Shell Thickness Dependence of Interparticle Energy Transfer in Core-Shell ZnSe/ZnSe Quantum Dots Doping with Europium
Liu et al. Nanoscale Research Letters
Shell Thickness Dependence of Interparticle Energy Transfer in Core-Shell ZnSe/ZnSe Quantum Dots Doping with Europium
Ni Liu 0
Shuxin Li 1
Caifeng Wang 0
Jie Li 0
0 College of Aeronautical Engineering, Binzhou University , Shandong 256603 , China
1 Anhui Key Laboratory of Nanomaterials, and Technology and Key Laboratory of Materials Physics, Institure of Solid State Physics, Chinese Academy of Sciences , Hefei 230031 , China
Low-toxic core-shell ZnSe:Eu/ZnS quantum dots (QDs) were prepared through two steps in water solution: nucleation doping and epitaxial shell grown. The structural and morphological characteristics of ZnSe/ZnS:Eu QDs with different shell thickness were explored by transmission electron microscopy (TEM) and X-ray diffraction (XRD) results. The characteristic photoluminescence (PL) intensity of Eu ions was enhanced whereas that of band-edge luminescence and defect-related luminescence of ZnSe QDs was decreased with increasing shell thickness. The transformation of PL intensity revealed an efficient energy transfer process between ZnSe and Eu. The PL intensity ratio of Eu ions (I613) to ZnSe QDs (IB) under different shell thickness was systemically analyzed by PL spectra and time-resolved PL spectra. The obtained results were in agreement with the theory analysis results by the kinetic theory of energy transfer, revealing that energy was transmitted in the form of dipole-electric dipole interaction. This particular method of adjusting luminous via changing the shell thickness can provide valuable insights towards the fundamental understanding and application of QDs in the field of optoelectronics.
Core-shell quantum dots; Energy transfer; Shell thickness; Fluorescence lifetime
Rare earth (RE) doped chalcogenide semiconductor
quantum dots have received particular attention in the
field of nanomaterials, due to their excellent photoelectric
properties, such as multispectral luminescence, long
fluorescent life, high luminous efficiency, low-gentle magnetic,
]. However, the absorption cross section of RE
ions is very small (the order of magnitude is 10− 21 cm− 2),
which leads to low luminescence efficiency . Moreover,
it is very difficult to directly stimulate transition of RE
ions, since the f-f transition belongs to the parity
forbidden transition according to the selection rule [
]. In order
to overcome the above mentioned restrictions, significant
research efforts have been devoted to the doping of RE
ions into luminescent matrix materials. The matrix
materials with large absorption cross-section can transfer
energy to RE ions, so as to indirectly enhance their
luminescence. This phenomenon is known as the “antenna
]. Various materials, such as fluorides, silicates,
and chalcogenide semiconductor quantum dots are
usually employed as matrix materials [
these, chalcogenide semiconductor quantum dots have
some unique properties, such as quantum size effect, high
fluorescence efficiency, large absorption cross section
(1.1 × 10− 18 cm− 2), light stability, rendering them as
excellent candidate materials [
]. Up to now, the
research efforts on RE doping in chalcogenide
semiconductor quantum dots were mainly focused on
tuning luminescence wavelength and improving PL
efficiency, by adjusting doping concentration, reaction
time, and other experimental parameters [
]. In the
research of dopant QDs, energy transfer was usually a
means of explaining spectral phenomena, but the intrinsic
mechanism of energy transfer was rarely explained.
In view of the above perspectives, the PL
characteristics and intrinsic energy transfer mechanism of
coreshell ZnSe:Eu/ZnS QDs were thoroughly explored in the
present work. The luminescence spectra of the ZnSe
host materials and Eu ions were investigated by
controlling shell thickness. The mechanism of energy transfer
between Eu ions and ZnSe/ZnS core-shell quantum dots
was systematically analyzed by time-resolved
fluorescence spectroscopy and energy transfer kinetic theory.
In this paper, ZnSe:Eu/ZnS core-shell quantum dots were
prepared through nucleation doping and epitaxial growth
method. The detailed preparation process was described
as follows: the mixture of zinc nitrate hexahydrate(Zn
(NO3)2.6H2O), europium(III) nitrate hexahydrate(Eu
(NO3)3.6H2O), and 3-Mercaptopropionic acid(MPA) with
a molar ratio of Zn2+/Eu/MPA = 1: 0.06: 20 prepared
under stirring in N2 atmosphere. Then 50 mL of 0.5 M
sodium selenohydride (NaHSe) solution was injected into
the precursor solution of Zn rapidly followed by
condensation at 100 °C under continuous stirring.
Afterwards, ZnSe:Eu nanoparticles were purified by
employing absolute ethanol and centrifugal precipitation.
