J. Phys. Chem. C 2007, 111, 5661-5666
5661
Low-Temperature Synthesis of Oil-Soluble CdSe, CdS, and CdSe/CdS Core-Shell
Nanocrystals by Using Various Water-Soluble Anion Precursors
Daocheng Pan, Qiang Wang, Shichun Jiang, Xiangling Ji,* and Lijia An*
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Graduate School of the Chinese Academy of Sciences, 5625 Renmin Street,
Changchun 130022, People’s Republic of China
ReceiVed: NoVember 23, 2006; In Final Form: February 2, 2007
Colloidal CdSe and CdS quantum dots were synthesized at low temperatures (60-90 °C) by a two-phase
approach at a toluene-water interface. Oil-soluble cadmium myristate (Cd-MA) was used as cadmium source,
and water-soluble Na2S, thiourea, NaHSe, Na2SeSO3, and selenourea were used as sulfur and selenium sources,
respectively. When a cadmium precursor in toluene and a selenium precursor in water were mixed, CdSe
nanocrystals were achieved at a toluene-water interface in the range of 1.2-3.2 nm in diameter. Moreover,
we also synthesized highly luminescent CdSe/CdS core-shell quantum dots by a two-phase approach using
poorly reactive thiourea as sulfur source in an autoclave at 140 °C or under normal pressure at 90 °C. Colloidal
solutions of CdSe/CdS core-shell nanocrystals exhibit a photoluminescence quantum yield (PL QY) up to
42% relative to coumarin 6 at room temperature.
Introduction
Metal and semiconductor nanocrystals play an important role
in the development of functional nanoscale materials and
devices.1 Synthesis of such nanocrystals is an important topic
in the field of material science. Through a two-phase approach,
some noble metal nanocrystals such as Au,2 Ag,3 Pt,4 and Pd5
nanocrystals have been synthesized at low temperatures. As two
important group II-VI semiconductor materials, CdSe and CdS
nanocrystals have received considerable interest of researchers
because of potential applications in light-emitting diode (LED),1c,d
solar cells,1f and biological labeling.1g Numerous approaches
such as homogeneous phase precipitation,6 reverse micelle,7 and
organometallic approach8 and its variants9 have been applied
to prepare CdSe and CdS nanocrystals. Moreover, highly
luminescent CdSe/CdS and CdSe/ZnS core-shell nanocrystals
have also been prepared through organometallic approach10 and
its variants.11 However, these reactions mentioned above were
all carried out in organic phase or aqueous phase, and both
nucleation and growth of the nanocrystals only happened in a
homogeneous system. It is very difficult for organic-phase
approaches to synthesize oil-soluble nanocrystals by using
various water-soluble precursors.
Recently, we developed a versatile two-phase approach to
synthesize highly luminescent CdS,12 extremely small CdSe,13
TiO2,14 ZrO2,15 and so forth nanocrystals. The oil-soluble CdSe
and CdS nanocrystals can be synthesized by using water-soluble
thiourea and selenourea as sulfur source and selenium source,
respectively. The nanocrystals have a very long nucleation and
growth stage because of a slow decomposition of thiourea and
selenourea. It was found that a slow nucleation does not always
lead to polydisperse nanocrystals if the growth time is long
enough.
In this paper, we prepared CdSe nanocrystals with a relatively
narrow size distribution through a rapid nucleation and a rapid
* To whom correspondence should be addressed. Phone: +86-43185262876 and +86-431-85262206. E-Mail: ljan@ciac.jl.cn (L.A.) and
xlji@ciac.jl.cn (X.J.).
growth using highly reactive NaHSe as selenium source or a
slow nucleation and a slow growth using poorly reactive Na2SeSO3 or selenourea as selenium source via a two-phase
approach at a toluene-water interface. Oil-soluble cadmium
myristate was used as cadmium source, and n-trioctylphosphine
oxide (TOPO) and n-trioctylphosphine (TOP) were used as
capping agents. Cadmium myristate, TOPO, and TOP can be
readily dissolved in toluene above 60 °C. The CdSe nanocrystals
were achieved at a liquid-liquid interface when a toluene
solution of cadmium myristate and an aqueous solution of
NaHSe were mixed under stirring. The surface of nanocrystals
was capped by a monolayer of n-trioctylphosphine oxide
(TOPO) and n-trioctylphosphine (TOP) as the nanocrystals start
to grow at a liquid-liquid interface. The size of resulting
nanocrystals is very small in the range of 1.2-3.2 nm in
diameter, because the duration of nucleation and growth for the
nanocrystals is very short.
