Phosphine-free synthesis of high quality ZnSe, ZnSe/ZnS, and Cu-, Mn-doped ZnSe nanocrystals

Article abstract of DOI:10.1039/b917674h

High quality zinc blende ZnSe and ZnSe/ZnS core/shell nanocrystals have been synthesized by two converse injection methods (i.e. zinc precursor injection or selenium precursor injection) when Se-ODE complex was chosen as the phosphine-free selenium precur

Full text of DOI:10.1039/b917674h

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PAPER
www.rsc.org/dalton | Dalton Transactions
Phosphine-free synthesis of high quality ZnSe, ZnSe/ZnS, and Cu-,
Mn-doped ZnSe nanocrystals†
Huaibin Shen,a,b Hongzhe Wang,a Xiaomin Li,a Jin Zhong Niu,a Hua Wang,a Xia Chen*b and Lin Song Li*a
Received 27th August 2009, Accepted 28th September 2009
First published as an Advance Article on the web 30th October 2009
DOI: 10.1039/b917674h
High quality zinc blende ZnSe and ZnSe/ZnS core/shell nanocrystals have been synthesized by two
converse injection methods (i.e. zinc precursor injection or selenium precursor injection) when Se-ODE
complex was chosen as the phosphine-free selenium precursor. Absorption spectroscopy, fluorescence
spectroscopy, X-ray diffraction (XRD), and transmission electron microscopy (TEM) were used to
characterize the as-synthesized ZnSe and ZnSe/ZnS nanocrystals. The quality of the as-prepared ZnSe
nanocrystals reached the same high level compared with the method using phosphine selenium
precursors since the quantum yields were between 40 and 60% and photoluminescence (PL) full width
at half-maximum (FWHM) was well controlled between 14 and 17 nm. The parameter window for the
growth of high quality ZnSe nanocrystals was found to be much broader and monodisperse ZnSe
nanocrystals were synthesized successfully even when the reaction temperature was set as low as
240 ◦C. As cores, such zinc blende ZnSe nanocrystals were also used to synthesize ZnSe/ZnS core/shell
nanocrystals with high fluorescence quantum yields of 70%. Cu2+ or Mn2+ doped ZnSe nanocrystals
were also synthesized by simply modifying this phosphine-free method. The emission range has been
extended to 500 and 600 nm with the use of Cu2+ and Mn2+ dopants compared with the emission
coverage of ZnSe at around 400 nm. This is the first totally “green approach” (i.e. phosphine-free
synthesis) for the synthesis of high quality ZnSe, ZnSe/ZnS, and Cu2+ or Mn2+ doped ZnSe
nanocrystals.
are now integrating green chemistry principles into their synthetic
1. Introduction
approaches for green nanoproducts,35 which means that not only
Semiconductor nanocrystals have attracted much attention be-
are the final products green but also the starting materials.
cause of their interesting properties.1-12 Most of the research has
focused on the CdSe nanocrystal, because of its high emission
efficiency and size-tuned photoluminescence (PL) in the visible
range.13-23 The highly toxic component cadmium, however, places
them in a disadvantageous position or even with a doubtful
future for applications in the field of bioimaging, light-emitting
devices, and solar cells.1-12 Therefore, attempts to develop non-
cadmium, phosphine-free “green” substitutes are currently a
priority in this field. Recently, some groups have reported the
synthesis of high quality ZnSe and related Cu-, Mn-doped ZnSe
nanocrystals with emitting windows from 400 to 610 nm.24-34
Therefore, these kinds of ZnSe based nanocrystals with or without
dopants would be potentially an ideal substitute for cadmium
based nanocrystals due to the ultimate elimination of the use of
toxic components. However, most of these methods still involve
the use of alkylphosphines (such as trioctylphosphine (TOP)
and tributylphosphine (TBP)) to make the complex precursors
between Se and TOP or TBP. It is well known that TOP and TBP
are hazardous, unstable, and expensive materials and generally
require glove box operations. An increasing number of researchers
Most recently, the Zou21 and Mulvaney22 groups first reported
the “green approach” phosphine-free synthesis of CdSe using
