 482 nm is approximately 120, which isan order of magnitude larger than for the ungatedspectral measurements (Figure 4). This increasedcontrast minimizes the influence of the ligand’semission and allows for more accurate determinationof the intensity ratios used in Equation 2.

C. Concentration Dependence

From Figure 7 we observe that, while the relativeintensity of the Dy emission increases as the concen-tration increases from 1 mol% to 10 mol%, it begins

5

FIG. 3: Schematic of experimental setup with both the gated (1) and ungated (2) spectrometers. F1: 355nm Laser line filter, F2: long-pass filter (400 nm cut-on), L1,L2,L3: Lenses, M1: mirror, Hub: Acton

Spectra Hub.

FIG. 4: Scaled emission spectra of[Y1−xDyx(acac)3(phen)] MCs for different Dy

concentrations. The spectra consist of four broadpeaks from the Dy ions superimposed on a broadbackground emission from the organic ligand. Thesmall peak at 532 nm is residual second harmonic

from the pump laser.

to decrease as the concentration is further increased.This behavior arises as the increase in concentra-tion initially is beneficial (providing more ions for

FIG. 5: Normalized emission spectrum from a[Y(acac)3(phen)] sample with 355 nm excitation.

The emission is broad and spans the whole spectralregion of interest.

luminescence), but as the concentration is furtherincreased quenching becomes dominate, and limitsthe emission from the material. Figure 8 demon-strates this effect in the relative intensity at 482 nmas a function of concentration, with a fit to a lognor-mal function as a guide for the eye. From Figure 8we find that the 10 mol% sample provides the most

6

FIG. 6: Normalized 482 nm and 520 nm emissionintensity from a [Y0.99Dy0.01(acac)3(phen)] sampleas a function of time. The emission at 482 nm isfound to be bi-exponential with lifetimes of 16 µsand 0.6 µs, while the 520 nm emission is found tofollow a single exponential with a lifetime of 0.6 µs.

The bi-exponential behavior of the 482 nmintensity is due to emission from both the

lanthanide and ligands, while the 520 nm intensityfollows a single exponential as it consists of

emission from the ligands only.

emission intensity.To further characterize concentration quenching

in [Y1−xDyx(acac)3(phen)], we measure the lumi-nescence intensity as a function of time after exci-tation to determine the luminescence lifetime at dif-ferent Dy concentrations. Figures 9a and 9b showthe emission intensity as a function of time after ex-citation for the different concentrations. From bothFigures 9a and 9b we find that the luminescence de-cays as a bi-exponential function with the fast decayoccurring with a concentration independent lifetimeof ≈ 0.6 µs and the slower decay having a concen-tration dependent lifetime, which decreases with in-creasing concentration. The observation of the longlifetime depending on the Dy concentration and theshort lifetime being invariant with concentration isadditional evidence that the short lifetime compo-nent is due to the ligands, while the long lifetimecomponent is due to the Dy ion.Using simple exponential fits to the data in Fig-

ures 9a and 9b we determine the lifetime as a func-tion of Dy concentration, which is shown in Figure8. From Figure 8 we find that as the concentra-tion increases the lifetime becomes shorter, with thelargest change occurring when going from 1 mol% to5 mol%.

FIG. 7: Scaled gated emission spectra of[Y1−xDyx(acac)3(phen)] MCs for different Dy

concentrations

FIG. 8: Relative intensity and lifetime of theemission at 482 nm as a function of Dy

concentration. The relative intensity is fit to alognormal function and the lifetime is fit to

Equation 4.

The influence of concentration quenching on lan-thanide luminescence lifetimes has been studied indepth [48, 62–66, 76–80] with the two main mecha-nisms of concentration quenching in solid-state ma-terials being: long-range Energy Transfer (LRET)[62–65, 80], and short-range Dexter electron ex-change [66]. These mechanisms result in differentfunctional behavior of the lifetime as a function ofconcentration. Assuming that the ion has an un-quenched lifetime of τ0, the lifetime τ (as a function

7

(a) (b)

FIG. 9: Normalized luminescence intensity as a function of time for different Dy concentrations atwavelengths of 482 nm (a) and 575 nm (b). Note that the fast decay seen at early times has a

concentration independent lifetime of ≈ 0.6 µs, which is consistent with emission from [Y(acac)3(phen)].

of concentration C) is:

τ =τ0

1 +(

CCH

)α , (3)

for LRET with CH being the half-quenching concen-tration and α being an exponent that depends on theenergy transfer mechanism (e.g. dipole-dipole inter-actions, quadrapole-dipole interactions). For Dexterelectron exchange the lifetime as a function of con-centration is given by:

τ =τ0

1 + kDe−(C/C0)−1/3, (4)

where c0 is the critical concentration [81] and kD isa material dependent constant. From the functionalforms of Equations 3 and 4 we note that LRET re-sults in the lifetime asymptotically approaching zero,while the Dexter mechanism has the lifetime asymp-totically approaching a nonzero value. Given thesedifferences in functionality of Equations 3 and 4, andthe observed behavior of the lifetime in Figure 8, weconclude that the Dexter mechanism’s concentrationdependence best matches the observed experimentaldata. Therefore we fit the lifetime data in Figure 8to Equation 4 and observe good agreement.The result of the experimental lifetimes follow-

ing Equation 4 is surprising, as typically the otherquenching mechanisms are more dominant [80] asthey are longer range interactions than Dexter elec-tron exchange. Typically Dexter electron exchangebased quenching doesn’t occur until the averagespacing of ions is on the order of 10’s of angstroms,

as the Dexter mechanism is proportional to the over-lap of the acceptor and donor wavefunctions [66].