Scheme 1 illustrates a proposed mechanism for formation of
CdSe nanocrystals at a toluene-water interface by using watersoluble NaHSe, Na2SeSO3, and selenourea as selenium precursors, respectively. Moreover, we also synthesized highly
luminescent CdSe/CdS core-shell nanocrystals using thiourea
as sulfur source through a two-phase approach in an autoclave
at 140 °C or under normal pressure at 90 °C.
Experimental Section
I. Chemicals. Cadmium oxide (99.5%), sodium borohydride
(99%), myristic acid (MA, 99.5%), oleic acid (OA, 90%), TOP,
and TOPO (tech, 90%) were purchased from Aldrich. Thiourea
(99%) and selenourea (98%) were obtained from Alfa, and
1-dodecanethiol (98%) and coumarin 6 (98%) were purchased
from Acrös.
II. Synthesis of Cd-MA. CdO (1.926 g, 15 mmol) and
myristic acid (7.5 g, 33 mmol) were loaded into a flask and
were heated to 210 °C for 10 min. An optically clear solution
was obtained. The crude product was recrystallized twice from
10.1021/jp0678047 CCC: $37.00 © 2007 American Chemical Society
Published on Web 03/24/2007
5662 J. Phys. Chem. C, Vol. 111, No. 15, 2007
SCHEME 1: A Proposed Mechanism for a Formation of
Oil-Soluble CdSe Nanocrystals at a Toluene-Water
Interface by Using Water-Soluble NaHSe, Na2SeSO3, and
Selenourea as Selenium Precursors, Respectively
toluene. The Cd-MA was dried in an oven and was used for
further reaction.
III. Synthesis of CdSe and CdS Nanocrystals using NaHSe
and Na2S as Selenium and Sulfur Precursors. Some reactions
were run under a nitrogen atmosphere to prevent oxidation of
the oxygen-sensitive selenide ions. As a typical example,
Cd-MA (0.2268 g, 0.4 mmol), TOPO (1 g), TOP (1 mL), and
toluene (10 mL) were placed in a flask and the mixture was
heated to 100 °C for 10 min until an optically clear solution
was obtained. A freshly prepared aqueous solution (10 mL,
0.02 M) of NaHSe16 or Na2S was swiftly injected into the above
flask under stirring. Immediately, crimson and green colloids
were produced for CdSe and CdS nanocrystals. The system was
kept at 80 °C for 20 min for the growth of CdSe or CdS
nanoparticles. At 20 min of reaction time, the second injection
of aqueous solution (10 mL, 0.02 M) of NaHSe or Na2S was
carried out dropwise (approximately one drop per second) to
the flask. When the reaction lasted for 60 min, organic phase
was separated from the crude solution. Insoluble solid was
separated by centrifugation and decantation prior to further
purification. The resulting nanocrystals in toluene solution were
precipitated with methanol and were further isolated by centrifugation and decantation. The purified nanocrystals were
redispersed in toluene for UV-vis, photoluminescence (PL),
transmission electron microscopy (TEM), and X-ray diffraction
(XRD) measurements without any size sorting.
IV. Synthesis of CdSe/CdS Core-Shell Nanocrystals in
an Autoclave. A two-step method was exploited to synthesize
CdSe/CdS core-shell nanocrystals. At first, the CdSe core
nanocrystals were prepared by a rapid nucleation and growth
using NaHSe as selenium source. Cd-MA (0.2 mmol), TOPO
(0.5 g), and toluene (10 mL) were added to a flask, and the
reaction mixture was heated to 60 °C for 10 min. An optically
clear solution was obtained. A freshly prepared aqueous solution
(10 mL, 0.02 M) of NaHSe was swiftly injected into the flask
under stirring. The reaction lasted for 30 min for the growth of
CdSe cores. In the second step, about 8 mL of the crude solution
of CdSe nanocrystals, Cd-MA (0.2 mmol), and TOPO (0.5 g)
were put into a 30 mL Teflon-lined stainless steel autoclave,
and it was heated to 80 °C for 5 min and then was cooled down
to room temperature. Thiourea (0.5 mmol) was dissolved in
10 mL of water and the solution was transferred into the
autoclave without stirring. The autoclave was sealed and
Pan et al.
maintained at 140 °C for 4 h. Finally, the autoclave was cooled
to the room temperature with tap water.
