Facile synthesis of highly luminescent Mn-doped ZnS nanocrystals

Article abstract of DOI:10.1021/ic201547g

Manganese-doped zinc sulfide quantum dots (Mn:ZnS d-dots) with high optical quality, pure dopant emission of 55-65% photoluminescence quantum yield, were synthesized in octadecene media with generic starting materials, namely, zinc (manganese) carboxylic acid salts, S powder, and dodecanethiol (DDT) based on a nucleation doping strategy. The optical properties and structure of the obtained Mn:ZnS d-dots have been characterized by UV-vis, photoluminescence (PL) spectroscopy, transmission electron microscopy (TEM), and X-ray diffraction (XRD). The resulting nearly monodisperse d-dots were found to be of spherical shape with a zinc-blende crystal structure. The influences of various experimental variables, including the reaction temperature for the MnS core nanocluster and ZnS host material, the amount of octadecene (ODE)-S, DDT, as well as Zn/Mn ratio have been systematically investigated. The use of DDT as capping ligand ensured the reproducible access to a stable small-sized MnS core. This paves the way for reproducibly obtaining highly luminescent d-dots. Programmed overcoating temperature for growth of ZnS shell was employed to realize balanced diffusion of the Mn ions in the d-dots.

Full text of DOI:10.1021/ic201547g

ARTICLE
pubs.acs.org/IC
Facile Synthesis of Highly Luminescent Mn-Doped ZnS Nanocrystals
Wenjin Zhang, Yan Li, Hua Zhang, Xinggui Zhou, and Xinhua Zhong*
State Key Laboratory of Chemical Engineering, Department of Chemistry, East China University of Science and Technology,
Shanghai 200237, China
ABSTRACT: Manganese-doped zinc sulfide quantum dots (Mn:ZnS d-dots) with
highopticalquality, puredopant emission of55À65%photoluminescence quantum
yield, were synthesized in octadecene media with generic starting materials, namely,
zinc (manganese) carboxylic acid salts, S powder, and dodecanethiol (DDT) based
on a “nucleation doping” strategy. The optical properties and structure of the
obtained Mn:ZnS d-dots have been characterized by UVÀvis, photoluminescence
(PL) spectroscopy, transmission electron microscopy (TEM), and X-ray diffraction
(XRD). The resulting nearly monodisperse d-dots were found to be of spherical
shape with a zinc-blende crystal structure. The influences of various experimental
variables, including the reaction temperature for the MnS core nanocluster and ZnS
host material, the amount of octadecene (ODE)-S, DDT, as well as Zn/Mn ratio
have been systematically investigated. The use of DDT as capping ligand ensured
the reproducible access to a stable small-sized MnS core. This paves the way for
reproducibly obtaining highly luminescent d-dots. Programmed overcoating temperature for growth of ZnS shell was employed to
realize balanced diffusion of the Mn ions in the d-dots.
