In this paper, we present the systematically experimental results on the influences of pH of the reaction medium, molar ratio of the precursors on the synthesis in aqueous phase of CdTe quantum dots (QDs) and CdTe/CdS QDs with core/shell structure. Under optimal synthesis conditions, water-soluble CdTe and CdTe/CdS QDs have been prepared that exhibit very strong photoluminescence peaking in the spectral range between 520 nm and 650 nm with narrow full width at half maximum (\(\sim 32\) nm in the short-wavelength emission case); depending on the emission range, most samples however exhibit the high luminescence quantum yields (\(\sim 40\)%). Moreover, the synthesis in aqueous phase shows some additional advantages: it is possible to prepare high quality CdTe QDs in large-scale (up to gram/reaction) with low cost, less toxic and short production time

Liem, Nguyen Quang

Thien, Trinh Duc

Toan, Pham Song

Vietnam Academy of Science and Technology: Journals Online

Communications in Physics, Vol. 20, No. 3 (2010), pp. 377-384
LARGE-SCALE SYNTHESIS OF CdTe QUANTUM DOTS
IN AQUEOUS PHASE
PHAM SONG TOAN AND NGUYEN QUANG LIEM
Institute of Material Science, VAST
TRINH DUC THIEN
Institute of Material Science, VAST
and
Hanoi National University of Education
Abstract. In this paper, we present the systematically experimental results on the influences of
pH of the reaction medium, molar ratio of the precursors on the synthesis in aqueous phase of
CdTe quantum dots (QDs) and CdTe/CdS QDs with core/shell structure. Under optimal syn-
thesis conditions, water-soluble CdTe and CdTe/CdS QDs have been prepared that exhibit very
strong photoluminescence peaking in the spectral range between 520 nm and 650 nm with narrow
full width at half maximum (∼32 nm in the short-wavelength emission case); depending on the
emission range, most samples however exhibit the high luminescence quantum yields (∼ 40 %).
Moreover, the synthesis in aqueous phase shows some additional advantages: it is possible to pre-
pare high quality CdTe QDs in large-scale (up to gram/reaction) with low cost, less toxic and short
production time.
I. INTRODUCTION
Semiconductor QDs or nanocrystals (NCs) have been intensely studied due to their
unique size-dependent properties and their promising applications as well. One of the most
highlight applications of semiconductor QDs is that it can be used as fluorescent probes
in biomedical labeling and ion detection [1]. In these applications, the water-soluble QDs
are required. Another strategic application of semiconductor QDs that requires a large
amount of materials is in solid state lighting industry, in which QDs serve as luminnophore
matrials to convert blue light from InGaN-based light emitting diodes (LED) into orange
emission and then giving rise eventually white light sources.
Many previous literatures have been proved that the hot injection of the Cd and
the Te precursors into the organic solvents such as trioctylphosphine oxide (TOPO), hex-
adecylamine (HDA) or 1-octadecene (ODE) [2], etc. can make very high quality CdTe
QDs whose luminescence quantum yield (QY) can be up to 85%. However, the CdTe QDs
prepared with TOPO or other mentioned organic solvent are usually capped by organic
ligands that make them water-insoluble. For many biomedical applications, these semicon-
ductor QDs need to be changed into hydrophilic state by doing ligand exchange. In fact,
the ligand exchange process requires rather complicated and time-consuming treatments
and in many cases it makes the QY decreased. An alternative way to obtain hydrophilic
QDs is to synthesize them directly in water [3-6]. In this synthesis, popular chemical
378 PHAM SONG TOAN, TRINH DUC THIEN, AND NGUYEN QUANG LIEM
agents such as cadmium bromide, 3-mecaptopropionic acid (MPA), tellurium powder and
sodium borohydride, etc. were used as precursors. The synthesis of semiconductor QDs in
water phase is not only simpler, less toxic and more bio-compatible than the hot-injected
organometallic ones but also very cost-effective (about 8 times less than that using ex-
pensive organic solvents [7]). Moreover, this method is feasible to manufacture QDs in
different quantum structure (e.g type II [8]) and is potential to produce in a large scale,
just by scale up the starting precursor amounts.
In the synthesis of semiconductor QDs in water phase, influences of pH of the
reaction medium, molar ratio of the precursors and surface passivation after making core-
only QDs must be studied in detail. In order to improve the luminescence efficiency and
the stability of obtained QDs, their surfaces must be passivated by a suitable material [9].
Luminescence of QDs passivated by inorganic shell is usually more robust than that of
