Luminescence properties of Tm3+ ions single-doped YF3 materials in an unconventional excitation region

Article abstract of DOI:10.1002/bio.3439

According to the spectral distribution of solar radiation at the earth's surface, under the excitation region of 1150 to 1350?nm, the up-conversion luminescence of Tm³⁺ ions was investigated. The emission bands were matched well with the spectr

Full text of DOI:10.1002/bio.3439

Received: 13 April 2017
DOI: 10.1002/bio.3439
Revised: 17 September 2017
Accepted: 1 November 2017
R E S E A R C H A R T I C L E
Luminescence properties of Tm3+ ions single‐doped YF3
materials in an unconventional excitation region
Yuan Chen1
Qing Liu1 Han Lin1 Xiaohong Yan1,2,3,4
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1 College of Science, Nanjing University of
Aeronautics and Astronautics, Nanjing, P. R.
China
Abstract
According to the spectral distribution of solar radiation at the earth's surface, under the excitation
region of 1150 to 1350 nm, the up‐conversion luminescence of Tm3+ ions was investigated. The
emission bands were matched well with the spectral response region of silicon solar cells,
achieved by Tm3+ ions single‐doped yttrium fluoride (YF3) phosphor, which was different from
the conventional Tm3+/Yb3+ ion couple co‐doped materials. Additionally, the similar emission
bands of Tm3+ ions were achieved under excitation in the ultraviolet region. It is expected that
via up‐conversion and down‐conversion routes, Tm3+‐sensitized materials could convert photons
to the desired wavelengths in order to reduce the energy loss of silicon solar cells, thereby
enhancing the photovoltaic efficiency.
2 Key Laboratory of Radio Frequency and
Micro‐Nano Electronics of Jiangsu Province,
Nanjing, P. R. China
3 College of Electronic Science and
Engineering, Nanjing University of Posts and
Telecommunications, Nanjing, P. R. China
4 School of Material Science and Engineering,
Jiangsu University, Zhenjiang, P. R. China
Correspondence
Xiaohong Yan, College of Science, Nanjing
University of Aeronautics and Astronautics,
Nanjing, 211106, P. R. China.
Email: xhyan@nuaa.edu.cn
KEYWORDS
down‐conversion, photoluminescence, solar energy, Tm3+‐ions single‐doped, up‐conversion
Funding information
National Natural Science Foundation of China,
Grant/Award Number: NSFC51032002; key
Project of the National High Technology
Research and Development Program
(‘863’Program) of China, Grant/Award
Number: 2011AA050526; Funding of Jiangsu
Innovation Program for Graduate Education,
Grant/Award Number: CXZZ13_0143
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1
INTRODUCTION
couples.[6–10] In the case of Tm3+/Yb3+ ion couples, among the most
reported articles about up‐conversion luminescence of Tm3+‐doped
materials, Yb3+ ions are usually doped as the indispensable
sensitizer,[11–13] and the energy transfer mechanisms between Tm3+
ions and Yb3+ ions are similar, described as phonon‐assisted
multiphoton stepwise summation process.[14] The process is initiated
Owing to worldwide increasing energy demand and environmental
pollution, research and development of renewable energy resources
have attracted much attention, with solar cells being a topic of great
interest. In recent years, great efforts have been devoted to improving
the photovoltaic efficiency of semiconductor solar cells. One of the
effective methods is modifying the incident solar spectrum via photon
frequency conversion, such as up‐conversion and down‐conversion,
which can reduce the sub‐bandgap transmission loss of long wave-
length and decrease the charge carrier thermalization loss of high‐
energy photons, respectively.[1–5] Benefiting from the unique and
abundant energy‐level structures, lanthanide ions have always been
considered as good candidates to modulate the solar spectrum, such
as Er3+ ions, Ho3+ ions, Tb3+/Yb3+ ion couples and Tm3+/Yb3+ ion
2
by one 980 nm photon absorption of a Yb3+ ion in the F7/2 ground
state (2F7/2
→
2F5/2). Subsequently, the excited Yb3+ ion transfers its
2
energy to a Tm3+ ion to excite it to the 3H5 excited state: F5/2
2
(Yb3+) + 3H6 (Tm3+) → F7/2 (Yb3+) + 3H5 (Tm3+), then Tm3+ ion
populates the 3F4 state by non‐radiative relaxation. Following that,
with the help of other excited Yb3+ ions, Tm3+ ions are promoted
from 3F4 state to 3F2 state, populating 3H4 state by non‐radiative
relaxation, and finally promoted from 3H4 state to 1G4 state.
However, by careful investigation of the Tm3+ ions energy‐level
diagram,[15] the energy‐level gap between 3H5 excited state and 3H6
ground state is about 8000 cm−1 (up to 1250 nm), which means that
the pump energy supported by 980 nm laser (up to 10,200 cm−1) will
be wasted partly. Furthermore, according to the air mass 1.5 global
