Spectroscopic peculiarities of CsCaI3:Tm2+ single crystals examined through one-photon and excited state excitation spectroscopy

Article abstract of DOI:10.1016/j.jallcom.2018.01.058

The CsCaI₃ single crystals doped with Tm²⁺ were grown by Bridgman technique. Divalent thulium luminescence related to the 4f¹³-4f¹³ intra-configurational and 4f¹²5d¹-4f¹³ inter-configurational transitions were analyzed as a function of temperature. Excited state excitation spectroscopy has been applied within a wide spectral range to examine peculiarities of transitions between different 4f¹²5d¹ excited states of Tm²⁺. This two-color excitation experiment was useful to assess d-d photon-absorption processes that are relevant to observed up-conversion phenomena attributed to effective anti-Stokes green luminescence around 550 nm. Furthermore, activation energy of 4f¹²5d¹ → 4f¹³ luminescence quenching was estimated and high value of ²F_5/2 → ²F_7/2 emission cross-section was determined in CsCaI₃:Tm²⁺. Our spectroscopic study was performed to complement and verify the previous investigation in this field.

Full text of DOI:10.1016/j.jallcom.2018.01.058

Accepted Manuscript
2+
Spectroscopic peculiarities of CsCaI :Tm single crystals examined through one-
3
photon and excited state excitation spectroscopy
Mirosław Karbowiak, Radosław Lisiecki, Piotr Solarz, Jarosław Komar, Witold Ryba-
Romanowski
PII:
S0925-8388(18)30059-8
10.1016/j.jallcom.2018.01.058
JALCOM 44522
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 24 September 2017
Revised Date: 3 January 2018
Accepted Date: 4 January 2018
Please cite this article as: Mirosł. Karbowiak, Radosł. Lisiecki, P. Solarz, Jarosł. Komar, W. Ryba-
2+
Romanowski, Spectroscopic peculiarities of CsCaI :Tm single crystals examined through one-photon
3
and excited state excitation spectroscopy, Journal of Alloys and Compounds (2018), doi: 10.1016/
j.jallcom.2018.01.058.
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ACCEPTED MANUSCRIPT
Spectroscopic peculiarities of CsCaI3:Tm2+ single crystals examined through one-
photon and excited state excitation spectroscopy
Mirosław Karbowiak1)*, Radosław Lisiecki2), Piotr Solarz2), Jarosław Komar2),
Witold Ryba-Romanowski2)
1) Faculty of Chemistry, University of Wrocław, F. Joliot-Curie 14, 50-383 Wrocław,
Poland
2) Institute of Low Temperature and Structure Research, Polish Academy of Sciences,
Okólna 2, 50-422 Wrocław, Poland
ABSTRACT
The CsCaI3 single crystals doped with Tm2+ were grown by Bridgman technique. Divalent
thulium luminescence related to the 4f13-4f13 intra-configurational and 4f125d1-4f13 inter-
configurational transitions were analyzed as a function of temperature. Excited state excitation
spectroscopy has been applied within a wide spectral range to examine peculiarities of transitions
between different 4f125d1 excited states of Tm2+. This two-color excitation experiment was
useful to assess d-d photon-absorption processes that are relevant to observed up-conversion
phenomena attributed to effective anti-Stokes green luminescence around 550 nm. Furthermore,
activation energy of 4f125d1 → 4f13 luminescence quenching was estimated and high value of
2
2F5/2 → F7/2 emission cross-section was determined in CsCaI3:Tm2+. Our spectroscopic study
was performed to complement and verify the previous investigation in this field.