V. Synthesis of CdSe Nanocrystals Using Selenourea as
Selenium Precursor. Loaded into a Teflon-lined stainless steel
autoclave, 0.1134 g of Cd-MA, 1.0 mL of oleic acid, and
10 mL of toluene were heated until Cd-MA was dissolved.
Then, the solution was cooled to room temperature. After the
0.013 g of selenourea was dissolved in 10 mL of N2-saturated
water, the solution was transferred to the autoclave. The
autoclave was sealed and maintained at 180 °C for 15-90 min.
Finally, the autoclave was cooled to room temperature with tap
water. The CdSe crude solution was precipitated with methanol
and was further isolated by centrifugation and decantation.
VI. Synthesis of CdSe/CdS Core-Shell Nanocrystals
under Normal Pressure. First, CdSe cores were obtained by a
two-phase approach in the autoclave using selenourea as
selenium precursor. The autoclave was maintained at 180 °C
for 18 min. Then, the CdSe crude solution was precipitated with
methanol and was further isolated by centrifugation and
decantation. The purified nanocrystals were redispersed in
10 mL of toluene. Finally, 10 mL of purified CdSe solution,
1.0 mL of oleic acid, and 0.1134 g of Cd-MA were added to
a flask, and the mixture was heated to 100 °C for 10 min. Forty
milligrams of thiourea was dissolved in 10 mL of water, and
this solution was injected into the flask under magnetic stirring
condition. The reaction mixture was kept at 90 °C for 150 min.
Aliquot solutions (0.20 mL) were taken from organic phase at
different reaction times for UV-vis and PL measurements. The
crude solution was purified with methanol prior to measurements.
VII. Synthesis of CdSe Nanocrystals Using Na2SeSO3 as
Selenium Precursor. Added to a flask were 0.1134 g of
Cd-MA, 0.5 g of TOPO, and 10 mL of toluene, and the mixture
was heated to 90 °C for 10 min. A 0.02 M aqueous solution
(10 mL) of Na2SeSO3 was injected into the flask for the
nucleation and growth of CdSe nanocrystals. The system is kept
at 90 °C for 180 min. UV-vis absorbance and PL spectra were
recorded at different reaction times. The Na2SeSO3 solution was
prepared according to the following reaction: Na2SO3 + Se f
Na2SeSO3. Placed into a flask were 0.39 g of selenium, 1.58 g
of Na2SO3, and 20 mL of water, and the mixture was heated to
90 °C for 10 h. After cooling to room temperature, the solution
was filtered off and diluted to 250 mL in a volumetric flask
with deionized water. The Na2SeSO3 solution was stored in dark
and was used within 3 days.
VIII. Synthesis of CdS Nanocrystals Using Thiourea as
Sulfur Precursor. Added to a flask were 0.2268 g of
Cd-MA, 1.0 mL of OA, and 10 mL of chlorobenzene, and the
mixture was heated to 90 °C for 10 min. A 0.1 M aqueous
solution (10 mL) of thiourea was injected into the flask for the
nucleation and growth of CdS nanocrystals. The system was
kept at 90 °C for 150 min. UV-vis absorbance and PL spectra
were recorded at different reaction times.
IX. Synthesis of DDT-Capped CdS Nanocrystals. Added
to a flask were 0.2268 g of Cd-MA, 1.0 mL of 1-dodecanethiol,
and 10 mL of toluene, and the mixture was heated to 100 °C
for 10 min. A 0.02 M aqueous solution (10 mL) of Na2S was
injected into the flask. The system was kept at 90 °C for 24 h.
The reaction did not happen. Then, a 0.1 M toluene solution of
phase-transfer reagent tetra-n-octylammonium bromide was
injected into the flask.
X. Characterization of Samples. UV-vis absorption and
PL spectra were recorded on a Shimadzu UV-2450 PC
spectrometer and a Shimadzu RF-5301 PC fluorometer with a
Synthesis of Oil-Soluble Core-Shell Nanocrystals
Figure 1. UV-vis absorption and PL spectra of TOPO-TOP capped
CdSe nanocrystals using NaHSe as selenium precursor.
resolution of 1.0 nm, respectively. Room-temperature PL
quantum yields (PL QYs) were calculated against coumarin 6
in ethanol as a standard sample (QY ) 0.78).17 The absorbances
of nanocrystal samples and standard sample at the excitation
wavelength (400 nm) are similar and small (about 0.05) to avoid
a self-absorbance.