elemental selenium as the chalcogenide precursor dispersed in
paraffin liquid or 1-octadecene (ODE). We also reported a new
efficient protocol for the phosphine-free synthesis of high quality
CdSe and CdSe/ZnS core/shell nanocrystals recently.36 The as-
prepared CdSe and CdSe/ZnS nanocrystals are of the same high
quality as the widely used phosphine-selenium precursors. The
PL quantum yields (QYs) of as-prepared CdSe/ZnS core-shell
nanocrystals could reach as high as 80% and generate stable
emissions covering the range from 510 to 650 nm. The total cost
saved can be as high as 50% to produce nanocrystals compared
with the method using phosphine-selenium precursors. So far,
there is still no report on the synthesis of high quality ZnSe
nanocrystals with such a “green approach” even though William
W. Yu et al. reported that Se could be dissolved in olive oil at
elevated temperatures to synthesize ZnSe nanocrystals.35 But the
quality of nanocrystals was still not as good as the ZnSe synthe-
sized with TBP-Se or TOP-Se. The reaction speed was so fast and
only 10 nm-sized ZnSe nanoparticles were able to be synthesized
and the QY was below 10%. In the case of the synthesis of high
quality ZnSe nanocrystals, however, phosphines were still needed
to dissolve Se powder and could not be eliminated.27,34,37 The
parameter window for the growth of high quality zinc chalcogenide
nanocrystals (> 310 ◦C) was found to be much narrower than that
of the corresponding cadmium chalcogenide nanocrystals with
aKey Laboratory for Special Functional Materials, Henan University,
Kaifeng, 475004, P. R. China. E-mail: lsli@ henu.edu.cn, bbbbz@163.com;
Fax: +86-378-3881358; Tel: +86-378-3881358
bCollege of Life Science, Jilin University, Changchun, 130021, P. R. China
† Electronic supplementary information (ESI) available: Experimental
details. See DOI: 10.1039/b917674h
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the use of the phosphine-selenium precursor.27 Activation of the
zinc precursor (zinc fatty acid salts) by alkylamines was found to
be critical for the formation of high quality ZnSe nanocrystals.
Here, we report a new efficient protocol for the synthesis of high
quality ZnSe, ZnSe/ZnS core/shell nanocrystals and Cu- or Mn-
doped ZnSe nanocrystals with a visible emitting window of 390–
610 nm. This approach eliminates not only the need for any air-
sensitive, toxic, and expensive starting materials such as TOP or
TBP, but also the use of aliphatic amines for activation such
as octadecylamine. Only low-cost, green, and environmentally
friendly reagents were used in the reaction. The as-prepared ZnSe,
ZnSe/ZnS core/shell nanocrystals, and Cu- or Mn-doped ZnSe
nanocrystals can reach the same high quality level compared with
the use of phosphine-selenium precursor as reported before.27
The parameter window for the growth of high quality ZnSe
nanocrystals was found to be much broader than that of the
corresponding ZnSe nanocrystals with the use of the phosphine-
selenium precursor. Highly monodisperse ZnSe nanocrystals were
synthesized successfully even when the reaction temperature was
2.2.2 Stock solution for Se precursor. It was made by de-
gassing Se (0.1578 g, 2 mmol), 20 mL of ODE in a 50 mL three-
neck flask, then it was heated to 220 ◦C for 180 min under nitrogen.
During this procedure, the color of the precursor changed from
transparent to orange, red, and finally turned into yellow. 0.2 M Se-
ODE solution was made for FT-IR and 1H NMR measurement,
The solution was conducted as follows: Se (2 mmol) and ODE
(10 mL) were mixed in a 25 mL flask. Under nitrogen flow and
stirring, the mixture was heated to 220 ◦C and maintained for
180 min, and then the reaction solution was cooled to room
temperature.
2.2.3 Stock solution for shell growth. The zinc precursor
solution (0.1 M) was prepared by dissolving ZnO (0.1628 g,
2 mmol) in oleic acid (4.51 g, 16 mmol) and paraffin oil (13.70 mL)
at 310 ◦C. The sulfur precursor solution (0.1 M) was prepared by
dissolving sulfur (0.064 g, 2 mmol) in ODE (20 mL) at 150 ◦C. All
precursor solutions were made under nitrogen flow.