Keywords:
Rare earth alloys and compounds
Optical absorption spectroscopy
Divalent thulium
CsCaI3
Excited state excitation
1

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1. Introduction
In recent years there has been a steady increase in the theoretical and experimental
studies of divalent lanthanide ions (Ln2+) doped in inorganic materials. Although the most
abundant data exist for divalent samarium, europium and ytterbium, which are most easily
reducible from the trivalent to divalent state [1], interconfigurational 4fN - 4fN-15d1 transitions of
other Ln2+ ions have also received appreciable attention owing to their technological applications
[2]. The intense and broad emission of divalent europium is desirable in phosphors [3,4,5,6]. The
Sm2+-doped materials are found as effective light conversion optical systems [7,8] and highly
sensitive optical temperature sensors [9]. The Tm2+ doped halides exhibit promising properties
for applications as efficient luminescent solar concentrators [10], whereas SrB4O7 host activated
with divalent thulium ions can be considered as a potential laser material and optical temperature
sensor [11]. Possible laser applications were recognized for 4f125d1-4f13 transitions of Yb2+ [12].
It is well known that the critical issue for stabilization of lanthanide ions in their divalent state is
the choice of appropriate host matrix [13]. In this regard a ternary halides CsCaX3 ( X = Cl, Br or
I) that crystallize in a perovskite structure and belong to orthorhombic crystalline system with
Ca2+ ions in the distorted octahedral coordination appear as very suitable hosts for stabilization
of divalent lanthanides. CsCaX3 crystals contain divalent sites, hence the Ln2+ ions can be
conveniently incorporated into host lattice. Several papers have been devoted to investigation of
spectroscopic properties of divalent lanthanide ions in halidoperovskites. The spectroscopic
properties of Eu2+ ions doped in the perovskites CsCaX3 and CsSrX3 (X = Cl, Br, and I), which
are considered as novel promising scintillators [14,15,16], were studied in [17]. The same
compounds doped with Yb2+ ions also exhibit properties suitable for their applications in high-
energy detection devices [18]. The comparative studies of photoluminescence properties of Yb2+
ions doped in the halidoperovskites CsMX3 (M = Ca, Sr; X = Cl, Br, I) were presented [19]
followed by systematic studies of the decay times and temperature dependence of the spin-
enabled and spin-forbidden transitions of Yb2+ ions in these hosts [18, 20].
Halidoperovskites doped with Tm2+ ions appear as particularly interesting systems since
different types of light emission were recognized in Tm2+-doped CsCaCl3, CsCaBr3 and CsCaI3
crystals. The occurrence of multiple emitting 4f125d1 states in Tm2+ was recently explained by
2

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using ab initio calculations [21]. Radiative and nonradiative relaxation processes were studied
and discussed for these systems [22,23,24]. The same authors reported on detailed analysis of
Tm-doped halide crystals absorption spectra in the region of 4f-4f and 4f-5d excitations [25]. It
has been shown that Tm2+ ions offer excellent advantage for the study of 4f-5d excitation
processes. In subsequent studies the upconversion (UC) phenomena have been observed in
CsCaI3:Tm2+ crystals under excitation at 810 nm [26,27]. It is unusual type of upconversion
phenomena involving the 4f-5d states of Tm2+. Understanding of 4f125d1 excited states are highly
essential to explore peculiarities of multiple emission and UC processes in materials doped with
divalent thulium ions. It has been recently proved that two-photon experiment is an useful
method to characterize lanthanide excited states including transitions between different 4fN-15d1
multiplets [28]. In particular it is especially suitable for probing the equilibrium distance offset
between different 4fn−4fn-15d1 excited states [28,29] Since for Tm2+ in CsCaI3 emission
originates not only from the lowest but also from the higher 4f125d1 excited state it is a very
suitable system for two-photon excited state excitation spectroscopy (ESE). UC process
involving exclusively 4f125d1 excited states has already been demonstrated in CsCaI3:Tm2+
system [26,27], but only using excitation at a single wavenumber. The ESE experiment has not
been reported.
The main aim of this study is to gain deeper insights into processes behind UC involving
4f125d1 excited states of Tm2+ ion in CsCaI3. Accordingly, in this work one photon excitation
spectroscopy and especially two-photon excited state excitation spectroscopy (ESE) are
employed to investigate and analyze 4f13 and 4f125d1 states responsible for activation of the
observed down-converted and up-converted Tm2+ luminescence. A significant decrease of the
4f125d1 → 4f13 luminescence lifetime was observed above T = 190 K and effective value of 2F5/2
2
→ F7/2 emission cross-section was estimated for CsCaI3:Tm2+ crystal. Our original results are
discussed in relation to previously obtained data for Tm2+ in CsCaI3. Moreover, the noticeable
differences between spectroscopic properties of investigated divalent thulium doped iodide
crystal and those reported for CsCaCl3:Tm2+ and CsCaBr3:Tm2+ are observed and discussed.