For obtaining ZnS shell by epitaxial growth method,
20 mg of ZnSe: Eu nanoparticles were added to 100 mL of
deionized water and were stirred in N2 atmosphere until
obtaining a clear and transparent solution. Then, zinc
acetate (Zn(AC)2.2H2O, 0.1 M)) and MPA (0.7 mL) with a
pH of 10.3 were added dropwise to the ZnSe: Eu solution
and were heated at 90 °C in N2 atmosphere until the
reaction completed. The same absolute ethanol and
centrifugal precipitation purification process was used.
Pure ZnSe:Eu/ZnS QDs were obtained which put into a
vacuum oven for further use. The samples used for
characterization were all re-dissolved in deionized water.
The size and morphology of ZnSe:Eu/ZnS QDs QDs
were investigated by transmission electron microscopy
(TEM) using Technai G2 operated at 200 kV. The XRD
of the sample powder was performed by wide angle
Xray scattering with graphite monochromatized high
intensity 0.148 nm Cu–Kα radiation. PL spectra were
measured at room temperature using Jobin Yvon
Fluorolog-3 system (Jobin Yvon Division Company,
France) and excitation wavelength was 365 nm. The
luminescence lifetime spectra of samples were measured
relative to FLS920 fluorescence spectrophotometer
equipped with a 450 W xenon lamp as the excitation
source, and the pulse frequency is 100 ns.
Results and Discussion
Figure 1a–o representatively shows the TEM results for
core ZnSe:Eu QDs and core-shell ZnSe:Eu/ZnS QDs with
different shell thickness. From the Fig. 1a–c, we can see
that the shape of ZnSe:Eu QDs are regular spherical, and
the average size is 2.7 nm. The high-resolution
transmission electron microscopy (HRTEM) demonstrates the
excellent crystallinity of the ZnSe:Eu QDs. When ZnS shell
is epitaxially grown on the surface of ZnSe:Eu QDs, the
size of the ODs became significantly larger, i.e., 3.6 nm
(1 ML), 4.6 nm (2 ML), 5.4 nm (3 ML), and 7.2 nm
(5 ML). As the thickness of the shell increases, the shape
of the quantum dots gradually becomes ellipsoid, but the
significant changing of lattice fringes in crystal boundaries
between ZnSe and ZnS was not obvious due to the
method of epitaxial growth.
In order to further improve the fluorescence efficiency
of the ZnSe:Eu QDs, the epitaxial shell growth of ZnS
on the core of ZnSe:Eu is prepared. The PL spectra of
core-shell ZnSe:Eu/ZnS QDs with different shell
thicknesses is depicted in Fig. 2a. Three characteristic
luminescence peaks of Eu are shown, which are ascribe
to 5D0 → 7F1(590 nm), 5D0 → 7F2(613 nm),
and 5D0 → 7F3(652 nm) [
], correspondingly. On the
other hand, another two luminescence peaks of ZnSe
QDs appeared, which are band-edge luminescence
(406 nm) with a relatively sharp full width at half
maximum (FWHM) and defect state luminescence (510 nm)
with broad FWHM [
]. With the increase of ZnS
shell thickness, the characteristic luminescence intensity
of Eu is enhanced. When the thickness of the shell is
3 ML, the three characteristic luminescence intensities
of Eu ions reach the maximum value, while the two
PL intensities of ZnSe QDs are reduced, as shown in
Fig. 2b. The PL intensity transformation of ZnSe:Eu
QDs indicates energy transfer between ZnSe and Eu.
The ratio of PL intensity integral of the Eu ion (I613)
to the band edge PL intensity integral (IB) of the
ZnSe quantum dot as well as the defect-related
luminescence intensity (ID) were calculated, respectively.
The results revealed that the energy transfer efficiency
varies with the thickness of the shell layer.
In particular, when ZnSe:Eu QDs are epitaxial coated
with ZnS shell, the lattice constants of the two
counterparts are not equal and the lattice continuity across the
interface is destroyed, resulting in lattice mismatch.
Because of lattice mismatch, ZnSe suffered compressive
stress at the interface and ZnS is subjected to tensile
stress, and the average lattice constant changed [
Consequently, the induced stress modifies the energy
level structure of the core-shell nanoparticles, which in
turn alters the electron energy level structure in the
nanocrystalline particles. Three possible steps are
considered for exciton recombination process: (i) radiation
recombination of excitons in host materials (including
the edge emission and defect emission of ZnSe QDs);
(ii) non- radiation recombination through heat
transfer loss; (iii) energy transfer between ZnSe host and
Eu ions, which enhanced PL intensity of Eu ions.
These three steps competed each other, resulted in
the simultaneously appearance of three PL peaks as
shown in Fig. 2a. The two types of fluorescence
transfer part of energy to the adjacent Eu ions during
radiation recombination process, which resulted in
characteristic luminescence peak at 613 nm of Eu and
that of the band-edge luminescence peak at 406 nm of
ZnSe with different ZnS shell thickness is shown in Fig. 4.