The XRD patterns were obtained using a Rigaku D/MAX2500 using Cu KR1 radiation and employing a scanning speed
of 0.02°/s in the range of 10-60°. XRD samples were prepared
by evaporating a drop of concentrated nanocrystal solution on
a glass plate.
J. Phys. Chem. C, Vol. 111, No. 15, 2007 5663
Figure 2. UV-vis absorption and PL spectra of the CdSe cores
synthesized under normal pressure at 60 °C using NaHSe as selenium
precursor and the corresponding CdSe/CdS core-shell nanocrystals
prepared in an autoclave at 140 °C using thiourea as sulfur precursor.
Results and Discussion
I. Synthesis of CdSe Nanocrystals Using Differently
Reactive NaHSe, Selenourea, and Na2SeSO3 as Selenium
Precursors. Figure 1 shows typical UV-vis absorption and PL
spectra of CdSe nanocrystals obtained through a two-phase
approach by using NaHSe as selenium precursor. The luminescence of TOPO-TOP capped CdSe nanocrystals is dominated
by near-band-edge luminescence. The Stokes shift is very small
(15 nm) and the fwhm (full width at half-maximum) is so narrow
(30 nm) which suggested a regular surface of particles and a
narrow size distribution. The CdSe nanocrystals is about
3.2 nm in diameter calculated from the first excitonic absorption
peak of UV-vis absorption spectrum.18 Definitely, the size of
CdSe nanoparticles is smaller than those obtained by organometallic route or its variants resulting from the more rapid
nucleation and growth, that is, NaHSe has a faster reaction rate
than selenium powder. To prepare CdSe nanocrystals with sizes
more than 2.0 nm, additional injections of precursor were
required. For comparison, the preparation of CdSe nanocrystals
was carried out at a lower temperature while keeping the other
conditions the same. The size of CdSe nanocrystals was smaller
(∼1.5 nm) when the reaction temperature was at 60 °C
(Figure 2). In general, the CdSe nanocrystals synthesized by
using NaHSe as precursor exhibit quantum yields of 8-10%
relative to coumarin 6 at room temperature.
Figure 2 shows UV-vis absorption and PL spectra of the
CdSe cores and corresponding CdSe/CdS core-shell nanocrystals. At room temperature, PL spectrum of the CdSe cores
shows a shallow trap emission because of the incomplete surface
passivation. The PL for CdSe/CdS core-shell nanocrystals is
dominated by near-band-edge emission, and quantum yield can
reach as high as 42% relative to coumarin 6, which indicates
the surface traps are markedly removed. The growth of the CdS
shell causes about 50 nm of red-shift in both UV-vis absorption
and PL spectra. Similar shifts due to the leakage of the exciton
into the shell have previously been reported for CdSe/ZnS10a,b
Figure 3. Temporal evolution of UV-vis absorption and PL spectra
of OA-capped CdSe nanocrystals using selenourea as selenium source.
or CdSe/CdS11c systems. It was found that TOP is not necessary
to prepare high-quality CdSe and CdSe/CdS core-shell nanocrystals. So, TOPO is used as sole capping agent to synthesize
CdSe and CdSe/CdS core-shell nanocrystals. Herein, we took
advantage of an autoclave to obtain a high temperature and
pressure for the growth of CdSe/CdS core-shell nanocrystals.
Meanwhile, CdSe/CdS core-shell nanocrystals under normal
pressure at 90 °C were also prepared. Measurements proved
that the two-phase synthesis in the autoclave is more reproducible than in a normal pressure because the stirring is not
necessary in the autoclave.
Figure 3 shows temporal evolution of UV-vis absorption
and PL spectra of CdSe nanocrystals in the presence of OA in
the size range of 1.2-3.0 nm using selenourea as selenium
source in the autoclave. CdSe nanocrystals cannot be obtained
for 15 min. During the first 20 min, the width of PL spectrum
decreases, and the width of PL spectrum markedly increases
with broadening of the size distribution during the reaction time
from 25 min to 90 min. The room-temperature PL QY of asprepared CdSe nanocrystals is in the range of 7-35% and has
a tendency to increase with increasing the nanocrystal size and
reaction time.