organically passivated one since inorganic epitaxial growth can eliminate efficiently both
the anionic and cationic dangling bonds on the QDs surface.
In this paper, we report results of our study on the influences of pH of the reaction
medium, molar ratio of the precursors on the synthesis of CdTe and CdTe/CdS core/shell
QDs in aqueous phase. Various characterization techniques such as transmission elec-
tron microscopy (TEM), X-ray diffraction (XRD), absorption and photoluminescence (PL)
spectroscopies were used showing the high quality of the obtained CdTe and CdTe/CdS
core/shell QDs.
II. EXPERIMENTAL
II.1. Synthesis of CdTe Core
Sodium borohydride (NaBH4, 99%), tellurium powder (Te, 99.8%), thiourea ((NH2)2CS,
97%) were purchased fromMerck, cadmium bromide (CdBr2, 99%), and 3-mercaptopropionic
acid (MPA, 99%) were purchased from Aldrich. All chemicals were used without additional
purification.
Firstly, 160 mg of Te powder and 100 mg of NaBH4 were put into a two-neck flask
and were degassed for 30 min and backfilled with nitrogen gas. And then, 2 ml of degased
distilled water was added into the flask through a syringe. The reaction mixture was
mixed by the ultrasonic generator for 30 minutes at the temperature of 50-60oC to get a
deep red transparent solution via the following chemical reaction:
2 NaBH4 + Te + 2 H20 = NaHTe + NaBO2 + 11/2 H2
For a typical synthesis of CdTe QDs, 12.5mM of CdBr2+ solution was mixed with
18.75 mM of MPA solution at the ratio of Cd:MPA = 1:1.5 (mol/mol) and the pH of this
solution was adjusted in range of 7 - 12 by addition of 1.0 M NaOH solution. After that,
the freshly prepared 0.625M NaHTe solution was injected quickly by a syringe into the Cd-
containing flask at the room temperature in nitrogen gas protection. The reaction mixture
changes instantly to golden yellow indicating the commencement of CdTe nucleation. The
as-prepared CdTe QDs emitted very weak luminescence peaking around 510 nm.
LARGE-SCALE SYNTHESIS OF CdTe QUANTUM DOTS IN AQUEOUS PHASE 379
II.2. Synthesis of CdTe/CdS Core/shell Structure
To stabilize the physical characteristics of QDs, excessive amounts of thiourea were
added into the solution containing CdTe cores and then the solution was annealed at 120oC
in few minutes or few hours to reach the expected average size. Herein, thioure was used
as the sulfur source due to its decomposition at high temperature and the S2− ions could
easily combine with the excessive Cd2+ ions in the initial solution to form CdTe/CdS
core/shell structures.
II.3. Characterization
The absorption and PL spectra were obtained by using Cary 5000 (Varian) UV-
vis-NIR spectrophotometer and iHR550 (Horiba) spectrometer equipped a CCD detector
(Synapse), respectively. In the PL measurement, a 377-nm LED was used as the excitation
source. The room-temperature PL QY of the prepared samples was estimated by compar-
ing the total integrated emission of the samples with that of Rhodamine 6G at the same
optical density [10]. The quantum yield of Rhodamine 6G has already been known ∼95%
[11]. The powder XRD patterns were received by using a Siemens D5000 powder X-ray
diffractometer equipped with a graphite monochromatized high-intensity Cu Kα radiation
(α = 1.54178 A˚ ). The TEM images were taken by a TEM system (Tecnai 20ST, Osaka
University, Japan).
III. RESULTS AND DISCUSSION
III.1. Influences of pH Value at the Cd:Te Ratio of 1:0.2
The pH of the reaction environment is an important factor to prepare CdTe QDs
in aqueous phase. To investigate effects of the pH value, we fixed the molar ratio of
Cd:MPA:Te at 1:1.5:0.2 for completed reaction. In the solution of MPA and Cd2+ mixture,
the Cd-MPA complexes were formed. It is supposed that the Cd-MPA complexes could
show the polymerization in low pH environments. Therefore, these complexes are insoluble
in acidic media due to the existence of Cd-thiol in the polymer state. On the other hand,
in high pH environments (pH≥ 7), the Cd-MPA complexes can exist in the molecular
state, and easily to be the Cd2+ sources.
In Figure 1, the absorption spectra of various CdTe QDs samples synthesized at
different pH (from 7 to 12) of the reaction medium are shown. It is realized that the CdTe
QDs prepared in neutral media (with pH∼7-8) show sharper, narrower excitonic peak and
suggested to have the better crystal quality. With increasing the pH value of the reaction
medium, the excitonic peak from CdTe QDs becomes unclear and broader. Therefore,
we suggest that Cd2+:MPA could be the most effective Cd2+ source for the formation of
CdTe QDs in the reaction environment of pH∼7-8.
III.2. Influences of the Molar Ratio between MPA and Cd at pH=7 of the
Reaction Environment