Given this spatial dependence it is necessary to es-timate the ion spacing in order to determine if Dex-ter electron exchange is a valid hypothesis. To do sowe first estimate the particle density using the latticevolume (see Table I), which gives a particle densityof np = 3.52×1026 m−3. This density represents thenumber of molecular complexes in a given volume,but not the density of Dy ions. To get the density ofDy ions we need to scale the particle density by thedoping percentage, which gives the ion density to beni = cnp, where c is the doping percentage. Whilea precise calculation of the mean inter-ion distance〈r〉, from the concentration is difficult, we can get

a rough estimate using the formula 〈r〉 ∼ 1/n1/3i .

Using this formula we get an inter-ion distance of65.7 A for the 1 mol% sample and 26.7 A for the 15mol% sample. These spacings suggest that the Dex-ter mechanism is a valid hypothesis for our observedconcentration dependence. However, further study(using more concentrations and a calculation of theexchange radius) is required to verify this result.

D. Temperature Dependence

1. Emission Spectra

With the concentration dependence of the emis-sion of [Y1−xDyx(acac)3(phen)] characterized, wenow turn to consider the influence of temperature on

8

FIG. 10: Emission spectra of a[Y1−xDyx(acac)3(phen)] (Dy concentration of 10mol%) at 14 different temperatures. The emission

displays strong thermal quenching.

its emission. To do so we use the gated spectroscopysystem to measure the temperature dependent emis-sion of [Y1−xDyx(acac)3(phen)] at 14 different tem-peratures ranging from 293 K to 423 K. Figure 10shows an example set of emission spectra for the10 mol% sample. From Figure 10 we find that theemission spectra display strong thermal quenching,which is seen in the emission from all concentrationstested.

We characterize the material’s thermal quenchingby plotting the emission intensity at 482 nm as afunction of temperature for all four concentrationsin Figure 11, with the intensity normalized such thatthe room temperature intensity is unity. From Fig-ure 11 we find that the intensity as a function of tem-perature decays exponentially with each concentra-tion’s curve being within uncertainty of each other.Fitting each curve to a simple exponential and tak-ing the weighted average of the decay constants overall four concentrations we find that the average de-cay constant is 36.89± 0.84 K.

To consider what this temperature dependencemeans practically, we assume that we have a detec-tion system capable of accurately measuring down to1% of the room temperature signal. Using the de-cay constant determined above, this dynamic rangegives a functional maximum temperature of approx-imately 463 K. However, for our current detectionsystem we find that for temperatures above 423 Kthe signal to noise ratio is unacceptably small (≈ 2for the 455 nm peak and ≈ 16 for the 482 nm peak).Thus our current maximum temperature limit is≈ 423 K.

FIG. 11: Normalized emission intensity at 482 nmas a function of temperature for different Dy

concentrations, with the intensity normalized tothe room temperature value. The emission shows

strong thermal quenching.

2. Luminescence Lifetime

In addition to the thermal quenching affectingthe emission intensity we also find that it af-fects emission lifetime, which is consistent withprevious measurements of other lanthanide-dopedcomplexes [77, 81–83]. We characterize the in-fluence of thermal quenching on the lifetime of[Y1−xDyx(acac)3(phen)] by measuring the lifetimeof the emission at 482 nm and 575 nm for the10 mol% sample, as shown in Figure 12. FromFigure 12 we find that thermal quenching in[Y0.9Dy0.1(acac)3(phen)] results in the lifetime de-creasing with increasing temperature.

This decrease in lifetime with increasing temper-ature is typically modeled as a barrier process withthe rate of energy loss from the excited state k, being

k = k0 + kNR(T ),

= k0 + k1e−∆ET /kT , (5)

where k0 is the excited state’s natural decay rate,kNR(T ) = k1e

−∆ET /kT is the rate of nonradiativeenergy transfer as a function of temperature T , with∆ET being the energy transfer barrier and k1 beinga rate related to non-radiative energy transfer fromthe excited state. Recalling that the lifetime of theexcited state is given by τ = 1/k, we use Equation5 to give the lifetime as a function of temperature:

9

FIG. 12: Lifetimes of the luminescence emission at482 nm and 575 nm as a function of temperaturefor [Y0.9Dy0.1(acac)3(phen)]. As the temperatureincreases the lifetimes decrease due to thermal

quenching.

TABLE II: Fit parameters determined fromlifetimes as a function of temperature. The

parameters for the lifetime at 482 nm and 575 nmare found to be within uncertainty of each other.

Parameter 482 nm 575 nmτ0 (µs) 11.6± 4.5 13.8 ± 3.9τ1(ns) 4.2± 2.5 5.3± 1.7

∆ET (10−3 eV) 200± 28 193± 13

τ =1

k0 + k1e−∆ET /kT,

=τ0

1 + τ0τ1e−∆ET/kT

, (6)

where τi = 1/ki.Fitting the lifetime data in Figure 12 to Equation

6 we determine the different parameters for both the482 nm and 575 nm peaks with the results tabulatedin Table II. From Table II we find that the parame-ters for both the 482 nm and 575 nm peak are withinuncertainty of each other. This is expected as bothpeaks correspond to transitions from the 4F9/2 en-ergy level.

3. Intensity Ratios

At this point we have characterized the emissionproperties of [Y1−xDyx(acac)3(phen)] for different

FIG. 13: Integrated peak ratio (455 nm/482 nm) asa function of temperature for four different Dy

concentrations

temperatures and concentrations. The next steptowards using [Y1−xDyx(acac)3(phen)] as a TCTphosphor is to characterize the excited state ratiosbetween the 4I15/2 and 4F9/2 states as a functionof temperature. This ratio is probed by measur-ing the ratio between the emission peak at 455 nm(4I15/2 → 6H15/2) and 482 nm (4F9/2 → 6H15/2).In our study – to improve the signal to noise ratio –we use the spectrally integrated peak intensities tocalculate the peak ratios with the 455 nm peak hav-ing an integration range of 450 nm to 460 nm andthe 482 peak having an integration range of 465 nmto 500 nm.

Using the gated spectra measured at different tem-peratures for each concentration we integrate thespectral peaks and calculate their ratios. The ra-tios of the integrated peaks are shown in Figure 13as a function of inverse temperature. Note that forthe 1:99 sample the intensity at temperatures above383 K becomes so weak as to make the ratios un-reliable. These ratios are therefore not used whenperforming fitting.