’ INTRODUCTION
incorporated into the crystal lattice with a large proportion of the
manganese remaining on the surface. This situation was sub-
stantially improved by Peng’s group with the development of a
new doping strategy by decoupling the doping from nucleation
and/or growth through nucleation-doping and growth-doping
approaches.21À24 Nucleation doping was realized by a mixed
dopant and host precursor during the nucleation. In an extreme
case, the nuclei could be formed with pure dopant cations and
followed by the growth of a host shell on the core. Under the
nucleation-doping strategy, MnSe nuclei were overcoated with
ZnSe shell. No residual ZnSe emission was observed, and the
doped NCs exhibited thermally stable highly efficient PL with
QY of 40À70%.21À24 For Mn:ZnS d-dots, they are a greener
material that is free from both Cd and Se, and popular nanopho-
sphors for electroluminescent devices.35,36 In comparison, the
synthesis of Mn:ZnS is not as advanced, and few examples
have been given for the synthesis of high quality Mn:ZnS d-dots
by the nucleation- or growth-doping strategy. Recently, high
quality Mn:ZnS d-dots or doped nanorods with PL QY around
35À50% have been prepared via the nucleation- or growth-
doping strategy.13À16
The critical role that dopants play in semiconductor devices
has stimulated research on the properties and the potential
applications of doped semiconductor nanocrystals (NCs), also
named doped quantum dots (d-dots). Colloidal d-dots have
drawn considerable attention in recent years because of their
unique optical, electronic, and magnetoptical properties and
consequently in potential applications as optoelectronic devices
and biomedical tags and so forth.1À5 Currently, a variety of
synthetic approaches have been developed for transition metal
ions (mainly Mn2+) doped zinc chalcogenides (including ZnSe
and ZnS) NCs,6À30 in which the transition metal ligand-field
excited states reside within the bandgap of the host semiconduc-
tor. Such materials have been shown to display nearly pure and
efficient sensitized dopant photoluminescence (PL). As an
emitter, these d-dots could not only replace CdSe or CdTe
quantum dots with nontoxic or low-toxic zinc, but also overcome
a couple of intrinsic disadvantages of undoped quantum dots
emitters, that is, strong self-quenching caused by their small-
ensemble Stokes shift, and sensitivity to thermal, chemical, and
photochemical disturbances.31À34
Herein, we report the preparation of highly luminescent Mn:
ZnS d-dots with use of DDT as a capping ligand under the
nucleation-doping strategy in octadecene media. Alkyl thiols are
soft Lewis base ligands for transition metal ions and thus can
form stable metal complexes to lower the reactivity of metal ions
precursors. Therefore, the DDT ligand can make Mn much less
reactive, and thus lead to form stable small-sized MnS core
Because of the potential high photoluminescence quantum
yields (PL QYs) from the Mn2+ 4T1À A1 transition and suitable
6
valence band edge of the host for application in electrolumines-
cent devices, the Mn:ZnS,6À16 and Mn:ZnSe17À30 d-dots have
attracted extensive attention. There has been a sustained effort
for more than a decade to enhance the dopant emission from
doped NCs. In the early stage, most reports on d-dots used a
colloidal route based on the simultaneous precipitation of both
host materials and dopants.6À9,17,18 All these studies have con-
cluded that only a small fraction of the initial manganese used is
Received: July 19, 2011
Published: September 19, 2011
r
2011 American Chemical Society
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ARTICLE
nanoclusters. The synthesis is based on a two-step approach:
(i) synthesis of small-sized MnS core nanoclusters, (ii) over-
coating ZnS shell around MnS core accompanied with the Mn
dopant diffusion and formation of Mn ions diffusion layer
between the MnS core and ZnS shell. We found that the use of
DDT as capping ligand ensured the reproducible access of stable
small-sized MnS core nanoclusters, and the programmed over-
coating temperature for ZnS host material growth realized the
balanced diffusion of Mn ions in the d-dots.
optical density.37,38 Also, the known QYs of the QDs in solution can be
used to measure the PL efficiencies of other QDs by comparing their
integrated emission. To conduct investigations with the transmission
electron microscopy (TEM), the NCs were deposited from dilute
toluene solutions onto copper grids with carbon support by slowly
evaporating the solvent in air at room temperature. TEM images were
acquired using a JEOL JEM-1400 transmission electron microscope
operating at an acceleration voltage of 120 kV. Powder X-ray diffraction
(XRD) was obtained by wide-angle X-ray scattering using a Siemens
D5005 X-ray powder diffractometer equipped with graphite monochro-
matized Cu Kα radiation (λ = 1.5406 Å). XRD samples were prepared
by depositing NC powder on a piece of Si (100) wafer.