Abbreviations used: NH4F, ammonium fluoride; TmCl3, thulium chloride;
Tm2O3, thulium oxide; XRD, X‐ray diffraction; YCl3, yttrium chloride; YF3,
yttrium fluoride; Y2O3, yttrium oxide
Luminescence. 2018;1–6.
wileyonlinelibrary.com/journal/bio
Copyright © 2018 John Wiley & Sons, Ltd.
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CHEN ET AL.
(AM 1.5G) solar spectrum at sea level, as shown in Figure 1, in the
infrared region, the solar radiation energy mainly distributes in three
regions: 1000 nm (950–1100 nm), 1250 nm (1150–1350 nm) and
1600 nm (1500–1750 nm), while the 1250 nm region is matched with
the corresponding energy‐level gap of Tm3+ ions very well. For a long
time, most attention has been attracted in the 1000 nm region (such as
980 nm) [16,17] and 1600 nm region (such as 1540 nm) [18–20], yet less
attention has been attracted in the 1250 nm region. In order to make
the best use of solar energy, the research of Tm3+ ions up‐conversion
luminescence under the excitation around 1250 nm will be meaningful.
Lots of published articles have depicted the absorption spectra around
1250 nm of Tm3+ ions in various Tm3+‐doped materials, such as LiYF4
crystals [21], chalcohalide glass [22] and tellurite glass [23,24], suggesting
that Tm3+‐sensitized materials might have potential up‐conversion
luminescence performance in the 1250 nm region of the solar spec-
trum. Additionally, Tm3+ ions also have the ability of down‐conversion
in the ultraviolet region, providing an opportunity to combine down‐
conversion and up‐conversion luminescence.
heating to prepare the stock rare earth solutions of 0.2 mol/L yttrium
chloride (YCl3) and thulium chloride (TmCl3) solutions, respectively.
Accurately weighed 0.0888 g of NH4F was dissolved into deionized
water as precipitator. In the synthesis of Y0.95Tm0.05F3, 3.80 ml of YCl3
aqueous solution and 0.20 ml of TmCl3 aqueous solution were mixed
under thorough stirring. Then the mixed aqueous solution of YCl3 and
TmCl3 was added to the aqueous solution of NH4F drop‐by‐drop, under
magnetic stirring. After stirring for about an hour, the colloidal solution
was transferred to a capped beaker for precipitating. Then the resulting
product was separated by centrifugation and cleaned by deionized water
three times. After that, the obtained product was collected and heated at
700°C for 6 h in the environment of hydrogen fluoride gas atmosphere.
The structure of the phosphor was investigated by X‐ray diffrac-
tion (XRD) using an Ultima‐III (Rigaku Corporation, Japan) equipment
provided with Cu tube with Kα radiation at 1.5406 Å, scanning in the
2θ range from 20° to 60° with 0.02° increments. The
photoluminescence excitation and emission spectra were measured
by the FLS920 spectrophotometer (Edinburgh Instruments Ltd.
England) equipped with one adjustable OPO (optical parametric
oscillator) laser and xenon lamp as the excitation sources. All spectra
measurements were performed at room temperature.
Fluorides have always been considered as excellent lanthanide‐
doped hosts, owing to the property of low phonon energy. In this
work, yttrium fluoride (YF3) phosphor was selected as the host mate-
rials, since its simple synthesis conditions which can be easily extended
to industrialization manufacture. By the method of co‐precipitation,
Tm3+ ions single‐doped YF3 phosphor was synthesized. Without the
conventional and indispensable Yb3+ ions, the up‐conversion and
down‐conversion luminescence were achieved by single Tm3+ ions.
Thus, excitation energy loss through energy transfer from Yb3+ ions
to Tm3+ ions can be avoided.
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RESULTS AND DISCUSSION
The composition and phase purity of the phosphor were examined by
XRD, shown in Figure 2. Compared withYF3 standard data (JCPDS No.
74‐0911) in Figure 2(a), all the diffraction peaks of the phosphor
(Figure 2b) can be indexed to the orthorhombic phase of YF3. The
dopant has little influence on the structure of the materials and does
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EXPERIMENTAL
The starting materials were yttrium oxide (Y2O3), thulium oxide
(Tm2O3), ammonium fluoride (NH4F) and hydrochloric acid. All reagents
were analytical grade, used without further purification, and deionized
water was used throughout. The synthesis of Y0.95Tm0.05F3 phosphor
was a method of co‐precipitation.
The procedure is described as follows. The stoichiometric amounts
of Y2O3 and Tm2O3 were dissolved in diluted hydrochloric acid by
FIGURE 1 The spectral distribution of solar radiation at the earth's
FIGURE 2 The standard data (JCPDS no. 74‐0911) of orthorhombic
surface
YF3 (a) and the XRD pattern of Y0.95Tm0.05F3 phosphor (b)