From Figure 13 we find that for all four concentra-tions the integrated ratio behaves as an exponentialwith inverse temperature, which is consistent withEquation 2. Fitting the ratios in Figure 13 to Equa-tion 2 we determine both the amplitude parameterA and the energy barrier ∆E, which are tabulatedin Table III.

From Table III we find that the energy differencesare all within uncertainty of each other and have a

10

TABLE III: Exponential fit parameters determinedfrom Figure 13 for different Dy concentrations

Dy Concentration (mol%) A ∆E (10−3 eV)1 2.30 ± 0.87 114 ± 145 1.06 ± 0.31 100.1 ± 9.410 1.08 ± 0.20 98.4 ± 6.215 0.98 ± 0.13 99.7 ± 4.3

weighted mean of 100.2(±3.2)× 10−3 eV. This en-ergy difference is significantly less than the expectedenergy difference of ≈ 0.15 eV (energy difference be-tween photons with wavelengths of 482 nm and 455nm). While the measured difference is less than thedifference expected due to the peak wavelengths, it isactually consistent with the energy difference deter-mined for Dy3+ in a water solution: ∆E ≈ 0.1147eV (energy difference between photons with wave-lengths of 475 nm and 455 nm) [43–46].To understand why the energy difference mea-

sured in this study (and seen in the literature[24, 28, 43–46, 84]) differs from the the value de-termined using the peak wavelengths, we note thatthe peak spacing (455 nm vs. 482 nm) does not di-rectly represent the spacing between the 4I15/2 and4F9/2 energy levels. In reality each broad spectralpeak consists of multiple overlapping smaller peaksthat arise due to transitions from the excited stateinto the Stark-split 6H15/2 ground state [45, 85].Therefore, to accurately determine the spacing be-

tween the 4I15/2 and 4F9/2 energy levels using peakpositions, we need to use transitions from the ex-cited states into the same 6H15/2 sub-level. Thishowever is difficult to accomplish when the transi-tions to the different sub levels are overlapping fromthermal and/or inhomogeneous broadening. Thisbroadening can be minimized either by using lowtemperatures [85] or a host material with a strongcrystal field [45]. Alternatively, by integrating overthe full peak (as we do in this study) we can accountfor all transitions from the 4I15/2 and 4F9/2 energy

levels into the 6H15/2 state and obtain an accurateenergy difference.Having determined the temperature response of

the intensity ratio, we can now calculate the sensitiv-ity of [Y1−xDyx(acac)3(phen)] and the temperatureresolution of our TCT technique. The sensitivity S,of a TCT phosphor is given by [86]

S =100

r

∂r

∂T, (7)

where r is the intensity ratio and the sensitivity isgiven in units of % K−1. To calculate the sensitiv-ity we first take the derivative of the intensity ratiofunction (Equation 2) and substitute in the average

FIG. 14: Sensitivity and temperature resolution ofour TCT method as a function of temperature.

amplitude and energy difference from Table I. Wethen divide the calculated derivative by the experi-mentally obtained ratio (for the 10 mol% samples)and compute Equation 7 with Figure 14 showingthe computed sensitivity. From Figure 14 we findthat the sensitivity has a maximum value of 1.5 %K−1, which is comparable to many other tempera-ture sensing phosphors [86].

In addition to calculating the sensitivity of ourphosphor we also consider the temperature resolu-tion ∆T , of our technique, which not only dependson the sensitivity of the phosphor, but also the ex-perimental uncertainty of the optical detection sys-tem. The temperature resolution can be defined asthe temperature change required to change the ratioby an amount equal to the experimental uncertainty.Mathematically this takes the form of

∆T = σr

(

∂r

∂T

)

−1

, (8)

where σr is the experimental uncertainty in the ratio.

Using the calculated derivative of the ratio and theratio uncertainty for the 10 mol% sample we calcu-late the temperature resolution and show the resultsin Figure 14. From Figure 14 we find that at roomtemperature the temperature resolution is approxi-mately 1.8 K and that it increases with temperature.This behavior is primarily due to the effects of ther-mal quenching, which is found to increase the rela-tive uncertainty of the measured ratio as the tem-perature is increased.

11

E. Photodegradation

In the previous section we measuredthe 455 nm/482 nm intensity ratios of[Y1−xDyx(acac)3(phen)] at different temperaturesand determined the calibration parameters for thematerial’s temperature response. While this analy-sis is the main purpose of our study we also observesignificant yellowing of the [Y1−xDyx(acac)3(phen)]powder during extended UV exposure, whichsuggests that the material undergoes significantphotodegradation.Given that [Y1−xDyx(acac)3(phen)] consists of ei-

ther a Dy or Y ion connected to organic ligands,photodegradation will occur solely due to the lig-ands, implying that the Dy spectral ratios should beunaffected (assuming the ligand stays transparent inthe 450 nm – 500 nm range). However, while the ra-tios should be unaffected, the overall intensity willbe decreased as the ligand acts as an “antenna” forthe Dy ion [52] and damage to the ligand will de-crease the efficiency of energy transfer between theligand and Dy ion.To characterize this decrease in energy transfer ef-

ficiency (and confirm that the ratios are unaffected)we measure the emission spectra between 450 nmand 500 nm for a 10 mol% sample as a function oftime during constant UV illumination at two differ-ent temperatures (293 K and 393 K). For both degra-dation and excitation we use the same 355 nm pulsedlaser with the fluence per pulse being 89 mJ/cm2