2. Experimental section
3

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CsCaI3 single crystals doped with 0.14 at.% and 0.68 at.% of Tm2+ (as determined with ICP-
OES spectroscopy) were grown by Bridgman technique. Pure CsCaI3 single crystal, prepared
from stoichiometric amounts of CsI and CaI2, was crushed and mixed with appropriate amount
of TmI2, then placed in a vitreous carbon crucible, which was put into silica ampoule and heated
for several hours at 720 K under high dynamic vacuum. After sealing under vacuum the ampoule
was lowered through the vertical furnace at 1015 K at a rate of 5 mm/h. TmI2 used in this
procedure was earlier synthesized via metallothermic route [30] in reaction of anhydrous TmI3
with Tm powder (99.9%, Sigma-Aldrich) at 1125 K in a sealed niobium tube. TmI3 was prepared
from Tm2O3 (99.99%, Stanford Materials) by the ammonium halide route [31]. The obtained
crystals were of good optical quality (Fig. S1). The crystals were cut and polished in the argon
glove-box and placed in airtight copper cuvette with quartz windows for absorption
measurements or sealed in a quartz tube under low-pressure of helium gas for emission
measurements.
The emission spectra were excited by a MDL-N-808-10W diode laser at 808 nm and
MXL-F-445-3W diode laser at 445 nm, respectively. The luminescence was dispersed by an
Optron Dongwoo monochromator with 750 mm focal length and detected by a Hamamatsu R955
photomultiplier or InGaAs detector depending on the spectral range. The excitation spectrum
was recorded with a Dongwoo Model Scanning System consisting of LPS-200X Xenon Lamp,
an excitation monochromator with 150 mm focal length, an emission monochromator having 750
mm focal length and a Hamamatsu R955 photomultiplier. Excited State Excitation spectra were
recorded using MDL-N-808-10W diode laser as first excitation source. Up-converted green
luminescence was monitored on Dongwoo DM711 monochromator and detected by a
Hamamatsu R3896 photomultiplier. The second excitation pulses fluently tuned in the required
spectral regions were delivered from Opotek Opolette 355 LD laser system. To record
luminescence decay curves a femtosecond laser (Coherent Model ‘‘Libra’’) that delivers a train
of 89 fs pulses at a centre wavelength of 800 nm and a pulse energy of 1 mJ with a repetition rate
regulated up to 1 kHz was used as an excitation source. To obtain light pulses at different
wavelengths (230-2800 nm) the laser is coupled to an optical parametric amplifier (Light
Conversion Model ‘‘OPerA’’). Luminescence decay curves were recorded with a grating
spectrograph (Princeton Instr. Model Acton 2500i) coupled to a streak camera (Hamamatsu
Model C5680). For spectroscopic measurement at low temperature the samples were placed in an
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Oxford Model CF 1204 continuous flow liquid helium cryostat equipped with a temperature
controller.
3. Results and discussion
CsCaI3 crystallizes in the orthorhombic Pbnm space group [32] and in dopped crystals
Tm2+ ions substitute for Ca2+ ions on a slightly distorted octahedral site (the actual site symmetry
for Tm2+ ions is C1).
Fig. 1 reveals the excitation spectrum of CsCaI3:0.14% Tm2+ visible luminescence
monitored at 547 nm and measured at T = 5K within 250 - 540 nm spectral region. The
excitation spectrum is quite complex and consists of numerous broad bands, while some of them
exhibit additional fine structure. The bands observed in the spectrum are due to transitions from
the ground 2F7/2 multiplet of the 4f13 configuration to the states of the 4f125d1 configuration [33].