’ EXPERIMENTAL SECTION
Chemicals. Zinc acetate (Zn(OAc)2, 99.99%), manganese chloride
(MnCl2, 99.99%), zinc diethyldithiocarbamate (ZDC, 98%), 1-dodeca-
nethiol (DDT, 99.9%), tetramethylammonium hydroxide pentahydrate
(TMAH), sulfur powder (99.99%), 1-octadecene (ODE, 90%), stearic
acid (SA, 95%), and oleylamine (OAm, 70%) were purchased from Aldrich.
All chemicals were used as received without any further purification.
Preparation of MnSt2 and Zinc Stock Solution. The manga-
nese precursor, manganese stearate (MnSt2), was prepared according to
a literature method.22 Typically, SA (20 mmol) was dissolved in 35 mL
of methanol with the aid of heating to get a clear solution. A solution
of TMAH (obtained by dissolving 20 mmol of TMAH in 15 mL of
methanol) was mixed with the SA solution. To this solution, 10 mmol of
MnCl2 dissolved in 15 mL of methanol was added dropwise with
vigorous stirring, and a white precipitate of MnSt2 slowly flocculated.
The precipitates were repeatedly washed with methanol and dried under
vacuum.
’ RESULT AND DISCUSSION
Formation of Small-Sized MnS Core Clusters. Herein, a
nucleation doping strategy was adopted for the preparation of
Mn:ZnS d-dots. In the first step, small sized MnS core NCs were
synthesized based on a hot-injection of S precursor ODE-S into
the noncoordinating solvent ODE system containing Mn pre-
cursor (MnSt2) and DDT at 250 °C. A ZnS shell was then
overcoated around the MnS cores with the introduction of
Zn(OAc)2 into the “S”-rich system containing the preformed
small-sized MnS core nanocluster. The resulting NCs can be
considered as a MnS/ZnS core/shell or MnS/Mn:ZnS/ZnS
quantum well structure from the structural viewpoint. For
simplicity, these NCs are called d-dots as done in previous
reports because of their similar emission properties.13,14,21À24
It is well-known that the emission features of dopant Mn2+
ions in d-dots are closely related to the distribution of Mn2+ ions
in the d-dots and the structure of the d-dots.13,14,21À24 For the
case of MnS/Mn:ZnS/ZnS quantum well structures, the PL of
dopant Mn ions is currently considered to come from the energy
transfer from photoexcited ZnS shells to the Mn ions in a
diffusion layer at the interface between the MnS core and the
pure ZnS overcoating layer. High PL QY of dopant Mn ions in
Mn:ZnS d-dots could be obtained by controlling the formation
of the Mn ions diffusion layer at the interface between the MnS
core and the ZnS outer layer. Accordingly, control of the size of
the MnS cores is the first key issue in this nucleation-doping
strategy to obtain highly luminescent dopant emission in Mn:
ZnS d-dots. Ideally, a small-sized core cluster would place the
dopant ions as close as possible to the center of the d-dots. This
shall result in emission centers as far away as possible from the
potential surface trap states of the NCs, and thus improve the
optical performance of the d-dots. A small core cluster also means
a relatively uniform environment of the doping ions in a d-dot.
Furthermore, a small-sized core might be more suited for forma-
tion of a high-quality diffusion layer at the interface between
the MnS core and the ZnS shell in comparison to a large-sized
MnS core because of the stronger quantum confinement effect.39
Therefore, we chose advisable reaction condition with use of DDT
as capping ligand to obtain small sized MnS core nanoclusters.
In recent years, alkyl amines have been used predominantly as
capping ligand for synthesis of Mn doped zinc chalcogenides
NCs.13À15,21À24 When manganese carboxylates were used as Mn
precursor, however, tuning the fine size of MnS nucleus was
found to be difficult when the solution was composed of pure
noncoordinating solvent (such as ODE), alkyl amines, carboxylic
acid, and their mixtures. In the case of the Mn:ZnSe system, one
simple way to suppress the formation of large-sized MnSe core
NCs via continuous growth is to add large excess amounts of the
The zinc stock solution (0.5 M) for ZnS shell growth was prepared by
dissolving 2.195 g (10 mmol) of Zn(OAc)2 in a mixed solvent system
containing 6.0 mL of OAm and 14.0 mL of ODE at 160 °C under
nitrogen flow. The obtained Zn stock solution was stored at 50 °C for
the following use.