(average intensity of 0.891W/cm2). Figure 15 showsthe scaled integrated intensity of the 482 nm peak(integration range 465 nm to 500 nm, smoothed forclarity) as a function of time for both 293 K and393 K, with the intensity scaled such that the initialintensity is 1.From Figure 15 we find that the scaled integrated

intensity of the 482 nm peak decays exponentiallywith time, with the higher temperature being foundto decay more quickly. Fitting the decay curves inFigure 15 to a simple exponential function we finddecay rates of γ = 1.40(±0.26) × 10−3 s−1 for the293 K curve and γ = 3.29(±0.56)× 10−3 s−1 for the393 K curve.Comparing the decay rates between the two tem-

peratures we find that by increasing the temperatureby 100 K the ligand decays 2.35 × more quickly thanat room temperature. This temperature dependencesuggests that photodegradation is due to an energybarrier process. To estimate this energy barrier wefirst assume that the decay rate depends on temper-ature as an Arrhenius function:

γ = γ0e−∆ED/kT , (9)

FIG. 15: Scaled integrated intensity (465 nm – 500nm) as a function of time during 355 nm exposurefor two different temperatures. The material is

found to photodegrade under 355 nm illuminationwith the decay rate increasing with temperature.Note that the data is smoothed using a binomial

algorithm for clarity.

where γ0 is the asymptotic decay rate and ∆ED isthe photodegradation energy barrier. With this as-sumption we calculate the ratio of decay rates to be,

γ1γ2

= exp

[

−∆ED

k

(

1

T1−

1

T2

)]

, (10)

where γ1 is the decay rate at temperature T1 and γ2is the decay rate at temperature T2. Equation 10can be rearranged to give the energy barrier to be

∆ED = −kT1T2

T2 − T1ln

(

γ1γ2

)

. (11)

Substituting in our experimental values we can esti-mate the energy barrier to be ∆ED = 0.084± 0.011eV. Note that a more accurate determination – us-ing degradation at many different temperatures – ofthis energy barrier is beyond the scope of this cur-rent study.In addition to determining the temperature de-

pendence of (acac)3(phen)’s photodegradation, wealso compare its photostability to other organic ma-terials by computing the commonly used figure ofmerit for organic dyes [87]:

FoM =B

σ(12)

=〈I〉

γ~ω, (13)

12

FIG. 16: Integrated ratio (455 nm/482 nm) as afunction of time during photodegradation. The

ratio is found to be invariant within experimentaluncertainty for the entire degradation time period.

where σ is the undamaged absorbance cross section,B is the degradation’s inverse quantum efficiency,〈I〉 is the average pump intensity, γ is the decayrate, and ~ω is the photon energy of the degrada-tion source. Using Equation 13 and the values fromour room temperature (293 K) measurement we de-termine a FoM of 1.13(±0.21)× 1021 cm−2, which ison the same order as a large number of organic ma-terials, but almost two orders of magnitude belowthe most photostable organic materials [87].With the influence of (acac)3(phen)’s photodegra-

dation on the emission intensity now determined, weturn to considering how (acac)3(phen)’s photodegra-dation affects the intensity ratio between the 455 nmpeak and the 482 nm peak. Using the same calcula-tion method described in Section IVD 3 we calculatethe integrated peak ratio as a function of tempera-ture for both temperatures as shown in Figure 16.From Figure 16 we find that for both 293 K and393 K the integrated ratio is invariant within uncer-tainty for the entire time frame of photodegradation.This result confirms our initial hypothesis that pho-todegradation won’t influence the intensity ratios.

F. Dynamic Heating Test

Having now determined the temperature depen-dence of the integrated ratio and shown that the ra-tio is invariant during ligand photodegradation, wenow demonstrate the use of [Y1−xDyx(acac)3(phen)]as a TCT phosphor. To do so we heat a sample(10 mol%) and measure the luminescence spectrum

FIG. 17: Temperature as a function of time asmeasured by a TC and calculated using TCT with[Y0.9Dy0.1(acac)3(phen)]. For comparison we alsoinclude the intensity at 482 nm as a function oftime. We find that as the phosphor intensityapproaches zero the calculated temperaturebecomes more noisy, with the peak accurate

temperature near 370 K.

and TC temperature as a function of time using thegated spectroscopy system.

After heating we compute the integrated inten-sity ratio as a function of time and calculate thetemperature using Equation 1 and the calibrationvalues. Figure 17 shows the calculated temperature,TC value, and the peak intensity at 482 nm as afunction of time. From Figure 17 we find that thecalculated temperature is within uncertainty of theTC temperature for temperatures up to about 370K after which point the calculated temperatures aremore noisy. This noise in temperatures is due to theemission intensity decreasing to the point where theSNR is too small to obtain accurate temperatures.

From Figures 11 and 17 it should be noted thatthe maximum accurate temperature is lower in Fig-ure 17 than in Figure 11. The reason for this differ-ence is related to the different camera settings be-tween the static calibration measurements and thedynamic heating measurements. During the calibra-tion measurements the camera gain is adjusted foreach temperature to account for the decrease in in-tensity, while during dynamic heating the gain isfixed for the entire run. This leads to the intensityquickly falling below the accurate detection limit.We are currently working on developing new soft-ware which will dynamically adjust the camera gainallowing for a larger temperature range.

13

V. CONCLUSIONS

We develop a TCT phosphor based on a[Y1−xDyx(acac)3(phen)] molecular crystals. Theemission (using an excitation wavelength of 355 nm)is found to contain four peaks originating from theDy ion and a broad background peak arising fromthe ligand. The emission from the Dy ion at roomtemperature is found to have a lifetime ranging from6 µs to 16 µs depending on Dy concentration, andthe emission from the ligand at room temperaturehas a lifetime of approximately 600 ns. This differ-ence in lifetime allows us to remove the ligand’s con-tribution from the emission spectra by using time-gated spectroscopy.

Using this technique we characterize the mate-rial’s emission for four different concentrations and14 different temperatures. From these measure-ments we find that the material displays both con-centration and thermal quenching, which resultsin decreased emission intensity and luminescencelifetimes at high concentrations and temperatures.With regards to concentration quenching we findthat the peak intensity occurs for a Dy concentrationof ≈ 10 mol% and that the lifetime as a function ofconcentration appears to follow the functionality ofDexter electron exchange quenching. While the es-timated ion spacing is found to be the correct orderof magnitude for the Dexter exchange mechanism,a more in depth study of the concentration depen-dence is needed to verify this hypothesis.