Octahedral ligand-field interaction in CsCaI3:Tm2+ results in splitting of 5d orbitals into t2g and
eg sets of levels. These levels are coupled to 2S+1LJ multiplets of 4f12 core originating from spin-
orbit and Coulomb interactions. The resulting states will be labeled as (2S+1LJ, t2g) and (2S+1LJ,
eg), respectively. The Coulomb interactions between the 5d electron and the 4f12 core electrons
additionally split these states into high-spin (S = 3/2) and low-spin (S = 1/2) states. The energy-
level diagram of Tm2+ in CsCaI3 as well as the radiative and nonradiative processes relevant to
the discussion presented in this study are shown in the inset in Fig. 1.
According to energy level diagram in Fig. 1 the first intense band observed in the
2
excitation spectrum with maximum at ~19375 cm-1 (516 nm) should be assigned to F7/2 → (3F4,
t2g) transitions, whereas the group of bands observed in the 20500 - 25000 cm-1 range, with
maxima at about 21680 cm-1 (461 nm), 22730 cm-1 (440 nm) and 23830 cm-1 (420 nm) are
attributed to 2F7/2→ (3H6, eg) transitions.
Near-infrared emission spectrum of Tm2+ was acquired in 1100-1200 nm spectral range
after excitation of CsCaI3:0.68% Tm2+ crystal at 445 nm. Fig. 2 (a) compares luminescence
spectra of Tm2+ in the near-infrared region measured at several different temperatures between 5
and 300 K. The spin-orbit interaction splits the 2F term of the ground 4f13 configuration of Tm2+
2
into the ground F7/2 and the excited 2F5/2 multiplet. Accordingly, the observed emission band is
2
2
related to f-f intra-configurational F5/2 → F7/2 transitions of Tm2+. It can be discerned that
influence of temperature on the infrared luminescence excited at 445 nm in CsCaI3:0.68% Tm2+
5

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is significant. Integrated luminescence intensity for the spectrum measured at T = 300 K is
smaller by a factor of six as compared to that recorded at T = 200 K. In the room-temperature
NIR emission spectrum of Tm2+ one pronounced peak with maximum at 8767 cm-1 is observed
and its FWHM value was determined to be 144 cm-1. The Füchtbauer-Ladenburg method was
used to estimate the emission cross-section. The dependence between the emission cross-section
σ
em and emission spectrum is described as:
β λ 5 I(
n2 cτ rad
λ
)
(1)
σ em (λ) =
8π
λ
I(
λ) dλ
∫
where I(
λ
) represents the experimental emission intensity at the wavelength
λ
, c is the light
velocity, n,
β and τrad are the refractive index of material, branching ratio and radiative lifetime
2
of the F5/2 multiplet, respectively. It was assumed that luminescence lifetime of 0.65 ms
2
measured for diluted sample is not significantly different than F5/2 radiative lifetime. Inset of
2
2
Fig. 2 presents simulated emission cross-section spectrum derived from the F5/2 → F7/2
luminescence of Tm2+ excited at 445 nm. The prominent line at 1141 nm dominates emission
cross-section spectrum with advantageously high peak value which amounts to σem = 6.07 x 10-20
cm2. For a comparison, significantly lower value of emission cross section σem = 3.50 x 10-20 cm2
was found for SrB4O7:Tm2+ [34].
Fig. 2 (b) presents the Tm2+ NIR emission spectrum measured at T = 5 K. At this
temperature only the lowest-energy crystal field sublevel of the emitting state is substantially
2
2
populated and four emission lines are expected for F5/2 → F7/2 transitions having in mind that
divalent thulium ions are coordinated by six I- ligands in a distorted octahedral sites of C1
symmetry in CsCaI3 lattice. In accordance with this expectation the emission spectrum presented
in Fig. 2 is dominated by four sharp lines located at 8634, 8721, 8773 and 8854 cm-1. It should
2
be mentioned that only two F5/2→2F7/2 emission lines identified at 8782 cm-1 and 8790 cm-1 by
J. Grimm et al. [22], were assigned by these authors as electronic transitions. The remaining
emission lines placed at longer wavelengths were found as vibronic sidebands. In a short-
wavelength part of our spectrum the additional structureless band appears at 8140 cm-1. In view
of a fast decay of this emission (τ≈1 µs) it may be associated with transitions between levels of
initial (3F4 ,t2g) and terminal (3H4 ,t2g) manifolds. The decay curves of observed luminescence
are presented in Fig. 2 (c,d). The 10 Dq crystal-field splitting of the 5d part of the 4f125d1
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electron configuration in CsCaI3:Tm2+ was estimated to be 8500 cm-1 and more than fifteen cut-
off phonons are needed to bridge the (3F4 ,t2g) - (3H4 ,t2g) energy gap. It is quite enough for (3F4
,t2g) components to become emissive levels and consequently, the excitation energy may be
down-converted to obtain NIR photons as result of d-d transitions.