Synthesis of Mn:ZnS d-Dots through Nucleation-Doping
Strategy. Typically, 0.1 mmol (62.2 mg) of MnSt2, 1.0 mL of DDT,
and 3.0 mL of ODE were loaded into a 50 mL three-neck flask and
degassed at 100 °C for 15 min under vacuum. Then the reaction system
was filled with N2, and the temperature was further raised to 250 °C to
get a colorless and clear solution. At this temperature, 0.5 mL of sulfur
solution in ODE (0.4 M), obtained by dissolving sulfur powder in ODE
at 120 °C, was injected into the reaction system. Immediately after the
injection of sulfur solution, the color of the reaction solution turned to
faint yellow, showing the formation of MnS nanoclusters. Two min after
the injection of sulfur solution, Zn stock solution was added dropwise to
the vigorously stirred solution via a syringe pump in a period of 2À3 h.
Immediately after the addition of Zn precursor to the S-rich system, the
solution color became golden yellow. To monitor the reaction, periodi-
cally small aliquots were taken for measuring UVÀvis and PL spectra.
When 1 mL of Zn stock solution was added into the reaction system,
reaction temperature was lowered down to 230 °C for the further
overgrowth of ZnS shell with addition of another 1 mL of Zn stock
solution. After finishing the addition of stock solution, the reaction
mixture stayed at 230 °C for another 20 min to ensure the complete
consumption of the Zn precursor. Finally, the reaction was cooled down
to room temperature, and the NCs were purified using methanol/
hexane extraction with the d-dots in the hexanes layer. This procedure
can reproducibly prepare Mn:ZnS d-dots with PL QY in the range of
55À65%.
Characterization. UVÀvis and PL spectra were obtained on a
Shimadzu UV-2450 spectrophotometer and a Cary Eclipse (Varian)
fluorescence spectrophotometer, respectively. The room-temperature
PL QY of the QDs were determined by comparing the integrated
emission of the QDs samples in solution with that of a fluorescent dye
(such as rhodamine 6G with a QY of 95% in ethanol) with an identical
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anionic precursor.21À24 In previous report for the synthesis of
Mn:ZnSe d-dots,24 when the Mn to Se precursor was up to 1:25,
small sized (less 2 nm) MnSe core NCs can be obtained with
long-term stability at a growth temperature of 250 °C. In contrast,
in the preparation of metal sulfide NCs, use of large excess of S
precursor is not valid for the formation of small sized MnS core
NCs. In previous reports,13,14 the resulting MnS core NCs is about
2.3 nm even terminating the reaction right after the mixing of Mn
and S precursors at high temperature.
To control the growth of NCs, selecting capping ligand is a key
issue, because a kind of appropriate ligand can commendably
limit growth of NCs to form smaller NCs cores. The coordina-
tion ability of the mercapto group is stronger than phosphorus,
oxygenous, nitrogenous groups and renders a strong binding
capacity with the surface atoms of the NCs. Therefore, the thiol
ligands can make Mn much less reactive, and thus lead to form
stable small-sized MnS core nanoclusters. In previous reports,40À42
thiols were successfully used as ligands for formation of high-quality
IÀIIIÀVI semiconductor NCs by balancing the reactivity of two
cationic precursors. For this reason, we chose DDT as capping
ligand to obtain stable small sized MnS core NCs under the
nucleation doping strategy for Mn:ZnS d-dots.