We also find that the effect of thermal quench-ing on [Y1−xDyx(acac)3(phen)]’s emission is con-sistent with results seen in other lanthanide-dopedcomplexes. We find that the emission intensity de-cays exponentially in the temperature range tested(with the 1/e intensity decay temperature being36.89± 0.84 K) and that the emission lifetime obeysEquation 6 with a thermal quenching barrier of≈ 0.2eV. Due to this thermal quenching we find that – forour current spectroscopy system – we can accurately

measure the emission to a temperature of ≈423 K.Additionally, we find that the organic ligand pho-

todegrades under UV illumination resulting in a de-crease of the Dy emission peaks as the ligand acts asan “antenna” for the Dy ion. Despite this decreasein luminescence efficiency, due to photodegradation,we find that the ratio between the 455 nm and 482nm peaks is unchanged.With the emission of [Y1−xDyx(acac)3(phen)]

characterized, we next demonstrate its use as a TCTphosphor by heating a 10 mol% sample dynamicallyand measuring the emission as a function of time us-ing our time-gated spectroscopy system. Based onthe emission spectra (and using the known calibra-tion parameters) we compute the temperature andcompare the calculated value to that measured bya thermocouple with both temperatures found to bewithin uncertainty of each other up to a temperatureof approximately 370 K, with the main limitationdue to the current spectroscopy systems software.To remove this limitation we are currently develop-ing improved software, which will allow for a highertemperature range with our initial target maximumtemperature being 423 K.In summary, we find that [Y1−xDyx(acac)3(phen)]

forms organic molecular crystals with appropri-ate spectroscopic properties for use as a TCTphosphor. Our next step – towards using[Y1−xDyx(acac)3(phen)] as a TCT phosphor duringshock compression of heterogeneous materials – is toperform fast laser heating of the phosphor in orderto demonstrate its ability to measure temperatureduring short time scales. At the same time we arealso testing other Dy3+-doped Yttrium ternary com-plexes to determine how their thermal performancecompares to [Y1−xDyx(acac)3(phen)].

ACKNOWLEDGEMENTS

This work was supported by the Air Force Officeof Scientific Research, Award # FA9550-15-1-0309to Washington State University.

[1] M. Chen, S. You, K. Suslick, and D. Dlott, “Hotspots in energetic materials generated by infaredand ultrasound, detected by thermal imaging mi-croscopy,” Rev. Sci. Instrum 85, 023705 (2014).

[2] M. Chen, S. You, K. Suslick, and D. Dlott,“Hot spot generation in energetic materials createdby long-wavelength infared radiation,” App. Phys.Lett. 104, 061907 (2014).

[3] R. Pokharel, J. Lind, A. Kanjarla, R. Lebensohn,S. Li, P. Kenesei, R. Suter, and A. Rollett, “Poly-

crystal plasticity: comparison between grain-scaleobservations of deformation and simulations,” An-nual Review of Condensed Matter Physics 5, 317–346 (2014).

[4] Q. An, W. Goddard, S. Zybin, A. Jaramillo-Botero,and T. Zhou, “Highly shocked polymer bonded ex-plosiuves at a nonplanar interface: hot-spot for-mation leading to detonation,” Journal of PhysicalChemistry C 117, 26551–26561 (2013).

14

[5] Y. Wu and F. Huang, “A microscopic model for pre-dicting hot-spot ignition of granular energetic crys-tals in response to drop-weight impacts,” Mechanicsof Materials 43, 835–852 (2011).

[6] P. Rae, H. Goldrein, S. Palmer, J. Field, andA. Lewis, “Quasi-static studies of the deforma-tion and failure of beta-hmx based polymer bondedexplosives,” Proceedings of the Royal Society a-Mathematical Physical and Engineering Sciences458, 743–762 (2002).

[7] Y. Gruzdkov and Y. Gupta, “Vibrational proper-ties and structure of pentaerythritol tetranitrate,”J. Phys. Chem. A 105, 6197–6202 (2001).

[8] Y. Gupta, “Recent developments to understandmolecular-changes in schocked energetic materials,”Journal De Physique IV 5, 345–356 (1995).

[9] A. Mellor, D. Wiegand, and K. Isom, “Hot-spot his-tories in energetic materials,” Structure and Prop-erties of Energetic Materials 296, 293–298 (1993).

[10] J. Field, “Hot-spot ignition mechanisms for explo-sives,” Acc. Chem. Res. 25, 489–496 (1992).

[11] J. Field, N. Bourne, S. Palmer, S. Walley, andJ. Smallwood, “Hot-spot ignition mechanisms forexplosives and propellants,” Philosophical Transac-tions of the Royal Society of London-MathematicalPhysical and Engineering Sciences 339, 269–283(1992).

[12] J. Field, G. Swallowe, and S. Heavens, “Ignitionmechanisms of explosives during mechanical defor-mation,” Proceedings of the Royal Society of Lon-don Series A 382, 231 (1982).

[13] R. Winter and E. Faber, “The role of localized plas-tic flow in the impact initiation of explosives,” Pro-ceedings of the Royal Society of London Series A343, 399–413 (1975).

[14] M. Chaudhri and J. Field, “The role of rapidly com-pressed gas pockets in the initiation of condensed ex-plosives,” Proceedings of the Royal Society of Lon-don 340, 113–128 (1974).

[15] S. N. Luo, B. J. Jensen, D. E. Hooks, K. J. Ramos,J. D. Yeager, K. Kwiatkowski, T. Shimada, D. A.Fredenburg, and K. Fezzaa, “Ultrafast, high res-olution, phase contrast imaging of shock responsewith synchrotron radiation: opportunities and chal-lenges,” in “APS Meeting Abstracts,” (2012).