2.2.4 Synthesis of copper stearate (CuSt2) and manganese
◦
set as low as 240 C. For ZnSe, the photoluminescence (PL) full
stearate (MnSt2). In
a typical synthesis, sodium stearate
width of half-maximum (FWHM) was well controlled between 14
and 17 nm. For Cu- or Mn-doped ZnSe nanocrytals, the FWHM
was around 70 and 50 nm, respectively. We believe this work could
provide the foundations for a simple, robust and high performance
version of the industrial scale synthesis of not only ZnSe core
and ZnSe/ZnS core-shell nanocrystals but also Cu- or Mn-doped
ZnSe based nanocrystals. The total cost saved could be as high
as 50% compared to nanocrystals synthesized with phosphine-
selenium precursor.
(20◦mmol) was dissolved in 80 g of methanol and heated to 50–
60 C until it became a clear solution, CuCl2 or MnCl2 solution
of 10 mmol in 10 g methanol was added dropwise with vigorous
stirring and precipitation of CuSt2 or MnSt2 slowly flocculated.
The precipitates were washed repeatedly with methanol three times
and then dried under vacuum.
2.3. Synthesis of ZnSe nanocrystals
Selenium precursor injection method - 1 mL Zn precursor I and
4 g paraffin oil in 25 mL flask was heated to 330 ◦C under nitrogen.
Next, 2mL Sestocksolution(containing0.2mmolSe)wasinjected
2. Experimental details
2.1. Chemicals
◦
into the above flask and temperature was lowered to 300 C for
nanocrystal growth. Aliquots were taken at different time intervals,
and UV-vis and photoluminescence (PL) spectra were recorded for
each aliquot. PL spectra covered 400–450 nm in the visible region.
Similar high quality ZnSe has been synthesized and previously
reported by Li et al.27 The difference in this synthesis was that
organic phosphines and aliphatic amines were avoided successfully
by the current method.
Zinc precursor injection method (Example 1) - 2 mL Se precursor
and 4 g paraffin oil was heated to 330 ◦C under nitrogen flow in a
25 mL flask. Next, 2 mL Zn precursor II solution was injected and
the temperature was lowered to 300 ◦C for nanocrystal growth.
Aliquots were taken at different time intervals, and UV-vis and
PL spectra were recorded for each aliquot. PL spectra covered the
region of 400–440 nm.
All reagents were used as received without further experimen-
tal purification. Zinc oxide (ZnO, 99.99% powder), sulfur (S,
99.98%, powder), 1-octadecene (ODE, 90%), oleic acid (OA, 90%),
decanoic acid (99%), and selenium (Se, 99.99%, powder) were
purchased from Aldrich. CuCl2, MnCl2, sodium stearate, hexanes
(analytical grade), paraffin oil (analytical grade), and methanol
(analytical grade) were obtained from Beijing Chemical Reagent
Ltd., China.
2.2. Preparation of precursors
2.2.1 Stock solution for Zn precursor. Solution for Zn precur-
sor I - A mixture (20 mL in total) of ZnO (0.16274 g, 2 mmol),
decanoic acid (8 mmol, 1.378 g), and 18.5 mL paraffin oil was
loaded in a 50 mL three-neck flask and heated to 300 C under
nitrogen to obtain a colorless clear solution.
Zinc precursor injection method (Example 2) - 2 mL Se precursor
and 4 g paraffin oil was heated to 310 ◦C under nitrogen gas flow
in a 25 mL flask. Next, 2 mL Zn precursor II solution was injected
◦
◦
and temperature was lowered to 300 C for nanocrystal growth.
Solution for Zn precursor II - A mixture (20 mL in total) of ZnO
(0.16274 g, 2 mmol), oleic acid (8 mmol, 2.256 g), and 17.5 mL
paraffin oil was loaded in a 50 mL three-neck flask and heated to
300 ◦C under nitrogen to obtain a colorless clear solution.
Solution for Zn precursor III - A mixture (100 mL in total) of
ZnO (1.6274 g, 20 mmol), oleic acid (80 mmol, 22.56 g), and 75 mL
paraffin oil was loaded in a 250 mL three-neck flask and heated to
300 ◦C under nitrogen to obtain a colorless clear solution.
Aliquots were taken at different time intervals, and UV-vis and
PL spectra were recorded for each aliquot. PL spectra covered the
region of 390–450 nm.
Zinc precursor injection method (Example 3) - 2 mL Se precursor
and 4 g paraffin oil was heated to 240 ◦C under nitrogen gas flow
in a 25 mL flask. Next, 2 mL Zn precursor I solution was injected
◦
and temperature was lowered to 220 C for nanocrystal growth.