Aside from f-f NIR emission of Tm2+ the visible luminescence of CsCaI3:0. 68% Tm2+
crystal was detected at T = 5 K upon excitation at 445 nm and 808 nm as it is displayed in the
Fig. 3. In fact, these spectra consist of one prominent structureless emission band centered in
green spectral region at 547 nm. When the (3F4, t2g) Tm2+ multiplet is excited at λ = 445 nm
down-converted emission occurs. The energy gap between the (3F4 ,t2g) ↔ (3H6 ,t2g) multiplets
was found to be 2700 cm-1 whereas a maximum energy of phonons in CsCaI3 is ħωmax.= 170 cm-1
[14]. Accordingly, efficient radiative transitions from the (3F4, t2g) multiplet take place. The
green Tm2+ emission is associated with a transition from the lowest excited state of the (3F4, t2g)
2
multiplet to the F7/2 Tm2+ ground state. The inset in Fig. 3 presents the magnified fragment of
emission spectrum recorded in 11500-13500 cm-1 range when excited at 445 nm. This emission
band peaked at 12100 cm-1 is associated with (3H6, t2g) → 2F7/2 transitions.
When the CsCaI3:0.68% Tm2+ crystal is excited at 808 nm the up-converted
luminescence is found (Fig. 3). In this case the (3H6, t2g) S = 3/2 multiplet is excited as a result of
the spin-forbidden transitions. The observed upconversion phenomena is atypical because the
adequate 4f125d1 Tm2+ excited states are involved to populate the divalent thulium luminescent
levels. Efficiency of the green up-converted emission in CsCaI3:0.68% Tm2+ is relatively high.
The up-converted emission with a maximum located at λ = 546 nm when excited at 12350 cm-1
(809.7 nm) has been reported by J. Grimm et. al. [24,27]. In contrast to the Tm2+ luminescence
presented in [24] our emission spectrum is characterized by a more intense band located at 485
nm (20618 cm-1) that can be attributed to transitions originating in the higher-energy levels of
(3F4, t2g) multiplet. It has been concluded that two-photon excitation process is responsible for
population of Tm2+ luminescent level in upconversion phenomena. Furthermore it has been
assumed that the Tm2+ up-converted luminescence in CsCaI3 mainly results from GSA (Ground
State Absorption) /ESA (Excited State Absorption) process but contribution of GSA/ETU
(Energy Transfer Upconversion) process is considered as well [27]. Upconversion phenomena in
Tm2+-doped iodide can be effective because excited state absorption and energy transfer
7

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upconversion processes involve 5d(t2g) → 5d(eg) excitation transitions [26] which are
characterized by quite high values of oscillator strength.
Up-conversion phenomena observed in CsCaI3:Tm2+ are activated by transitions between
4f125d1 excited states. In order to get more detailed insight into these transitions a two-color
excitation experiment was performed. The set-up consisting of two laser sources, namely a CW
AlGaAs- diode laser and a tunable pulsed OPO laser was used to record excitation spectra in
wide spectral regions. When CW λ = 808 nm excitation wavelength is employed, a ground-state
absorption occurs and the (3H6, t2g) multiplet is continuously populated. An additional pulsed
excitation at varied wavelengths stimulate the already excited Tm2+ ions to populate the higher
energy 4f125d1 states. A second photon is resonant with the energy separation between involved
Tm2+ excited states. Eventually, excited state excitation spectra of CsCaI3:0.68% Tm2+
monitoring anti-Stokes green emission at 547 nm were measured and are presented in Fig. 4.