Figure 4 shows UV-vis absorption and PL spectra of OAcapped CdSe cores synthesized at 180 °C under high pressure
and corresponding CdSe/CdS core/shell nanocrystals synthesized
under normal pressure at 90 °C and temporal evolution of PL
spectra of CdSe/CdS core-shell nanocrystals during growth of
CdS shell. The CdSe cores are about 1.6 nm in diameter18 and
show a band-edge emission as well as a shallow trap emission
because of the incomplete surface passivation. It was found that
5664 J. Phys. Chem. C, Vol. 111, No. 15, 2007
Pan et al.
Figure 6. UV-vis absorption and PL spectra of TOPO-TOP capped
CdS nanocrystals using Na2S as sulfur source.
Figure 4. UV-vis absorption and PL spectra of OA-capped CdSe
cores synthesized under high pressure at 180 °C and corresponding
CdSe/CdS core/shell nanocrystals synthesized under normal pressure
at 90 °C and temporal evolution of PL spectra of CdSe/CdS coreshell nanocrystals during growth of CdS shell.
Figure 5. Temporal evolution of UV-vis absorption and PL spectra
of TOPO-capped CdSe nanocrystals using Na2SeSO3 as selenium
source.
TABLE 1: Sizes, FWHMs, and QYs for CdSe Nanocrystals
Synthesized Using Different Sources
precursors
reactivity
sizes (nm)
fwhm (nm)
QYs (%)
NaHSe
selenourea
Na2SeSO3
high
medium
low
1.2-2.0
1.2-3.0
2.0-3.2
30
28-45
30
8-10
7-35
1-3
the trap emission could be removed completely if the reaction
time is more than 30 min when CdS shell is formed on the
CdSe cores, and the continuous red-shift was observed in the
UV-vis absorption and PL spectra of CdSe/CdS core-shell
nanocrystals with the growth of CdS shell. The quantum yields
of CdSe/CdS core-shell nanocrystals can be as high as 3040%. Although these quantum yields of CdSe/CdS core-shell
nanocrystals synthesized under normal pressure are markedly
lower than those synthesized under high pressure in the
autoclave (60-80%),13 the growth CdS shell under normal
pressure is more convenient to observe temporal evolution of
absorption, PL spectra, and quantum yields of nanocrystals,
because it is difficult to take aliquot solution from the autoclave
at high temperature. Moreover, the growth CdS shell under
normal pressure has a lower reaction temperature and can
markedly eliminate the isolated nucleation of CdS, that is,
forming CdS nanocrystals instead of CdSe/CdS core-shell
nanocrystals.
Figure 5 shows temporal evolution of UV-vis absorption
and PL spectra of TOPO-capped CdSe nanocrystals using poorly
reactive Na2SeSO3 as selenium source. The nanocrystals have
a very long nucleation and growth stage because the Na2SeSO3
was slowly decomposed. The size of nanocrystals was easily
controlled by varying reaction time. The reaction is more
reproducible and controllable than that using highly reactive
NaHSe as selenium source. The fwhm in the PL spectra of CdSe
nanocrystals is at about 30 nm during the reaction time from
20 min to 180 min. This suggests that no evident defocusing
and focusing of the size distribution is happening. In general, a
fast nucleation and a slow growth are favorable for preparation
of monodisperse nanocrystals under diffusion control because
the small nanocrystals have a faster rate than the big nanocrystals.19 When highly reactive NaHSe was used as selenium
source, the nanocrystals with a narrow size distribution could
be obtained because of a very fast nucleation rate. When poorly
reactive Na2SeSO3 was used as selenium source, the nanocrystals with a narrow size distribution could also be obtained
because of a very slow growth rate. In case of medium reactive
selenourea as selenium source, the CdSe nanocrystals do not
also show a broad size distribution. So, the narrow size
distributions that are independent of precursors always can be
obtained by a two-phase approach. However, the Ostwald
ripening is more evident for those reactions using highly reactive
precursors, such as NaHSe and selenourea.
For CdSe nanocrystals using differently reactive NaHSe, Na2SeSO3, and selenourea as selenium sources, we compared their
sizes, fwhm, and quantum yields in Table 1.