Aliquots were taken at different time intervals, and UV-vis and PL
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spectra were recorded for each aliquot. PL spectra ranging from
400 nm to 435 nm were obtained.
solution turned slightly yellowish but it intensified as the reaction
progressed. After 30–120 min at 280 ◦C, a solution of 0.4 mL zinc-
decanoic acid was injected. The reaction mixture was cooled to
240 ◦C and another 4 mL zinc-decanoic acid was added dropwise.
2.4. Large scale synthesis of ZnSe nanocrystals using the zinc
precursor injection method
2.8. Characterization
100 mL Se precursor (contains 10 mmol, 0.79 g Se) and 100 mL
paraffin oil were mixed and heated to 310 ◦C under nitrogen gas
flow in a 500 mL flask. Next, 50 mL Zn precursor III solution was
injected and the temperature was lowered to 300 ◦C for nanocrystal
growth. The nanocrystals were precipitated twice through adding
the solution into acetone, separated through spinning at 6000 rpm
for 20 min, and then dried in vacuum overnight. ZnSe nanocrystals
(~ 1.3 g) were obtained finally.
UV-vis absorption and PL spectra were recorded using an Ocean
Optics spectrophotometer (mode PC2000-ISA). PL spectra were
recorded using an excitation wavelength of 365 nm. PL quantum
yields (QYs) of nanocrystals were measured relative to LD 423
(QY = 68% in ethanol), coumarin 540 (QY = 78% in ethanol),
and rhodamine 590 (QY = 95% in methanol), respectively. All
the QY data of nanocrystals and dyes were collected through
SPEX F212. The NC PL QYs were calculated using the formula
as QYNC = QYdye(INC/Idye)(nNC)2/(ndye)2(1-10-ODdye/1-10-ODNC),
where I is the spectrally integrated emission intensity, n is the
refractive index, and OD is the optical density of the NC or
the dye samples (indicated by a subscript). The optical density of
the NC samples at the excitation wavelength was within the 0.02–
0.05 range. X-ray diffraction (XRD) studies of NCs were carried
out with a Philips X¢ Pert Pro X-ray diffractometer using Cu-Ka
2.5. Synthesis of ZnSe/ZnS core/shell nanocrystals
The growth of ZnS shell was achieved by the “successive ion layer
adsorption and reaction (SILAR)” method reported before.14,36
A
typical SILAR synthesis was performed as follows: 8 mL of ODE
and 0.4 g of ODA were loaded into a 50 mL reaction vessel. The
ZnSe nanocrystals in hexanes (4.0 nm in diameter, 3 ¥ 10-7 mol)
were added, and the system was kept at 100 ◦C under N2-flow for
30 min to remove the hexanes and other undesired materials of
low vapor pressure. Subsequently, the solution was heated up to
240 ◦C under N2-flow where the shell growth was performed. The
sequence of the injection solution for each step was as follows:
S–Zn–S–Zn, i.e. ZnSe with 2-layer ZnS core-shell nanocrystals
were obtained. It was found that a period of 10 min between
each addition was sufficient for the reaction to be completed,
because the UV-vis and PL spectra showed no further changes
after this period of time. At last, the optical property could be
further improved after reflux at 200 ◦C for about one hour, i.e.
narrow PL FWHM at room temperature. The PL quantum yields
of nanocrystals exceeded 70% after refluxing. For purification,
10 mL of hexanes was added and the unreacted compounds
and by-products were removed by successive methanol extraction
(at least three times).
˚
radiation (l = 1.54 A). The specimens were prepared as follows:
about 20 mg of the purified nanocrystals were dissolved in about
0.5 mL of toluene and then dropped onto a low-scattering quartz
sample-holder and dried in air and kept overnight in a vacuum
desiccator. To get a better signal-to-noise ratio, the XRD data were
collected at a scan rate of 16 s with 0.2◦ per step. Transmission
electron microscopy (TEM) studies were performed using a JEOL-
100CXII electron microscope operating at 100 kV and a JEOL
JEM-2010 electron microscope operating at 200 kV. The specimens
were prepared as follows: about 10 mL of a particle solution was
dropped onto a 200-mesh copper grid, and was dried overnight
at ambient conditions. IR spectra were obtained using an FT-IR
instrument (Nicolet, AVATAR 360). The specimens were prepared
by directly loading about 10 mg of sample onto a NaCl window.