[16] B. J. Jensen, K. J. Ramos, A. J. Iverson, J. Bernier,C. A. Carlson, J. D. Yeager, K. Fezzaa, and D. E.Hooks, “Dynamic experiment using impulse at theadvanced photon source,” Journal of Physics: Con-ference Series 500, 042001 (2014).

[17] B. J. Jensen, S. N. Luo, D. E. Hooks, K. Fezzaa,K. J. Ramos, J. D. Yeager, K. Kwiatkowski, T. Shi-mada, and D. M. Dattelbaum, “Ultrafast, highresolution, phase contrast imaging of impact re-sponse with synchrotron radiation,” AIP Advances2, 012170 (2012).

[18] J. Yeager, S. Luo, B. Jensen, K. Fezzaa, D. Mont-gomery, and D. Hooks, “High-speed synchrotron x-ray phase contrast imaging for analysis of low-z com-posite microstructure,” Composites Part A: AppliedScience and Manufacturing 43, 885 – 892 (2012).

[19] S. N. Luo, B. J. Jensen, D. E. Hooks, K. Fez-zaa, K. J. Ramos, J. D. Yeager, K. Kwiatkowski,and T. Shimada, “Gas gun shock experimentswith single-pulse x-ray phase contrast imaging anddiffraction at the advanced photon source,” Reviewof Scientific Instruments 83, 073903 (2012).

[20] “http://www.dcs-aps.wsu.edu/,” .[21] “New research opportunities in dynamic com-

pression science,” Tech. rep., Institute for ShockPhysics, Washington State University (2012).

[22] A. Z. M. Al-Juboori, “Rare earth (Sm3+ and Dy3+)-doped gadolinium oxide nanomaterials for lumines-cence thermometry,” Physica Scripta (2013).

[23] A. Heyes, S. Seefeldt, and J. Feist, “Two-colourphosphor thermometry for surface temperaturemeasurement,” Optics & Laser Technology 38, 257–265 (2006).

[24] A. Khalid and K. Kontis, “Thermographic phos-phors for high temperature measurements: Prin-ciples, current state of the art and recent applica-tions,” Sensors 8, 5673–5744 (2008).

[25] K. Kontis, “A review of some current researchon pressure sensitive and thermographic phosphortechniques,” The Aeronautical Journal 3162, 495–508 (2007).

[26] S.W., Allison, and G. Gillies, “Remote thermom-etry with thermographic phosphors: Instrumenta-tion and applications,” Review Scientific Instru-ments 68, 2615–2650 (1997).

[27] M. Cates, S. Allison, S. Jaiswal, and D. Beshears,“YAG:Dy and YAG:Tm fluorescence to 1700 ◦c,”in “The 49th International Instrumentation Sym-posium – The Instrumentation, Systems, and Au-tomation Society,” (Orlando, FL, 2003).

[28] G. Gross, A. Smith, and M. Post, “Surface ther-mometry by laser induced fluorescence,” ReviewScientific Instruments 60, 12 (1989).

[29] K. Kontis, Y. Syogenji, and N. Yoshikawa, “Sur-face thermometry by laser induced fluorescence ofDy3+ :YAG,” The Aeronautical Journal 106, 453–457 (2002).

[30] K. Kontis, “Surface heat transfer measurements in-side a supersonic combustor by laser induced flu-orescence,” Journal of Thermophysics and HeatTransfer 17, 320–325 (2003).

[31] R. Hasegawa, I. Sakata, H. Yanagihara, B. Johans-son, A. Omrane, and M. Alden, “Two-dimensionalgas phase temperature measurements using phos-phor thermometry,” App. Phys. B 88, 291–296(2007).

[32] G.Sarner, A. Omrane, H. Seyfried, M. Richter,H. Schmidt, and M. Alden, “Laser diagnostics ap-plied to a full-size fighter jet afterburner,” in “Eu-ropean Combustion Meeting,” (2005).

[33] J. Feist, A. Heyes, and S. Seefeldt, “Thermographicphosphors for gas turbines: instrumentation devel-opment and measurement uncertainties,” in “11thInternational symposium on Application of LaserTechniques to Fluid Mechanics,” (Lisbon, Portugal,2002).

15

[34] J. P. Feist, A. L. Heyes, and J. R. Nicholls, “Phos-phor thermometry in an electron beam physicalvapour deposition produced thermal barrier coat-ing doped with dysprosium,” in “Proceedings ofthe Institution of Mechanical Engineers,” , vol. 215(2001), vol. 215, pp. 333–341.

[35] J. P. Kumar, G. Ramgopal, Y. Vidya, K. Anan-tharaju, B. D. Prasad, S. Sharma, S. Prashantha,H. Nagaswarupa, D. Kavyashree, and H. Nagab-hushana, “Green synthesis of Y2O3:Dy3+ nanophos-phor with enhanced photocatalytic activity,” Spec-trochimica Acta Part A: Molecular and Biomolecu-lar Spectroscopy 149, 687 – 697 (2015).

[36] S. K. Gupta, M. Mohapatra, V. Natarajan, andS. V. Godbole, “Photoluminescence investigationsof the near white light emitting perovskite ceramicSrZrO3:Dy3+ prepared via gel-combustion route,”International Journal of Applied Ceramic Technol-ogy 10, 593–602 (2013).

[37] M. Jayasimhadri, B. V. Ratnam, K. Jang, H. S. Lee,B. Chen, S.-S. Yi, J.-H. Jeong, and L. Rama Moor-thy, “Combustion synthesis and luminescent prop-erties of nano and submicrometer-size Gd2O3:Dy3+

phosphors for white leds,” International Journal ofApplied Ceramic Technology 8, 709–717 (2011).

[38] B. Ratnam, M. Jayasimhadri, K. Jang, H. Sueb Lee,S.-S. Yi, and J.-H. Jeong, “White light emissionfrom NaCaPO4:Dy3+ phosphor for ultraviolet-basedwhite light-emitting diodes,” Journal of the Ameri-can Ceramic Society 93, 3857–3861 (2010).