When the combination of these both aforementioned excitation sources was used the green UC
emission signal was certainly enhanced comparing to CW single – laser excitation. This may
indicate that ESA (3H6, t2g) → (3F4, t2g) contribution in UC process is highly relevant in
CsCaI3:Tm2+ crystal.
Two-color excited state excitation experiment can be utilized to examine equilibrium
distance offset the involved 4f125d1 states. It was assumed that electronic transitions between two
excited states with comparable equilibrium bond lengths are related to ESE sharp spectral line
while transitions between contracted and expanded excited states give rise to broad bands [28].
Our excited state excitation spectrum displayed in Fig. 4 consists of a few broad bands (labeled
A-D) placed within 6830-8330 cm-1, 8665-9340 cm-1, 11095-13825 cm-1 and 14100- 16815 cm-1
spectral ranges. The lower-energy ESE (A-B) bands can be attributed to (3H6, t2g) → (3F4, t2g)
transitions and the remaining broadened bands (C-D) located at higher energy may be associated
with two-photon absorption transitions to 4f125d(t2g/eg)1 mixed states. At lower energy sides of A
and B ESE bands sharp and narrow lines can be discerned, which begin to appear at 6987 cm-1
and 8766 cm-1, respectively. These sharp components are related to transitions characterized by
no change in potential energy surfaces equilibrium position of the various 4f125d(t2g)1 excited
states. In accordance with that for low-energy f-d states of Tm2+ in CsCaI3 there is rather similar
Tm-ligand equilibrium distance in their potential energy surface. The adequate set of ESE bands
attained during a two-photon excitation experiment was recently observed by M. de Jong. et. al.
8

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[28] for CsCaCl3:Tm2+ and CsCaBr3:Tm2+ crystals. The onset of two observed groups of lines
has been then established at 7100 cm-1 and 9000 cm-1. It means that (3H6, t2g) → (3F4, t2g) ESE
bands observed for CsCaI3:Tm2+ are slightly shifted toward lower energy in relation to the
chloride and bromide counterparts. It stems from more covalent character of iodide host as
compared to chloride or bromide hosts as well as from differences in a crystal-field strength.
Both these factors exert strong effect on barycenter of 4f125d1 configuration and energies of d-d
transitions. The ESE band measured for CsCaI3:Tm2+ contain more narrow and sharp lines than
it is observed in CsCaCl3:Tm2+. This finding may be a consequence of a higher site symmetry of
Tm2+ ion in chloride (C4h) than in iodide (C1) crystal. As compared to previous results [28] our
excited state excitation experiment was performed using higher-energy second excitation
photons and ESE spectra were acquired up to 16800 cm-1. Actually, above 11000 cm-1 broad and
structureless bands appear, centered at 12037, 13492, 14804 and 15782 cm-1. These intense ESE
bands may correspond to two-photon absorption from (3H6, t2g) state to higher-energy 4f125d1
states. The high intensity of ESE bands results from a large value of d-d two-photon absorption
cross-section [35]. The spectral character of these broad bands implies that larger difference
between equilibrium distance in potential energy surface of lower- and higher-energy 4f125d1
states occurs in CsCaI3:Tm2+.
Fig. 5 displays results of an experiment assessing the impact of temperature on thulium
excited state relaxation dynamics. The visible luminescence decay curves were measured in a
region 5 – 300 K at λdet.=547 nm under femtosecond pulse excitation at λexc.= 450 nm. The
relaxation of f-d (3F4, t2g) states in CsCaI3:Tm2+ may result from radiative transitions and
nonradiative multiphonon transitions. The energy separation between (3F4, t2g) and (3H6, t2g)
states of Tm2+ was found to be 2700 cm-1 and roughly sixteen cut-off phonons are needed to
bridge this energy gap. Accordingly, in this case the nonradiative relaxation should be effectively
suppressed. Estimated experimental lifetime of green Tm2+ emission (τ ≈ 1.3 µs) is practically
constant from T = 5 K up to 190 K. Subsequently, at the higher temperature the lifetime is
significantly shortened and amounts to 7 ns at room temperature. It is worth noticing that a
single exponential time dependence was recognized for all analyzed decay curves. The
luminescence decay curves measured at T=5K, 77 K and 298 K are presented in a lower part of
Fig 5. The exponential decay function I=I0+Aexp(-t/τexp.) was used to determine the adequate
experimental lifetimes.