II. Synthesis of CdS Nanocrystals Using Differently
Reactive Na2S and Thiourea as Sulfur Precursors. In this
part, we made two CdS nanocrystal samples by using highly
reactive Na2S and poorly reactive thiourea as sulfur source. In
Figure 6, PL spectrum of TOPO-TOP capped CdS nanocrystals
using Na2S as sulfur source shows a shallow trap emission
resulting from lower capping densities, and similar trap emission
has been observed for TOPO-capped CdS using thiourea as
sulfur precursor.12a A sharp first excitonic absorption peak at
400 nm and a narrow PL band of CdS nanocrystals indicate
that nanocrystals have a narrow size distribution. This reaction
has a poor reproducibility when Na2S is used as sulfur source
resulting from a too fast reaction rate. For each time, the size
of CdS nanocrystals might be different at the same temperature
and reaction time. It is difficult to control the nanocrystal sizes
by reaction time.
To control the nanocrystal sizes by reaction time, the reaction
rate must be slowed down. Herein, the poorly reactive thiourea
was used as sulfur source to synthesize CdS nanocrystals in a
water/chlorobenzene two-phase system. Figure 7 shows the
Synthesis of Oil-Soluble Core-Shell Nanocrystals
J. Phys. Chem. C, Vol. 111, No. 15, 2007 5665
Figure 7. Temporal evolution of UV-vis absorption and PL spectra
of OA-capped CdS nanocrystals using thiourea as sulfur source. The
reaction time is 30, 35, 40, 45, 60, 75, 90, 110, 130, and 150 min from
bottom to top, respectively.
Figure 9. UV-vis absorption spectrum of DDT-capped CdS nanocrystals.
Figure 8. XRD patterns of TOPO-TOP capped CdSe and CdS
nanocrystals synthesized using NaHSe and Na2S as selenium and sulfur
precursors. Vertical lines indicate pure CdSe and CdS reflections (top:
zinc blende, CdSe; bottom: zinc blende, CdS). The crystallite sizes
were calculated from the (111) peak.
The CdSe and CdS nanocrystals could also be synthesized
using cadmium acetate as cadmium source instead of Cd-MA.
However, their PL QYs are much lower than those synthesized
using Cd-MA as cadmium source. If a strong ligand such as
1-dodecanelthiol (DDT) was used as capping agent, the reaction
could not happen even at 100 °C for 24 h because long-chain
aliphatic thiols/cadmium complexes are more stable than CdSe
and CdS and inhibit the nucleation process9b. However, if we
added some tetra-n-octylammonium bromide as phase-transfer
reagent, the reaction happened immediately. The nucleation and
growth could happen if a large number of sulfide ions were
transferred into organic phase by the phase-transfer agents, and
CdS nanocrystals were possibly formed in organic phase instead
of at the toluene-water interface. Figure 9 shows the UV-vis
absorption spectrum of DDT-capped CdS nanocrystals. The
sharp first excitonic absorption peak also indicates that the size
distribution is relatively narrow. The size of DDT-capped CdS
nanocrystals is about 2.0 nm calculated from the first excitonic
absorption peak of UV-vis absorption spectrum.
temporal evolution of UV-vis absorption and PL spectra of
OA-capped CdS nanocrystals during the reaction time from 30
min to 150 min. The trap emission was not observed for OAcapped CdS nanocrystals. The band-edge absorption and PL
peaks of CdS nanocrystals show a slow and continuous redshift with the reaction time. The nanocrystals have a very slow
growth rate resulting from a slow decomposition rate of thiourea.
This nucleation and growth processes are similar to those used
Na2SeSO3 to make CdSe nanocrystals. The fwhm in the PL
spectra of CdS and CdSe nanocrystals does not change much
with reaction time. Ostwald ripening did not happen for these
two cases, which indicated the size distributions have no
defocusing or focusing in nanocrystal synthesis.
The crystal structures of CdSe and CdS nanocrystals are
confirmed by the X-ray powder diffraction patterns in Figure
8. For CdSe and CdS nanocrystals, the peak positions match
well with the theoretical values of the cubic structure of CdSe
(JCPDS No.19-0191) and CdS (JCPDS No.10-0454), respectively. No characteristic peaks of other impurities were observed,
and all the reflections could be indexed to the pure cubic phase
CdSe and CdS. In general, a high reaction temperature is
favorable for formation of hexagonal nanocrystals, and a low
reaction temperature usually leads to cubic nanocrystals. The
broad diffraction peaks indicate the smaller size of nanocrystals.