[39] J. Kaur, Y. Parganiha, V. Dubey, D. Singh, andD. Chandrakar, “Synthesis, characterization andluminescence behavior of ZrO2:Eu

3+, Dy3+ withvariable concentration of Eu and Dy doped phos-phor,” Superlattices and Microstructures 73, 38 –53 (2014).

[40] Y. Akita, T. Harada, R. Sasai, K. Tomita, andK. Nishiyama, “Emission properties of Ln (Eu, Tb,Dy, Er)-doped {Y2O3} nanoparticles synthesized bysurfactant-assembly and their applications in visiblecolor-tuning,” Journal of Photochemistry and Pho-tobiology A: Chemistry 299, 87 – 93 (2015).

[41] B.R. Anderson, R. Gunawidjaja, P. Price, andH. Eilers, “Spectroscopic determination of ther-mal impulse in sub-second heating events usinglanthanide-doped oxide precursors and phenomeno-logical modeling,” Journal of Applied Physics, 120,083102 (2016).

[42] R. Gunawidjaja, B.R. Anderson, H. D. y Riega, andH. Eilers, “Sub-second laser heating of thermal im-pulse sensors,” AIP Conf. Proc. p. pending publica-tion (2016).

[43] R. K. Carnall W.T., “Electronic energy levels in thetrivalent lanthanide aquo ions,” J. Chem Phys. 49,4424–4442 (1968).

[44] W. Carnall, “The absorption and fluorescence spec-tra of rare earth ions in solution,” in “Handbookon the physics and chemistry of rare earths,” ,K. Gschneidner and L. Eyring, eds. (Elsevier BV,Amsterdam, 1979), chap. 24.

[45] J.-C. G. Bunzli and S. V. Eliseeva, “Basics of lath-anide photophysics,” in “Lanthanide Luminescence:Photophysical, Analytical and Biological Aspects,”, P. Hanninen and H. Harma, eds. (Springer-Verlag,2010).

[46] J. Bunzli, “Lanthanide luminescence for biomedicalanalyses and imaging,” Chem. Rev. 110, 2729–2755(2010).

[47] N. F. W. F. Sager and F. A. Serafin, “Substituenteffects on intramolecular energy transfer. I. absorp-tion and phosphorescence spectra of rare earth β-diketone chelates,” J Phys. Chem 69, 1092 (1965).

[48] D. Dexter, “A theory of sensitized luminescence insolids,” J. Chem. Phys. 21, 836 (1953).

[49] H. Xin, M. Shi, X. C. Gao, Y. Y. Huang, Z. L. Gong,D. B. Nie, H. Cao, Z. Q. Bian, F. Y. Li, and C. H.Huang, “The effect of different neutral ligands onphotoluminescence and electroluminescence proper-ties of ternary terbium complexes,” The Journal ofPhysical Chemistry B 108, 10796–10800 (2004).

[50] S. I. Klink, “Synthesis and photophysics of light-converting lanthanide complexes,” Ph.D. thesis,University of Twente (2000).

[51] M. Latva, H. Takalo, V.-M. Mukkala, C. Matach-escu, J. C. Rodriguez-Ubis, and J. Kankare, “Corre-lation between the lowest triplet state energy level ofthe ligand and lanthanide(III) luminescence quan-tum yield,” Journal of Luminescence 75, 149 – 169(1997).

[52] J. Feng, L. Zhou, S.-Y. Song, Z.-F. Li, W.-Q. Fan,L.-N. Sun, Y.-N. Yu, and H.-J. Zhang, “A study onthe near-infrared luminescent properties of xerogelmaterials doped with dysprosium complexes,” Dal-ton Trans. pp. 6593–6598 (2009).

[53] A. Adronov and J. M. Frechet, “Light-harvestingdendrimers,” Chem Comm pp. 1701–1710 (2000).

[54] R. S. Dickens, J. A. K. Howard, C. L. Maupin, J. M.Moloney, D. Parker, R. D. Peacock, J. P. Riehl, andG. Siligardi, “Ground and excited state chiropticalproperties of enantiopure macrocyclic tetranaphthyllanthanide complexes: controlled modulation of thefrequency and polarisation of emitted light,” New J.Chem. 22, 891–899 (1998).

[55] J. Bunzli, “Luminescent probes,” in “LanthanideProbes in Life Chemical and Earth Sciences: The-ory and Practice,” , J. Bunzli and G. Choppin, eds.(Elsevier, 1989), pp. 219–293.

[56] N. M. Shavaleev, S. V. Eliseeva, R. Scopelliti, andJ.-C. G. Bunzli, “N-aryl chromophore ligands forbright europium luminescence,” Inorganic Chem-istry 49, 3927–3936 (2010). PMID: 20302272.

[57] S. V. Eliseeva and J.-C. G. Bunzli, “Lanthanideluminescence for functional materials and bio-sciences,” Chem. Soc. Rev. 39, 189–227 (2010).

[58] L. N. Puntus, K. A. Lyssenko, I. S. Pekareva, andJ.-C. G. Bunzli, “Intermolecular interactions as ac-tors in energy-transfer processes in lanthanide com-plexes with 2,2’-bipyridine,” The Journal of Phys-ical Chemistry B 113, 9265–9277 (2009). PMID:19522489.

16

[59] J.-C. G. Bunzli, “Review: Lanthanide coordina-tion chemistry: from old concepts to coordinationpolymers,” Journal of Coordination Chemistry 67,3706–3733 (2014).

[60] J.-C. G. Bunzli, “Lanthanide light for biology andmedical diagnosis,” Journal of Luminescence 170,Part 3, 866 – 878 (2016). Light, Energy and Life.

[61] J.-C. G. Bunzli, “On the design of highly lumines-cent lanthanide complexes,” Coordination Chem-istry Reviews 293–294, 19 – 47 (2015). 41st In-ternational Conference on Coordination Chemistry,Singapore, July 2014.