The effect of using DDT in controlling the size of the formed
MnS core is distinctive. When DDT was chosen as capping ligand
in the preparation of MnS core NCs with the use of manganese
stearate and ODE-S as corresponding Mn/S precursors, the
absorption edge of the MnS clusters stayed around 360 nm with a
corresponding particle size smaller than 2 nm even after the
heating time was up to 1 h (solid curves in Figure 1); while in the
control experiment without the presence of thiol ligands, the
formed MnS nanocluster can grow larger sharply via continuous
growth and lead to less reproducibility of the synthetic method
(dotted curves in Figure 1). The access of stable small sized MnS
core nanoclusters with the use of DDT as capping ligand paves
the way for preparation of highly luminescent Mn:ZnS d-dots
reproducibly. It was found that when we repeated our synthetic
approach 10 times, the PL QYs of the obtained Mn:ZnS d-dots
could be in the range of 55À65%.
Optical Properties of Mn:ZnS d-Dots. As shown in
Figure 2A, the UVÀvis absorption spectra of Mn:ZnS d-dot
samples show the characteristics of a quantum well structure
without the distinguished absorption onset,22 and the featureless
absorption at the UV region should be related to the absorption
of the ZnS host. A strong PL band is observed at about 587 nm,
6
which is ascribed to the Mn2+ ion 4T1À A1 transition.43,44 The
highest PL QY of 65% can be obtained when the nominal Zn/Mn
ratio is 10:1, and the corresponding photograph of the emission
color is shown in the inset of Figure 2A. Compared with that of
previously reported Mn:ZnS d-dots,6À10 the trap state emission,
appearing near 400À500 nm, is greatly suppressed in our samples,
implying preparation of the high-quality Mn-doped QDs.
It was found that the Mn ion dopant PL intensity increased
rapidly with increase of ZnS shell thickness as shown in
Figure 2B. With the overgrowth of ZnS shell around the pre-
formed MnS core nanoclusters, faint Mn PL was observed. When
the nominal Zn/Mn ratio was less than 5 (i.e, the ZnS shell
around MnS is thin), the PL intensity of the resulting MnS/ZnS
was relative low (PL QY is less than 10%). When the Zn/Mn
ratio was greater than 5:1, with the increasing of ZnS shell
thickness the PL QYs of the resulting Mn:ZnS increased sharply
and approached the highest value of 65% when the Zn/Mn ratio
Figure 1. Temporal evolution of UVÀvis absorption spectra of MnS
nanoclusters synthesized with (solid curves) and without (dotted curves)
the use of DDT ligand.
Figure 2. (A) Typical UVÀvis absorption and PL (λex = 300 nm) emission spectra of Mn:ZnS d-dots. (Inset) Digital photograph of Mn:ZnS d-dots
under irradiation of a UV lamp. (B) The dependence of PL intensity of Mn:ZnS d-dots on the nominal Zn/Mn ratio.
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and (311) planes as shown in Figure 3E, meaning that it is cubic
(zinc blende) phase (JCPDS card no. 77-2100), which supports
the hypothesis that Mn2+ is more facile to incorporate in cubic
ZnSe NCs than in wurtzite and rock-salt crystals.19 No diffraction
peaks from manganese impurities were detected in the sample. In
addition, we can conclude that doping Mn2+ into the host NCs
does not bring about a phase transformation of the crystal
structure.