1H NMR analysis was used to identify the chemical compositions
of the Se precursors. 1H NMR spectra were recorded on a Bruker
AVANCE 400 MHz spectrometer. For standard samples, about
10 mg of standard sample (ODE or 0.2 M Se-ODE) was dissolved
in CDCl3 (~ 0.8 mL) for 1H NMR measurement. Mass spectrum
(MS) analysis was carried out at 70 eV on a Finnigan TraceDSQII
using a direct insertion technique or GC insertion.
2.6. Preparation of Cu doped ZnSe nanocrystals
Host ZnSe nanocrystals were synthesized according to the zinc
precursor injection method reported above. After a desired size of
ZnSe was reached (such as PL at 400 nm), the temperature of the
reaction mixture was cooled to 60 ◦C. Then 5 mL Se precursor
was added and the temperature was increased to 180 ◦C, then
0.5 mL of CuSt2 in paraffin oil (0.001 M) was added dropwise. As
the temperature increased to 240 ◦C, a blue-green emission was
observed. Once the desired emission was reached, an extra amount
of Zn precursor I was added and the temperature was increased
to 260 ◦C for annealing.
3. Results and discussion
For the synthesis of ZnSe nanocrystals, we used two approaches
to do the injection: selenium precursor injection or zinc precursor
injection. As reported before,27 only the selenium precursor
injection method could synthesize high quality ZnSe nanocrystals
with the use of phosphine-selenium precursor. Aliphatic amines
had to be chosen as the reagent to activate the zinc carboxylate
precursors too. But with the use of current phosphines-free
selenium precursor, both the selenium precursor injection and
the zinc precursor injection methods have been demonstrated to
successfully synthesize high quality monodisperse ZnSe nanocrys-
tals. Typically, the ZnSe nanocrystals grew very fast with the
selenium precursor injection method and the absorption peaks
could shift to 393 nm after a 10-second reaction while FWHM of
2.7. Preparation of Mn doped ZnSe nanocrystals
In a typical experiment, MnSt2 (0.02 g, 0.032 mmol), 0.02 g ODA,
and 12 g paraffin oil were loaded in a 50 mL three-neck flask,
◦
degassed by purging argon and heated to 300 C. MnSt2 started
◦
to dissolve at about 100 C and a clear solution was obtained at
250 ◦C. Then, 4 mL (0.1 M) Se sol◦ution was injected swiftly into
the above reaction mixture at 300 C. The color of the resulting
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PL was 15 nm. When the reaction lasted between 1 and 3 min,
the FWHM decreased to around 14 nm. As the reaction lasted
longer, the FWHM began to broaden and it reached around 20 nm
after 10 min. Its PL covers the range from 405 to 445 nm (See
ESI,† Fig. S1), and the QY could reach between 20 to 40% (See
ESI,† Fig. S2). For the zinc precursor injection method, the first
absorption peak was at 385 nm (Fig. 1) when the reaction lasted
10 s and the FWHM of PL was 17 nm. Absorption and PL were
red shifted as the reaction continued and the FWHM decreased
further and was between 14 and 17 nm for the whole reaction as
the PL covered the range between 390 and 450 nm. If the injection
temperature was increased to 330 ◦C, the reaction went even faster,
the absorption peak shifted to 392 nm after 10 s. Because more
precursors were used to form the nuclei in the early stages, its PL
only reached 440 nm compared to 450 nm for the injection at
310 ◦C (Fig. 1). That is to say, a larger number of nuclei in the
early stage resulted in a slower nanocrystal growth rate, and thus
the final nanocrystals were smaller, and vice versa.18 The FWHM
was broad, but it was still controlled between 14 to 20 nm. As is
well known, saturated acids with short chains generally have more
activity than those with longer chains,27 such as decanoic acid is
more active than oleic acid. Therefore, for further testing whether
high quality ZnSe could be synthesized at a lower temperature,
oleic acid was replaced by decanoic acid to make the Zn precursor.
The results indicated that highly monodisperse ZnSe could still be
synthesized when the temperature was as low as 240 ◦C. It was
noticed that the reaction went slowly in the early stages of the
reaction; absorption and PL were very weak when the reaction
only lasted 10 s. Both absorption and PL turned strong after 1 min
from the initial injection. The absorption peak turned sharper as
the reaction lasted longer and the PL covered 400 to 435 nm
(Fig. 1).