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Effect of thermal energy on Tm2+ transition rates has been assessed with the Arrhenius
plots defined as
− E



q 
W = Aexp
(2)


kT


equation (1) can be transformed to
Eq
ln(W ) = ln(A) −
kT
(3)
where W is the transition rate and A (s-1) denotes a transition rate given by the intercept of the
line at 1/T →0 and is attributed to pre-exponential factor, k is Boltzmann constant and T is
temperature. The parameter Eq (quenching energy, (cm-1)) is obtained from the fit of the (2) line.
The lowest energy states of the 4f125d1 Tm2+ configuration are thermally populated with
temperature increasing and dynamics of 4f125d1-4f13 vibronic transitions is more effective.
Consequently, an increase of relaxation rate is observed as it is presented in inset of Fig. 5. The
activation energy was estimated to be 2697±15 cm-1. It means that our finding is highly
consistent with the reported ∆E = 2700 cm-1 for energy gap between (3F4, t2g) and (3H6, t2g) states
in CsCaI3:Tm2+. The comparable experimental lifetime of 1 µs at T = 10 K was determined for
CsCaI3:1 at.% Tm2+ by J. Grimm et. al. [24]. Furthermore, these authors have observed that
green radiative transitions are effective up to about 250 K and above this temperature the up-
converted emission rapidly drops. This finding is consistent with our results revealing substantial
decrease of measured lifetime already around T=200 K. It was documented that radiative rate
constant R of (3F4, t2g) → 2F7/2 emission is the most prominent relaxation path in the iodide up to
220 K. Above this temperature nonradiative relaxation to (3H6, t2g) state is a dominant process
[24]. It is worth noticing that the presented spectroscopic results for CsCaI3:0.68% Tm2+ crystal
are comparable to those obtained for CsCaI3:0.14% Tm2+, hence no significant differences were
found for the sample containing lower divalent thulium concentration.
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4. Conclusions
The spectroscopic properties of Tm2+ - doped CsCaI3 single crystals were investigated in
relation to previously reported data. Novel and valuable results were achieved particularly
pertinent to excited state excitation experiment performed in a wide spectral range. Acquired
excited excitation spectra are useful for analysis of d-d photon-absorption processes relevant
especially in observed up-conversion phenomena. The 4f125d → 4f13 luminescence was analyzed
as a function of temperature. The calculated activation energy was found to be fully in
compliance to (3F4, t2g) ↔ (3H6, t2g) excited states energy gap. Moreover Füchtbauer–Ladenburg
2
2
method was applied to estimate high value of emission cross section for NIR F5/2 → F7/2
transition in CsCaI3:0.68% Tm2+. The noticeable differences between spectroscopic properties of
investigated divalent thulium doped iodide crystal and those reported for CsCaCl3:Tm2+ and
CsCaBr3:Tm2+ can be underlined. Emission bands measured for CsCaI3:Tm2+ are located at
2
lower energies in relation to chloride and bromide counterparts. We observed that the F5/2
→
2F7/2 emission intensity decreases significantly at T=250 K in CsCaI3:Tm2+ whereas this efficient
quenching process occurs at T=100 K for CsCaCl3:Tm2+ and at T=150 K for CsCaCl3:Tm2+. The
green up-converted emission is observed for all halides but this luminescence is especially
efficient in CsCaI3:Tm2+ since a large (3F4 ,t2g) - (3H6 ,t2g) energy gap is high enough for effective
radiative transitions. Our excited state excitation experiment reveals some differences in the
(ESE) spectra measured for iodide crystals and these studied for chloride and bromide. ESE
bands related to different excited 4f125d1 states in CsCaI3:Tm2+ are shifted toward lower energy
as compared to those recorded for chloride and bromide counterparts. These variations in energy
level structure of the f–d states in Tm2+-doped halides are relevant for efficiency of processes
involved in upconversion phenomena as well.