The crystallite sizes of TOPO-TOP capped CdSe and CdS
nanocrystals calculated using the Sherrer formula20 are 2.8 and
2.4 nm, respectively, which are smaller than those determined
from TEM images.
Conclusions
In summary, a two-phase approach under mild conditions has
been developed to synthesize hydrophobic CdSe and CdS
quantum dots with a narrow size distribution using various
water-soluble precursors. Room-temperature photoluminescence
spectra of nanocrystals show the near-band-edge emission for
CdSe nanocrystals and the shallow trap emission for CdS
nanocrystals. A two-phase approach combined with the autoclave was applied to synthesize highly luminescent CdSe/CdS
core-shell nanocrystals. Through our two-phase approach,
nanocrystals with narrow size distributions could be obtained
by a slow nucleation and a slow growth using poorly reactive
Na2SeSO3 or selenourea as selenium source or a rapid nucleation
and a rapid growth using highly reactive NaHSe as selenium
source. So, it provides a simple and controllable synthetic route
to prepare binary oil-soluble nanocrystals under mild conditions
by using various water-soluble precursors.
Acknowledgment. This work was supported by the National
Natural Science Foundation of China for General (General:
90101001, 20674085, 20674086; Key: 50633030; Creative
Research Group: 50621302), “863” Project (2006AA03Z224),
the Special Pro-Funds for Major State Basic Research Projects
(2002CCAD4000), the Special Funds for Major State Basic
Research Projects (No. 2003CB615600) and the Distinguished
Young Fund of Jilin Province (20050104), the Project (KJCX2SW-H07) from the Chinese Academy of Sciences, and the
5666 J. Phys. Chem. C, Vol. 111, No. 15, 2007
International Collaboration Project (04-03GH268, 200507022) from Changchun City and Jilin Province, China.
References and Notes
(1) (a) Andres, R. P.; Bein, T.; Dorogi, M.; Feng, S.; Henderson, J. I.;
Kubiak, C. P.; Mahoney, W.; Osifchin, R. G.; Reifenberger, R. Science
1996, 272, 1323. (b) Gittins, D. I.; Bethell, D.; Schiffrin, D. J.; Nichols, R.
J. Nature 2000, 408, 67. (c) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A.
P. Nature 1994, 370, 354. (d) Coe, S.; Woo, W. K.; Bawendi, M.; Bulovic,
V. Nature 2002, 420, 800. (e) Klein, D. L.; Roth, R.; Lim, A. K. L.;
Alivisatos, A. P.; McEuen, P. L. Nature 1997, 389, 699. (f) Huynh, W. U.;
Peng, X. G.; Alivisatos, A. P. AdV. Mater. 1999, 11, 923. (g) Bruchez, M.;
Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013.
(2) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R.
J. Chem. Soc., Chem. Commun. 1994, 7, 801.
(3) Kang, S. Y.; Kim, K. Langmuir 1998, 14, 226.
(4) Horswell, S. L.; Kiely, C. J.; O’Neil, I. A.; David, J.; Schiffrin, D.
J. J. Am. Chem. Soc. 1999, 121, 5573.
(5) Chen, S. W.; Huang, K.; Stearns, J. A. Chem. Mater. 2000, 12,
540.
(6) (a) Rogach, A. L.; Kornowski, A.; Gao, M. Y.; Eychmuller, A.;
Weller, H. J. Phys. Chem. B 1999, 103, 3065. (b) Chemseddine, A.; Weller,
H. Ber. Bunsenges. Phys. Chem. 1993, 97, 636. (c) Vossmeyer, T.; Katsikas,
L.; Giersig, M.; Popovic, I. G.; Diesner, K.; Chemseddine, A.; Eychmuller,
A.; Weller, H. J. Phys. Chem. 1994, 98, 7665.
(7) (a) Kortan, A. P.; Hull, R.; Opila, R. L.; Bawendi, M. G.;
Steigerwald, M. L.; Carroll, P. J.; Brus, L. E. J. Am. Chem. Soc. 1990,
111, 1327. (b) Petit, C.; Pileni, M. P. J. Phys. Chem. 1988, 92, 2282. (c)
Herron, N.; Wang, Y.; Eckert, H. J. Am. Chem. Soc. 1990, 112, 1322. (d)
Khomane, R. B.; Manna, A.; Mandale, A. B.; Kulkarni, B. D. Langmuir
2002, 18, 8237.