For a better comparison of the selenium precursor and the zinc
precursor injection methods, the reaction speed vs. the PL peak
positions and the PL peak positions vs. the FWHM have been
collected and are shown in Fig. 2. It is clearly shown in Fig. 2a
that the fastest reaction speed was obtained with the selenium
precursor injection. The slowest reaction speed was obtained
with the zinc precursor injection method at a 240 ◦C injection
temperature. When the injection temperature for the zinc precursor
injection method was increased to 310 ◦C, the best conditions
were obtained with the broadest coverage of PL. From Fig. 2b,
PL’s FWHM turned broader when the reaction lasted longer by
the selenium precursor injection method; on the contrary, PL’s
FWHM was narrower with the zinc precursor injection method
when the reaction length increased. It showed especially good
◦
results with FWHM between 14 to 17 nm at 310 C and its QY
could reach 40 to 60% (See ESI,† Fig. S2).
In this new synthesis method, selenium needs to be dissolved
in ODE and forms an active complex in its precursor (such as
=
the Se O complex) before the synthesis of ZnSe nanocrystals.
In Zou’s previous report, a mechanism was described as: first
Se was reduced to H2Se gas and the long alkane chains were
oxidized to alkene chains, then the Cd complex reacted with
H2Se to form CdSe nanocrystals.21 It seems a new mechanism
has been found in this study and ODE may act as a reducing
agent. To test this hypothesis, FTIR spectra of ODE, 0.1 M Se-
ODE, and 0.2 M Se-ODE have been collected and shown in
Fig. 3a. The FTIR spectrum shows the characteristic peaks of
the ODE vinyl group out-of-plane C–H bend located at 909.7
-1
-1
=
and 991.2 cm , C C stretch located at 1641.4 cm , and C–H
stretch located at 3077.6 cm-1. For 0.1 M Se-ODE’s IR spectra, the
absorption bands related to ODE have decreased significantly. All
the characteristic bands for ODE disappeared in 0.2 M Se-ODE’s
IR spectra. Moreover, 1H NMR and MS have been collected and
are shown in Fig. S3 and S4.† A new kind of shift related to
1
=
C C’s H has been observed from the H NMR spectrum, located
at 5.4 ppm. On comparison of the MS of pure ODE and 0.2 M
Se-ODE, the peak related to ODE (m/z 252.2) was weaker in
Se-ODE and showed a new peak at m/z 331.2 corresponding to
the combination between Se and ODE. Through IR, NMR, and
MS results, it confirmed an ODE molecule must form some kind
of complex with a Se molecule. A proposed structure is shown in
Fig. 3b, and the formation of this complex led Se to dissolve totally
in ODE. This may be the reason why it showed higher reactivity
than the use of the Se-TBP precursor for the synthesis of high
quality ZnSe nanocrystals.27 Therefore, the reaction temperature
could be decreased further to as low as 240 ◦C compared to 300 ◦C
with the use of Se-TBP.27,34
Fig. 1 UV-vis absorption and photoluminescence spectra collected
during the growth of ZnSe nanocrystals with different conditions. Top:
330 ◦C, the growth time of ZnSe QDs were 10 s (3.3 nm), 1 min (3.5 nm),
3 min (5.2 nm), 5 min (5.4 nm), 10 min (6.0 nm), 30 min (7.9 nm) and
60 min (8.6 nm); Middle: 310 ◦C, the growth time of ZnSe QDs were 10 s
(3.1 nm), 1 min (3.5 nm), 3 min (4.2 nm), 5 min (5.3 nm), 10 min (5.9 nm),
30 min (7.5 nm) and 60 min (9.6 nm) and Bottom: 240 ◦C, the growth
time of ZnSe QDs were 1 min (3.2 nm), 3 min (3.8 nm), 5 min (4.5 nm),
10 min (5.2 nm), 30 min (7.2 nm) and 60 min (7.5 nm). The fractions of
ZnSe nanocrystals were diluted to a similar absorption intensity for the
measurement. Note that the spectra are normalized and shifted for clarity.
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Fig. 2 Growth kinetics of different conditions during the synthesis of ZnSe nanocrystals: (a) the reaction speed vs. the PL peak positions and (b) the
PL peak positions vs. the FWHM. Including the selenium precursor injection at 330 ◦C (squares); the zinc precursor injection at 330 ◦C (stars), 310 ◦
C
(triangles), and 240 ◦C (dots).