CHEN ET AL.
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not introduce any detectable impurity phase, suggesting that the Tm3+
ions have replaced the site of the Y3+ ions in YF3 phosphor. Besides,
according to the XRD data, it could be calculated that the crystallinity
of the phosphor compound is 98.34%, while the average particle size is
about 90 nm.
The photoluminescence excitation spectrum of the Y0.95Tm0.05F3
phosphor was obtained by monitoring the 800 nm emission line,
ascribed to the 3H4
→
3H6 transition of Tm3+ ions. As shown in
Figure 3, there are two intense excitation bands around 1164 and
1218 nm, which were chosen as the excitation source wavelength to
pump the Tm3+ ions.
Figure
4
depicts the photoluminescence spectrum of
Y0.95Tm0.05F3 under the excitation of 1218 nm. The 800 nm infrared
and 700 nm red emission bands of Tm3+ ions are observed, originated
from the transitions of 3H4 → 3H6 and 3F2 → 3H6 of Tm3+ ions, respec-
tively. It should be noted that c‐Si solar cells exhibit intense spectral
responses around 800 nm, suggesting that the Tm3+‐doped material
in this work, owing to its wide 800 nm infrared emission band, might
be an ideal candidate to convert long wavelength photons to the
desired ones which can be directly absorbed by c‐Si solar cells, thus
improving the photovoltaic efficiency.
FIGURE
4
The photoluminescence spectrum of Y0.95Tm0.05F3
phosphor, under the excitation of 1218 nm
Figure
5
depicts the photoluminescence spectrum of
Y0.95Tm0.05F3 under the excitation of 1164 nm. Similar to Figure 4,
the emission bands centered at 800 nm and 700 nm were also
observed. Furthermore, a slight blue emission signal at 475 nm was
observed, assigned to the 1G4
→
3H6 transition of Tm3+ ions. The
difference in the photoluminescence spectra between Figure 4 and
Figure 5 suggests that the pump photon energy of 1218 nm cannot
afford the population of 1G4 state of Tm3+ ions to generate 475 nm
blue emission signal, yet the photon energy of 1164 nm barely does.
As shown in Figure 6, by monitoring emission line at 475 nm,
assigned to the 1G4
→
3H6 transition of Tm3+ ions, the
photoluminescence excitation spectrum of Y0.95Tm0.05F3 phosphor
was observed. Among those excitation bands mentioned earlier, the
peak at 315 nm was chosen as the pump source wavelength, obtaining
the photoluminescence spectrum shown in Figure 7. In contrast to
FIGURE
5
The photoluminescence spectrum of Y0.95Tm0.05F3
phosphor, under the excitation of 1164 nm
FIGURE
3
The photoluminescence excitation spectrum of
FIGURE
6
The photoluminescence excitation spectrum of
Y0.95Tm0.05F3 phosphor, by monitoring the 800 nm emission line
Y0.95Tm0.05F3 phosphor, by monitoring the 475 nm emission line