With the increase of ZnS shell thickness, the average
lifetime of donor ZnSe QDs decreases exponentially as
fastacting energy transfer for enhanced stress in core-shell
structure. Concomitantly, the acceptor Eu average lifetime
increases as it receives transferred photon energy.
According to the kinetic theory of energy transfer, the
ratio of ZnSe band edge PL intensity (IB) to that of Eu
ion (IE) as a function of the ZnS shell thickness can be
calculated by time-resolved PL spectra [
steady-state excitation conditions, the energy transfer
rate for ZnSe-Eu can be expressed according to Eq. 1:
W ZnSe−Eun1 ¼ τ2
where WZnSe − Eu is the energy transfer rate of
ZnSeEu; τ2 is the lifetime of Eu ions (I613); n1 and n2 are the
number of excited ions of ZnSe and Eu ion level,
respectively. The macroscopic energy transfer rate can be
expressed as follows:
where τ0 is the lifetime of the bare ZnSe QDs when the
ZnS shell thickness is 0 ML and τ1 is the lifetime of
ZnSe band edges (IB). The ratio between band-edge
emission intensity (IB) of ZnSe QDs to that of Eu ions
(I613) can be expressed as follows:
electrons transitions in Eu ions from 7F0 state to 5D0
], as shown in Fig. 3.
The time-resolved PL spectra of ZnSe:Eu/ZnS
coreshell QDs is an important means to detect energy
transfer between them [
]. The fluorescence lifetime of the
where γ1 and γ2 are the emissive coefficients.
Comparing the experimental ratio of I613/IB (red bar
graph) with the theoretical results (black bar graph), we
can conclude that the ratio calculated by the
luminescence kinetics model agree well with the
experimental results, as shown in Fig. 5. It also demonstrates
the energy transfer efficiency increased with the increase
of shell thickness.
No radiation energy transfer mainly takes place via
the interaction between multipolar moments. When
the distance between the host and the guest is
relatively short, the energy can be transferred from the
host (donor: ZnSe) to the guest (acceptor: Eu)
through multipole interaction [
]. The mechanism of
energy transfer between donor and acceptor can be
corroborated by considering the fluorescence intensity
and lifetime of the donor and the acceptor. The
fluorescence lifetime of the multipole moment can be
expressed according to Eq. (4):
φðtÞ ¼ exp
s c0 τ0
where τ0 is the fluorescence lifetime of the donor
without dopant, c is the doping concentration of acceptor,
cd0istaisncet(hce0 ¼cr3it.ic4aπlR30c)o。ncDeniftfrearteinotnS vreallauteesdstatnod cforrititchael
interaction of different multipolar moments [
corresponds to electric dipole-electric dipole interaction for
s = 6, dipole-quadrupole interaction for s = 8, and
quadrupole-quadrupole interaction for s = 10,
respectively. The fitting results for different s values are
depicted in Fig. 6. The ratio of band-edge luminescence
intensity and fluorescence lifetime is well matched with
the fitting results for s = 6, which indicates the existence
of energy transfer between the donor of ZnSe and Eu
acceptor by electric dipole-electric dipole mode. These two
of the interactions for cross relaxation are electrostatic
The ZnSe:Eu/ZnS (QDs) were prepared by wet
chemical method via nuclear doping followed by
epitaxial ZnS shell growth. The morphology and
structure of core-shell ZnSe:Eu/ZnS QDs were clearly
revealed by TEM and XRD results. The
photoluminescence (PL) spectra of ZnSe:Eu/ZnS QDs with
different thickness of ZnS shell showed that the PL
intensity of the Eu characteristic luminescence peak
increased while that of characteristic luminescence
and defect luminescence of ZnSe decreased,
illustrating an effective energy transfer between ZnSe and
Fig. 6 Fitting diagram of experimental and theoretical values of I.
Eu. The intrinsic mechanism of energy transfer with
different ZnS shell thickness was systematically
investigated through time-resolved spectra and energy
transfer dynamics theory. The results revealed that
energy was transmitted in the form of dipole-electric
I613: The PL intensity integral of the Eu ion; IB: The band edge PL intensity
integral of ZnSe; ID: The defect-related luminescence intensity integral of
ZnSe; PL: Photoluminescence; QDs: Quantum dots; TEM: Transmission
electron microscopy; XRD: X-ray diffraction
The authors thank Qiancheng Zhang for proofreading the manuscript.
This work was funded by the Natural Science Foundation of Shandong Province
(no. ZR2017PF011), the National Natural Science Funds of China (no. E020701),
and the Doctoral Scientific Research Foundation of Binzhou University
This study has nothing to do with human participants or health-related
LN designed and conducted the experiments and analyses, and drafted the
manuscript. LSX analyzed the data and supervised this study. WCF and LJ
conceived the project, organized the paper, and edited the manuscript. All
authors read and approved the manuscript.
The authors declare that they have no competing interests.
Springer Nature remains neutral with regard to jurisdictional claims in published
maps and institutional affiliations.
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