Fig. 4 Photoluminescence spectra of ZnSe/ZnS core-shell nanocrystals.
Inset is the absorption and photoluminescence spectra of the ZnSe core
nanocrystal for comparison.
Fig. 3 (a) FTIR spectra of ODE, 0.1 M Se-ODE, 0.2 M Se-ODE.
(b) A possible mechanism to show the possible reaction between Se and
ODE. Note: The dashed red lines are only used as a guide.
To increase PL stability and QY of ZnSe nanocrystals, ZnSe
nanocrystals synthesized at 310 ◦C with the zinc precursor
injection method have been used as cores to synthesize ZnSe/ZnS
core/shell nanocrystals. As shown in Fig. 4, its PL is centered
at 414 nm. The highest PL intensity could be reached after two
layers of ZnS overcoating and the QY of as-prepared core/shell
nanocrystals could reach as high as 70%. QY dropped with further
coatings of ZnS layers and only 38% QY was obtained with
three ZnS layers. To further characterize the structures of ZnSe
and ZnSe/ZnS core-shell nanocrystals, their crystallographic
properties were determined using powder X-ray diffraction (XRD)
(Fig. 5). According to the X-ray powder diffraction pattern, the
peak positions of the ZnSe nanocrystals are in agreement with bulk
ZnSe in zinc-blende structure and ZnSe nanocrystals synthesized
by the phosphine-selenium precursor method.27 Three obvious
Fig. 5 X-ray diffraction patterns and related TEM images of (a) 9.2 nm,
(b) 7.5 nm, (c) 5.0 nm ZnSe, and (d) 6.5 nm ZnSe/ZnS core-shell with
2 layers ZnS overcoating on 5.0 nm ZnSe.
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Fig. 6 TEM images and photoluminescence spectra related to (left) Cu2+ doped ZnSe and (right) Mn2+ doped ZnSe nanocrystals. Inset: photos
demonstrating the color-tunable photoluminescence of ZnSe with Cu2+ or Mn2+ dopant, all samples were excited with a 365 nm UV lamp.
diffraction peaks located at 27.4, 45.6, and 54.0◦ corresponded
to the (111), (220), and (311) planes of zinc blende phase ZnSe.
Also it is clearly shown that XRD peaks became narrower as the
size of the ZnSe nanocrystals increased. An obvious peak shift
to the standard ZnS position has been observed when ZnS shells
were grown onto the ZnSe core and the zinc blende structure
was kept after the coating process. As described above, typical
ZnSe nanocrystals synthesized at 310 ◦C with the zinc precursor
injection method have a standard deviation of about 5% obtained
by TEM image analysis (Fig. 5). The size of the ZnSe nanocrystals
with PL at 414 nm averaged 5 nm. Obvious size-dependent PL
emission was observed. When PL reached 445 nm, an average
size was counted around 9.6 nm. Fig. S5† shows the HRTEM
image of ZnSe nanocrystals, the (111) planes are separated by
d = 0.32 nm, which is consistent with that of zinc blende ZnSe.
For ZnSe/ZnS core/shell nanocrystals, 5 nm ZnSe nanocrystals
as core was increased to 6.5 nm after overcoating two ZnS layers.
The HRTEM image of ZnSe/ZnS nanocrystals is shown in Fig.
S6† and the (111) planes are separated by d = 0.31 nm, which
is consistent with that of zinc blende ZnS. All the ZnSe and
ZnSe/ZnS nanocrystals in HRTEM images have well-resolved
lattice images, which imply good crystallinity.