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CHEN ET AL.
FIGURE 7 The photoluminescence spectrum of Y0.95Tm0.05F3 phosphor, under the excitation of 315 nm
Figures 4 and 5, the same emission bands at 475 nm, 700 nm and
800 nm could also be detected. As we know, the spectral response
of c‐Si solar cells is mainly located in the range of 400 to 1000 nm.
Thus, the Tm3+ ‐doped material in this work could absorb photons in
the wavelength range of both ultraviolet (315 nm) and infrared (1164
and 1218 nm) regions, where c‐Si solar cells exhibit poor performance,
to generate 475, 700 and 800 nm emission to match the correspond-
ing spectral response of c‐Si solar cells.
multiphoton stepwise process is predominant.[14] For the reason that
Tm3+ ions exhibit low absorption cross‐section, in the conventional
experimental operation, Yb3+ ions are usually co‐doped as sensitizer,
thus facilitating efficient Tm3+ ions up‐conversion luminescence.
Taking one of the previous published articles as an example.[13]
Pumped by the 980 nm laser, the excitation of Tm3+ ions was achieved
by energy transfer fromYb3+ ions toTm3+ ions. Absorbing 980 nm pho-
2
tons (up to 10200 cm−1), Yb3+ ions in the F7/2 ground state were
2
Since Dieke and Crosswhite provided the energy‐level schemes of
rare earth ions in 1963,[25] a large number of followers have carried out
further studies and gradually developed the relevant theories in
excited‐state dynamics, emission profiles, energy transfer
mechanisms and so on. For example, in the analysis of Tm3+ ions up‐
conversion luminescence mechanisms, the phonon‐assisted
excited to F5/2 state, transferring energy to Tm3+ ions continuously.
Since the energy of excited Yb3+ ions did not resonantly match with
the 3H5 excited state of Tm3+ ions (~8100 cm−1), pump energy of
about 2000 cm‐1 had to be wasted through energy release as lattice
phonons.[26] The Tm3+ ions were promoted from the 3H6 ground state
to the 3H5 excited state, populating the 3F4 state by non‐radiative
FIGURE 8 The possible up‐conversion luminescence mechanisms in the system of Y0.95Tm0.05F3

CHEN ET AL.
5
relaxation (energy loss by phonons), then promoted from 3F4 state to
3F2 state, populating 3H4 state by non‐radiative relaxation, and finally
promoted from 3H4 state to 1G4 state.
of Tm3+ ions and enhancing the photovoltaic efficiency of solar cells
in the future.
In this work, pumped at 1164 and 1218 nm, as a result of the
absence of Yb3+ ions, the realization of up‐conversion luminescence
could be achieved by Tm3+ ions themselves.[27] According to the
energy‐level diagram of Tm3+ ions, pump photon energy of excitation
sources and energy matching conditions,[28] the possible
up‐conversion luminescence mechanisms in the system of
ACKNOWLEDGEMENTS
This work was supported by the National Natural Science Foundation
of China (NSFC51032002), the key Project of the National High
Technology Research and Development Program (‘863’Program) of
China (No. 2011AA050526), the Funding of Jiangsu Innovation
Program for Graduate Education (No. CXZZ13_0143) as well as the
Fundamental Research Funds for the Central Universities.
Y0.95Tm0.05F3 can be illustrated as shown in Figure 8. As depicted in
Figure 8(a), under the excitation of 1218 nm (up to 8200 cm−1), Tm3+
ions were promoted from the 3H6 ground state to 3H5 excited state
(energy‐level gap around 8100 cm−1). In contrast to the energy loss
of about 2000 cm−1 under excitation of 980 nm, the energy of
1218 nm pump source was resonantly transferred to Tm3+ ions,
increasing energy utilization efficiency. Subsequently, part of Tm3+
ORCID
Yuan Chen http://orcid.org/0000-0001-6817-0856
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How to cite this article: Chen Y, Liu Q, Lin H, Yan X. Lumines-
cence properties of Tm3+ ions single‐doped YF3 materials in an
unconventional excitation region. Luminescence. 2018;1–6.
https://doi.org/10.1002/bio.3439
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