PL Properties of Mn:ZnS d-Dots Dependent on Synthetic
Conditions of MnS Core Nanoclusters. The synthetic condi-
tions of the MnS cores nanoclusters were found to play a critical
role in determining the PL properties of the resulting Mn:ZnS
d-dots. First, the reaction temperature for the synthesis of the
MnS core nanoclusters influences the PL QY of the resulting
d-dots greatly. The experimental results revealed that to obtain
highly luminescent Mn dopant emission in the Mn:ZnS d-dots,
the optimal reaction temperature for introducing the sulfur
precursor solution ODE-S into the reaction flask for formation
of MnS NCs was 250 °C. If the temperature was too low or too
high relative to 250 °C, the PL intensity (normalized to the same
absorbance at the excitation wavelength) of the dopant emission
in the resulting d-dots was significantly lower than that obtained
at 250 °C as shown in Figures 4A and 4B. The PL intensity of the
resulting Mn:ZnS d-dots at 210 °C is less than 1/10 of that at
250 °C (Figure 4B). Too low temperature was not adequate for
the formation of MnS nanoclusters, while too high temperature
would lead to formation of large-sized MnS core NCs and
accelerate the decomposition of DDT. These effects would
deteriorate the PL QY of the resulting d-dots, since a large sized
MnS core NC is not favorable for the Mn diffusion and formation
of the diffusion layer because of the weak quantum confinement
effect.39 As shown in Figure 4C, the dopant PL intensity of the
resulting d-dots was not sensitive to the reaction time, and high
PL QY can be kept with reaction time extending up to 10 min.
After that, with further extension of the growth time, it actually
started to decrease. This feature of the PL QY not being sensitive
to the reaction time can bring a high reproducibility of this
synthetic approach to obtain highly luminescent d-dots. If not
specified, the d-dots mentioned in this report were all based on
such MnS nanoclusters formed at 250 °C for about 2 min.
Influence of Amount of ODES. A stable and small-sized MnS
core nanocluster is a prerequisite for high-quality Mn:ZnS d-dots
under the nucleation-doping strategy. Because of the strong
coordination ability of DDT to free metal ions and surface atoms
of MnS NCs, DDT can favorably limit the growth of NCs to
obtain small-sized MnS core nanoclusters. On one hand, DDT
acts as a kind of appropriate ligand, on the other hand it can
simultaneously support a sulfur source after thermal decomposi-
tion during the reaction. Consequently, we tried to use DDT
alone as a sulfur source to prepare Mn:ZnS d-dots. The experi-
mental results revealed that with the use of DDT alone as sulfur
source, the PL QY of the resulting d-dots was very low. If the
amount of DDT was greater than 2 mL, a yellow precipitation
was formed within the reaction mixture. Therefore, DDT acting
as sole sulfur source should not be suitable for obtaining high-
quality Mn:ZnS d-dots, and the optimal amount of DDT was
chosen as 1.0 mL.
Figure 3. TEM images of Mn:ZnS d-dots with Mn/Zn precursor ratios
of (A) 1:1.2, (B) 1:5, and (C), 1:10. (D) Typical XRD patterns of the
Mn:ZnS d-dots with Mn/Zn precursor ratios of 1:10. The line spectrum
(bottom) indicated the reflections of zinc blende bulk ZnS.
was 10:1. With further increase of the ZnS thickness, the PL QY
decreased gradually. Thisobserved dependenceof the PL intensity
of the Mn:ZnS d-dots on Zn/Mn ratio can be explained by the Mn
dopant PL mechanism. The PL of Mn ions comes from the energy
transfer from photoexcited ZnS shells to the Mn ions in a Mn ion
diffusion layer located between the MnS core and the pure ZnS
overcoating layer.22 For the Mn:ZnS d-dots synthesized by the
nucleation doping strategy, the formation of the diffusion layer was
considered to derive from the diffusion from the MnS core into the
ZnS shell during the growth process. When the Zn/Mn precursor
ratios were low (less than 5:1), that is, the ZnS shell is thin, Mn2+
ion emission centers are closer to the surface of the d-dots, where
a higher possibility exists to quench the Mn PL by surface traps, so
a weak Mn ion PL band is observed. With overcoating the ZnS
shell, the Mn ion diffusion layer is deeper placed in the d-dots, and
a strong PL band is thus observed.