Furthermore, we tested doping some ions into ZnSe nanocrys-
tals for the visible range emission based on this new established
method. Cu2+ and Mn2+ have been selected here to generate
emission at the green and orange ranges, respectively. For the
synthesis of Cu2+ doped ZnSe, it was found that the reactivity
of ZnSe decided the difficulty of the doping process. If the size
of ZnSe nanocrystals was very small (such as PL at 398 nm
with an average size of 2.9 nm), it had a high reactivity and it
was very easy to adsorb Cu2+ onto its surface. Then, it would be
easy to overcoat ZnSe on the outermost and form Cu:ZnSe/ZnSe
core/shell structures. Once the size of ZnSe increased to 3.6 nm
(PL at 410 nm) and above, it was difficult to adsorb Cu2+ ions on the
surface of ZnSe. The experimental results indicated a promising
result with Cu2+ dope◦d into ZnSe when the temperature was set
between 200 and 240 C. The band gap emission of ZnSe would
be replaced by the Cu dopant emission, which covered the range
between 480 nm and 520 nm with an average PL FWHM around
70 nm. When Cu:ZnSe was overcoated with a certain thickness of
ZnSe and ZnS, the QY could be increased to as high as 20%. A
photo of Cu2+ doped ZnSe nanocrystals with emission at 515 nm
and its corresponding TEM image have been shown in Fig. 6. The
inset shows the model of Cu2+ doped ZnSe nanocrystals, the PL of
Cu:ZnSe nanocrystals is the recombination of an excited electron
in the conduction band of a ZnSe host nanocrystal and the hole
from the d-orbit of a copper ion in the same nanocrystal.30,38,39
Thus, the PL of Cu:ZnSe nanocrystals shifts to red as the size
of the host increases. For Mn2+ doped ZnSe nanocrystals, MnSe
was first synthesized at high temperature (such as 300 ◦C). Then,
Zn precursor was added slowly after a certain time. The PL was
controlled at around 570 nm or 600 nm when the addition of Zn
precursor occurred after 30 min or 60 min from the beginning
of the reaction, respectively. The surface layer of MnSe formed
between MnSe and ZnSe may contribute to the emission.31 Its
emission is covered from 570 nm to 610 nm with QY as high
as 40%. The TEM image and real emitting photo (PL at 578
nm) of as-prepared Mn2+ doped ZnSe nanocrystals under UV
lamp irradiation are shown in Fig. 6. The inset is the model
of Mn2+ doped ZnSe nanocrystals, MnSe core was synthesized
first followed by the growth of the ZnSe shell. For simplicity,
these nanocrystals can also be called doped nanocrystals because
of their similar emission properties.30 The diameters of the as-
synthesized Cu2+ and Mn2+ doped ZnSe nanocrystals were 9.2 nm
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The Royal Society of Chemistry 2009
Dalton Trans., 2009, 10534–10540 | 10539
©

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and 7.5 nm, respectively. Their high resolution TEM images
are shown in Fig. S7 and S8,† the existence of lattice planes
confirmed the good crystallinity of the Cu2+ and Mn2+ doped
ZnSe nanocrystals.
4 P. Peng, D. J. Milliron, S. M. Hughes, J. C. Johnson, A. P. Alivisatos
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4. Conclusion
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In summary, we have used two injection approaches (i.e. the
zinc precursor injection and the selenium precursor injection) to
synthesize high-quality zinc blende ZnSe nanocrystals by using
Se-ODE complex as the phosphine-free selenium precursor. It
indicated that the zinc precursor injection approach had a better
quality control than the selenium precursor injection approach.
This Se-ODE complex showed very high reactivity for the
synthesis of ZnSe nanocrystals; thereafter, it totally eliminated the
use of aliphatic amines to activate the zinc carboxylate precursors
compared to a “must-have” agent by the ZnSe syntheses with
phosphine-Se precursor. And it also demonstrated that the zinc
precursor injection could lower the required temperature for the
reaction, even as low as 240 ◦C. Moreover, high quality ZnSe/ZnS
core/shell nanocrystals have been successfully synthesized. We
also extended this method further to synthesize Cu2+ or Mn2+
doped ZnSe. The emission range has been extended to 500 and
600 nm with the use of Cu2+ and Mn2+ dopants compared to
the emission coverage of pure ZnSe at around 400 nm. With such
good coverage and emission properties, it has the potential to fully
replace cadmium based emission in the visible range. This is a total
“green approach” (i.e. phosphine-free synthesis) for the synthesis
of high quality ZnSe, ZnSe/ZnS, and Cu2+ or Mn2+ doped ZnSe
nanocrystals. A large scale synthesis has also been successfully
carried out and a single batch could make as high as 1.3 g of
high quality ZnSe nanocrystals. It provides a bright future for the
actual use of nanocrystals in the industrial fields of biolabeling,
solar cells, light emitting devices, etc.
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Acknowledgements
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This work was supported by the research project of the Na-
tional Natural Science Foundation of China (20771035), Hi-
Tech Research and Development Program of China (863 plan,
2006AA03Z3592), Innovation Scientists and Technicians Troop
Construction Projects of Henan Province, and the Scientific
Research Foundation for the Returned Overseas Chinese Scholars,
State Education Ministry.
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©