[62] T. Forster, “Delocalized excitation and excitationtransfer,” in “Modern Quantum Chemistry. Istan-bul Lectures. Part III: Action of Light and OrganicCrystals,” , vol. 3 (Academic Press, 1965), pp. 93–137.

[63] T. Forster, “10th spiers memorial lecture. transfermechanisms of electronic excitation,” Discussions ofthe Faraday Society 27, 7–17 (1959).

[64] T. Forster, “Zwischenmolekulare energiewanderungund fluoreszenz,” Ann. Phys. 2, 55 (1948).

[65] T. Forster, “Energiewanderung und fluoreszenz,”Naturwissenschaften 33, 166–175 (1946).

[66] D. Dexter and J. H. Schulman, “Theory of con-centration quenching in inorganic phosphors,” J .Chem. Phys. 22, 1063 (1954).

[67] D. Dexter and W. Heller, “Capture and collisionprocesses for excitons in alkali halides,” Phys. Rev.84, 377 (1651).

[68] W. R. Heller and A. Marcus, “A note on the prop-agation of excitation in an idealized crystal,” Phys.Rev. 84, 809 (1951).

[69] S. Sato and M. Wada, “Relations between in-tramolecular energy transfer efficiencies and tripletstate energies in rare earth β-diketone chelates,”Bulletin of the Chemical Society of Japan 43, 1955–1962 (1970).

[70] J. Tang and P. Zhang, Lanthanide Single MoleculeMagnets (Springer, 2015).

[71] G.-J. Chen, C.-Y. Gao, J.-L. Tian, J. Tang,W. Gu, X. Liu, S.-P. Yan, D.-Z. Liao, andP. Cheng, “Coordination-perturbed single-moleculemagnet behaviour of mononuclear dysprosium com-plexes,” Dalton Trans. 40, 5579 (2011).

[72] W. C. Choy and D. Tao, “Rare-earth complexes andits applications in organic light emitting diodes,” in“Solid State Chemistry Research Trends,” , R. W.Buckley, ed. (Nova Science Publishers Inc, 2007),chap. 2.

[73] H. Brito, O. Malta, M. Felinto, and E. Teotonio,“Luminescence phenomena involving metal eno-lates,” in “The Chemistry of Metal Enolates,” ,J. Zabicky, ed. (Wiley, 2009), The Chemistry ofFunctional Groups, chap. 3.

[74] C. Y. Peng, H. J. Zhang, J. B. Yu, Q. G. Meng, L. S.Fu, H. R. Li, L. N. Sun, and X. M. Guo, “Synthe-sis, characterization, and luminescence properties of

the ternary europium complex covalently bonded tomesoporous sba-15,” J Phys. Chem B 109, 15278(2005).

[75] H. Xin, Y. Ebina, R. Ma, K. Takada, and T. Sasaki,J Phys. Chem B 110, 9863 (2006).

[76] V. R. Kharabe, A. H. Oza, and S. J. Dhoble,“Synthesis, PL characterizations and concentra-tion quenching effect in Dy3+ and Eu3+ activatedLiCaBO3 phosphor,” Luminescence 30, 432–438(2015).

[77] J. J. Joos, K. W. Meert, A. B. Parmentier, D. Poel-man, and P. F. Smet, “Thermal quenching andluminescence lifetime of saturated green Sr1−xEux

Ga2S4 phosphors,” Optical Materials 34, 1902–1907(2012).

[78] O. Meza, E. Villabona-Leal, L. Diaz-Torres, H. De-sirena, J. Rodriguez-Lopez, and E. Perez, “Lu-minescence concentration quenching mechanism inGd2O3:Eu

3+,” J Phys. Chem A 118, 1390–1396(2014).

[79] G. Ju, Y. Hu, L. Chen, X. Wang, and Z. Mu, “Con-centration quenching of persistent luminescence,”Physica B: Condensed Matter 415, 1 – 4 (2013).

[80] J. R. Lakowicz, Principles of FluorescenceSpectroscopy (Springer, 2006).

[81] J. Bierwagen, S. Yoon, N. Gartmann, B. Walfort,and H. Hagemann, “Thermal and concentration de-pendent energy transfer of Eu2+ in SrAl2O4,” Op-tical Materials Express 6 (2016).

[82] J. Ueda, P. Dorenbos, A. J. Bos, A. Meijerink,and S. Tanabe, “Insight into the thermal quenchingmechanism for Y3Al5O12:Ce

3+ through thermolu-minescence excitation spectroscopy,” J. Phys. ChemC 119, 25003–25008 (2015).

[83] R. Yu, D. S. Shin, K. Jang, Y. Guo, H. M. Noh,B. K. Moon, B. C. Choi, J. H. Jeong, and S. S. Yi,“Luminescence and thermal-quenching properties ofDy3+-doped Ba2CaWO6 phosphors,” Spectrochim-ica Acta A 125, 458–462 (2014).

[84] J. Feist and A. Heyes, “The characterization ofY2O2S:Sm powder as a thermographic phosphorfor high temperature applications,” Measure ScienceTechnology 11, 942–947 (2000).

[85] Y. Bi, C. Chen, Y.-F. Zhao, Y.-Q. Zhang, S.-D.Jiang, B.-W. Wang, J.-B. Han, J.-L. Sun, Z.-Q.Bian, Z.-M. Wang, and S. Gao, “Thermostabilityand photoluminescence of Dy( III ) single-moleculemagnets under a magnetic field,” Chem. Sci. 7, 5020(2016).

[86] C. D. S. Brites, P. P. Lima, N. J. O. Silva, A. Mil-lan, V. S. Amaral, F. Palacio, and L. D. Carlos,“Thermometry at the nanoscale,” Nanoscale 4, 4799(2012).

[87] B. R. Anderson, S.-T. Hung, and M. G. Kuzyk,“Wavelength dependence of reversible photodegra-dation of disperse orange 11 dye-doped pmma thinfilms.” J. Opt. Soc. Am. B 32, 1043–1049 (2015).