Morphology and Structure. TEM was used to monitor and
characterize the overall size of the obtained d-dots, and the
thickness of the shell can be calculated correspondingly. The size
of the obtained MnS nanoclusters under the experimental
condition was too small to be detected using TEM. Figure 3
shows the TEM images of the corresponding Mn:ZnS d-dots
with nominal Mn/Zn precursor ratios of 1:2, 1:5, and 1:10. The
corresponding average diameters of the d-dots were determined
to be 2.9 ( 0.3, 3.6 ( 0.3, and 4.5 ( 0.4 nm, respectively. TEM
images demonstrate that all of the as-obtained NCs are of near-
spherical shape and possess high size uniformity. Taking the
MnS core diameter of 2.0 nm and the thickness of ZnS mono-
layer as 0.32 nm, the ZnS shell layer correspond to 1.4, 2.5, and 4
monolayers, respectively.
The fact that DDT cannot serve as the sole sulfur source for
the preparation of high-quality Mn:ZnS d-dots pushed us to use
ODE-S as another sulfur source. ODE-S should be one of the
best choices to serve as sulfur source, since it has high reactivity to
metal ions and is air-stable and thus conveniently processable. In
The wide-angle XRD patterns of the obtained Mn:ZnS d-dots
with Mn/Zn precursor ratios of 1:9.6 display the (111), (220),
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Figure 5. (A) PL spectra of Mn:ZnS d-dots dependent on the amount
of ODE-S, and (B) summary of PL peak intensity and peak position
dependent on the amount of ODE-S.
from 594 to 587 nm. With the increase of ODE-S, the cationic
precursor, Mn fatty acid salts should be consumed rapidly. This is
favored for formation of small-sized MnS nanoclusters, and the
formed small MnS nanoclusters should be stable in the reaction
solution because of the existence of an excessively high concen-
tration of sulfur monomer in the solution. As described above the
formation of small-sized MnS core nanoclusters is the prerequi-
site for obtaining highly luminescent dopant emission in the Mn:
ZnS d-dots. However, the Mn ion emission reduced system-
atically when the amount of ODE-S was higher than 0.2 mmol.
The presence of excess amount of ODE-S may result in the S-rich
surface of the obtained Mn:ZnS d-dots. The formation of
dangling bonds of the S2À species would cause the PL emission
to deteriorate and thus brings about the decrease of PL intensity.
Influence of Overcoating Temperature of ZnS Shell.
Effective “dopant diffusion” during overcoating of host material
is critical for obtaining high-quality dopant PL in d-dots via the
nucleation doping strategy. Undoubtedly, the overcoating tem-
perature (Toc) for ZnS host material growth is one of the most
important factors in determining the dopant diffusion process,
and thus Toc has a significant influence on the optical properties
of resulting Mn:ZnS d-dots. As show in Figure 6, with the same
type of MnS nanoclusters cores, at a relatively low Toc of 210 °C,
dopant Mn2+ PL emission can be observed only after the Zn
precursor stock solution was added up to a Zn/Mn ratio of 3:1;
after that the PL intensity increased continuously but slowly
within the experimental ZnS layer frame. On the other hand,
when the Toc was set at 250 °C, with the introduction of the first
Figure 4. PL spectra (A) and relative PL intensity (B) of Mn:ZnS d-dots
depending on the reaction temperature for synthesizing MnS core
nanoclusters, and (C) PL spectra of Mn:ZnS d-dots depending on the
reaction time of MnS core nanocluster growing at 250 °C.
the following experiment, we investigated the optimal amount of
ODE-S with the fixation of other reaction variables as stated in
the Experimental Section. Figure 5A shows that the PL intensity
of the resulting Mn:ZnS d-dots is heavily dependent on the
amount of ODE-S used, and the corresponding PL intensity and
peak position are summarized in Figure 5B. In principle, with
increase of the ODE-S amount, the Mn ion PL intensity
increased rapidly and approached the highest intensity at
ODE-S of 0.2 mmol. Simultaneously, with increase of the
ODE-S amount the PL peak position blue-shifted monotonically
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diffusion of Mn ions is not necessary; then a lower growth
temperature is only needed for the thickening of the ZnS shell,
but the diffusion is stopped. As a result, a continuous growth of
the ZnS host lattice would provide an additional passivation layer
for the dopant Mn emission center, which can further enhance
the PL emission.