(8) (a) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem.
Soc. 1993, 115, 8706. (b) Katari, J. E. B.; Colvin, V. L.; Alivisatos, A. P.
J. Phys. Chem. 1994, 98, 4109. (c) Peng, X. G.; Manna, L.; Yang, W. D.;
Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000,
404, 59. (d) Manna, L.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc.
2000, 122, 12700.
Pan et al.
(9) (a) Peng, Z. A.; Peng, X. G. J. Am. Chem. Soc. 2001, 123, 183. (b)
Qu, L. H.; Peng, Z. A.; Peng, X. G. Nano. Lett. 2001, 1, 333. (c) Yu, M.
W.; Peng, X. G. Angew. Chem., Int. Ed. 2002, 41, 2368. (d) Qu, L.; Peng,
X. G. J. Am. Chem. Soc. 2002, 124, 2049.
(10) (a) Hines, M. A.; Guyot-Sionnest, P. J. Phys. Chem. 1996, 100,
468. (b) Dabbousi, B. O.; RodriguezViejo, J.; Mikulec, F. V.; Heine, J. R.;
Mattoussi, H.; Ober, R.; Jensen, K. F.; Bawendi, M. G. J. Phys. Chem. B
1997, 101, 9463. (c) Peng, X. G.; Schlamp, M. C.; Kadavanich, A. V.;
Alivisatos, A. P. J. Am. Chem. Soc. 1997, 119, 7019. (d) Talapin, D. V.;
Rogach, A. L.; Kornowski, A.; Haase, M.; Weller, H. Nano. Lett. 2001, 1,
207. (e) Lee, J.; Sundar, V. C.; Heine, J. R.; Bawendi, M. G.; Jensen, K. E.
AdV. Mater. 2000, 12, 1102.
(11) (a) Mekis, I.; Talapin, D. V.; Kornowski, A.; Haase, M.; Weller,
H. J. Phys. Chem. B 2003, 107, 7454. (b) Reiss, P.; Bleuse, J.; Pron, A.
Nano Lett. 2002, 2, 781. (c) Li, J. J.; Wang, Y. A.; Guo, W. Z.; Keay, J.
C.; Mishima, T. D.; Johnson, M. B.; Peng, X. G. J. Am. Chem. Soc. 2003,
125, 12567. (d) Reiss, P.; Carayon, S.; Bleuse, J.; Pron, A. Synth. Met.
2003, 139, 649.
(12) (a) Pan, D. C.; Jiang, S. C.; An, L. J.; Jiang, B. Z. AdV. Mater.
2004, 16, 982. (b) Wang, Q.; Pan, D. C.; Jiang, S. C.; Ji, X. L.; An, L. J.;
Jiang, B. Z. Chem. Eur. J. 2005, 11, 3843.
(13) Pan, D. C.; Wang, Q.; Jiang, S. C.; Ji, X. L.; An, L. J. AdV. Mater.
2005, 17, 176.
(14) Pan, D. C.; Zhao, N. N.; Wang, Q.; Jiang, S. C.; Ji, X. L.; An, L.
J. AdV. Mater. 2005, 17, 1991.
(15) Zhao, N. N.; Pan, D. C.; Nie, W.; Ji, X. L. J. Am. Chem. Soc.
2006, 128, 10118.
(16) Klayman, D. L.; Griffin, T. S. J. Am. Chem. Soc. 1973, 95, 197.
(17) Reynolds, G. A.; Drexhage, K. H. Opt. Commun. 1975, 13, 222.
(18) The average diameter (D) of CdSe nanocrystals was calculated by
using the following equation: D ) (1.6122 × 10-9) λ4 - (2.6575 × 10-6)
λ3 + (1.6242 × 10-3) λ2 - 0.4277λ + 41.57, and λ is the wavelength of
the first excitonic absorption peak. Yu, W. W.; Qu, L. H.; Guo, W. Z.;
Peng, X. G. Chem. Mater. 2003, 15, 2854.
(19) Sugimoto, T. AdV. Colloid Interface Sci. 1987, 28, 65.
(20) Nanda, J.; Sapra, S.; Sarma, D. D.; Chandrasekharan, N.; Hodes,
G. Chem. Mater. 2000, 12, 1018.