Appendix A. Supplementary data
Supplementary data related to this article can be found at ...
Acknowledgment
The National Science Centre (Poland) supported this work under research project DEC-
2012/07/B/ST4/00581.
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Figures
T=5K
,
λdet.=18280 cm-1
Excitation spectrum CsCaI3:Tm2+
*
(468nm)
(440nm)
(420nm)
*
*
(516nm)
*
20000
25000
30000
35000
40000
Wavenumber [cm-1]
Fig. 1 One-photon excitation spectrum of CsCaI3:0.68% Tm2+ recorded at 5 K while monitoring
emission at λ = 547 nm. Inset presents energy-level diagram of Tm2+ in CsCaI3.
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CsCaI3:0.8 at.% Tm2+
Exc. 445 nm
CsCaI3:0.8 at.% Tm2+
6
4
2
Exc. 445 nm
T=5K
300K
250K
200K
5K
0
1100
1150
1200
λ σnm
2F5/2-2F7/2
2F5/2-2F7/2
a)
b)
8000
8500
9000
8000
8500
9000
9500
Wavenumber [cm-1]
Wavenumber [cm-1]
CsCaI3:0.8 at.% Tm2+
CsCaI3:0.8 at.%Tm2+
τ
=0.92
µs
τ
=0.65 ms
2F5/2
-1
=8140 cm
det.
c)
λ
d)
0
1000
2000
3000
Time [µs]
0
2
4
6
Time [
µ
s]
2
Fig. 2. a) Impact of temperature on F5/2-2F7/2 Tm2+ emission spectra excited at 445 nm - inset
shows the emission cross-section spectrum. b) Emission spectrum measured at T=5K. c) Decay
curve of luminescence detected at λ=8767 cm-1. d) Decay curve of luminescence detected at
λ=8140 cm-1.
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CsCaI3:Tm2+
Exc. 445 nm
T=5K
11500 12000 12500 13000 13500
ν 1cm-1
1
DC
Exc. 445 nm
UC
Exc. 808 nm
10000
15000
20000
25000
Wavenumber [cm-1]
Fig. 3. Low-temperature (T=5K) emission spectra of CsCaI3:0.68% Tm2+ crystal excited at 445
nm and 808 nm.
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2+
CsCaI :Tm
T=5K
3
λ
=547 nm
A
det.
B
7000
8000
C
9000
10000
Wavenumber [cm-1]
2+
CsCaI :Tm
3
T=5K
λ
=547 nm
det.
D
10000
12000
14000
16000
Wavenumber [cm-1]
Fig. 4. Excited state excitation spectra of CsCaI3:0.68% Tm2+ measured at T = 5K within 6000-
17000 cm-1 spectral range. The luminescence was monitored at λ=547 nm.
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1
0.1
1.5x108
Eq=2697 cm-1
1.0x108
5.0x107
0.0
0.01
0
100
200
300
T [K]
0
100
200
300
Temperature [K]
e-1
e-3
e-5
e-7
e-9
T=5K
τ
=1.3 µs
T=220K
τ
=0.46 µs
T=298K
τ
=0.007 µs
0
2
4
6
8
Time [µs]
Fig. 5.Upper- Lifetime of the CsCaI3:0.68% Tm2+ luminescence monitored at 18280 cm-1 and
plotted versus temperature. The inset shows the activation energy Eg = 2697 cm-1 (± 15 cm-1).
Lower – Decay curves of thulium luminescence measured at different temperatures.
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•
•
•
•
Excited state excitation spectra are presented for Tm2+ in CsCaI3.
The 4f125d1 → 4f13 luminescence was analyzed in the function of temperature.
Activation energy of 4f125d1 → 4f13 luminescence quenching was estimated.
A high value of 2F5/2 → 2F7/2 emission cross-section was determined.