’ CONCLUSION
In summary, highly emissive Mn:ZnS d-dots with PL QY in
the range of 55À65% were synthesized based on “nucleation-
doping” strategy in ODE media. By judiciously choosing DDT
as capping ligand, formation of stable small-sized MnS core
nanoclusters ensures the reproducible production of high-quality
d-dots through the construction of a dopant Mn ions diffusion
layer at the interface of the MnS core and ZnS host shell.
Programmed overcoating temperatures for ZnS host growth
were optimized to control the “lattice-diffusion” of the Mn ions
incorporated into the d-dots.
Figure 6. Normalized PL intensity of Mn:ZnS d-dots dependent on
ZnS shell overcoating temperature.
’ AUTHOR INFORMATION
batch of Zn stock solution, the color of the reaction solution
turned to golden yellow instantly, which indicated the fast growth
of the ZnS shell. The PL intensity increased fast and reach a sharp
maximum at about a Zn/Mn ratio of 5:1; following that the PL
intensity decreased fast. When the Toc was set at 230 °C, the
intensity of the dopant PL increased steadily and reached the
highest value at a Zn/Mn ratio of 10:1. With further introduction
of Zn stock solution, the color of the reaction mixture changed to
orange slowly from the initial yellow, and the solution turned to
turbid gradually accompanied with the reduction of PL intensity.
However, the PL QY did not reach very high (<30%) under a
Toc of 230 °C. In our procedure for overcoating ZnS host materials,
a programmed overcoating temperature was adopted at different
shell growth stages as shown in Figure 6. In the first stage, higher
temperature (250 °C) was adopted before the Zn/Mn ratio
approached 1:5; then the temperature was lowered to 230 °C for
continuous overgrowth of the ZnS shell. Experimental results
revealed that higher PL QY can be obtained with use of a two
stage temperature than a single temperature (230 °C or 250 °C).
The influence of Toc on the dopant PL emission of the
resulting d-dots is mainly derived from the influence on the
dopant Mn ion diffusion. If Toc is too low (such as 210 °C) to
spur the diffusion of Mn ions, dopant Mn ions were retained
mostly within the interior lattice of the core nanoclusters, and an
effective diffusion layer could not formed; and thus the structure
behaved like a core/shell structure with MnS core and ZnS shell
layer. In this case no effective energy transfer from host materials
to dopant ions and thus poor PL properties would be observed.
In contrast, when the Toc is too high, a large amount of dopant
Mn ions undergo a diffusion process. A high concentration of
dopant ions were placed right next to each other within the
center of the d-dots, and the strong MnÀMn interaction is not
favored for the high PL.8,32,45 In another extreme case, Mn ions
migrated to the surface of the d-dots or even ejected out of the
d-dots. This will result in the increase of the nonradiative
relaxation rate from the Mn ions to the quenching center, thus
quenching PL. The advantage of overgrowth of the ZnS shell
under two temperature stages could be explained by controlled
diffusion. In the beginning stage, higher temperature benefits the
diffusion of Mn ions to form the diffusion layer around the thin
shell of d-dots. With the formation of the diffusion layer, further
Corresponding Author
*E-mail: zhongxh@ecust.edu.cn. Fax: +86 21 6425 0281.
’ ACKNOWLEDGMENT
The work is financially supported by the National Natural
Science Foundation of China (Nos. 20771037 and 20871047),
State Key Laboratory of Chemical Engineering (No. SKL-ChE-
09C01), the Fundamental Research Funds for the Central
Universities, and the Program for Professor of Special Appoint-
ment at Shanghai Institutions of Higher Learning for financial
support.
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