As mentioned, the Cd-MPA complexes take the important role as the Cd2+ source
for the formation of CdTe QDs. The generation of these complexes strongly depends
on the molar ratio between MPA and Cd2+. Figure 2a shows the absorption spectra
of CdTe QDs prepared with different molar ratio of Cd:MPA at the same pH=7 of the
380 PHAM SONG TOAN, TRINH DUC THIEN, AND NGUYEN QUANG LIEM
Fig. 1. Absorption spectra of CdTe QDs (Cd:MPA:Te of 1:1.5:0.2) in different
pH values of the reaction environments.
Fig. 2. Absorption and PL spectra of CdTe QDs with different molar ratio of Cd
and MPA (Cd:Te ratio of 1:0.5; pH=7).
reaction environment. In addition, the steady PL spectra of these samples with almost the
same absorbance were presented in Figure 2b. It is obvious that the samples prepared with
Cd:MPA ratio from 1:1 to 1:1.5 exhibit sharper excitonic absorption peaks and more robust
PL signal. These results suggest that at this Cd:MPA ratio range, the generated Cd-MPA
LARGE-SCALE SYNTHESIS OF CdTe QUANTUM DOTS IN AQUEOUS PHASE 381
complexes were the most effective Cd2+ source. To explain such kind of behaviors, we
suppose the mechanism of the formation of Cd-thiol complexes as the following equations:
RSH +OH− → SR− +H20 (1)
Cd2+ + SR− → (Cd− SR)+ (2)
(Cd− SR)+ + SR− → Cd(SR)2 (3)
The generated thiolate ions in the eqn.(1) can react with Cd2+ ions to form monoth-
iol complexes following the eqn.(2). In addition, the monothiol complexes can further react
with other thiolate ions to create dithiol complexes as described in eqn.(3). As a result,
when the Cd:MPA ratio is more than 1:1.5, the amount of the monothiol complexes will
be dominant than that of dithiol complexes. On the other hand, when the Cd:MPA ratio
is equal to or less than 1:2, the dithiol complexes might be dominant in the Cd-MPA
complexes. It is easy to understand that the free Cd2+ ions could be released more easily
from monothiol complexes than from dithiol complexes. Therefore, higher MPA ratios
limited the release of the free Cd2+ ions for participation in the nucleation and crystal
growth of CdTe QDs [12].
III.3. Influence of the Cd-to-Te ratio
The Cd-to-Te ratio is the main factor which determines the concentration of QDs in
the solution. When this ratio is changed, the average size of CdTe QDs is nearly the same.
Figure 4 shows the absorption spectra of CdTe QDs samples prepared with different ratio
of Cd:Te and annealed in 5 minutes at 120oC. The obvious absorption peaks were observed
and almost unchanged at 460 nm. Only the size distribution and the concentration of QDs
in solution were increased proportionally with increasing reacted Te molecules. When the
Cd:Te ratio is smaller than 1, the black precipitations attributed to the oxide of tellurium
was observed. Therefore, the Cd:Te should be chosen to be larger than 1. In addition, the
excessive Cd molecules are necessary for further coating CdS. Moreover, the concentrations
of the precursors have small effects on the growth rate and optical properties of CdTe QDs
and can be negligible [12]. For this reason, it enables the convenient large-scale production
of CdTe QDs.
III.4. X-ray diffraction spectrum and TEM images
Based on the obtained results, high PL QY (∼40%) CdTe QDs samples could be
produced in the neutral media of pH∼7 with molar ratio of Cd:MPA:Te about 1:1.5:0.5.
Figure 4 shows the PL spectra of Rhodamin 6G and that of CdTe QDs used to determine
the PL QY. With core/shell structure, PL QY of CdTe/CdS QDs increased significantly.
We have prepared CdTe QDs with different sizes for different spectral emissions.
Figure 5 shows the flashed-photos of typical CdTe QDs obtained with different annealing
times ranging from 30 minutes to 6 hours.
It is obvious that after making the CdS shell to form CdTe/CdS core/shell structures
the PL QY of QDs increased. Figure 6a shows the XRD patterns of the CdTe/CdS sample
annealed in 540 minutes at 120oC, in which the peaks at diffractive angles of 24.16o,
40.31o and 46.82o were observed corresponding to the cubic zinc-blende CdTe and CdS
nanocrystals. Thus, in nano-sized cubic CdTe were formed instead of hexagonal one. Note
382 PHAM SONG TOAN, TRINH DUC THIEN, AND NGUYEN QUANG LIEM
Fig. 3. Absorption spectra of CdTe QDs (Cd:MPA = 1:1.5; pH = 7) with different
molar ratio of Cd and Te.
Fig. 4. PL spectra of Rh6G and CdTe QDs used to determine the QY of CdTe
QDs; namely ∼40% in this case.
that the XRD pattens may include background caused by ligand or unwanted substance
which is hard to remove completely.
In addition, the TEM image of another sample was presented in Figure 6b. From
this image, the average size of QDs is observed about 2-3 nm.
LARGE-SCALE SYNTHESIS OF CdTe QUANTUM DOTS IN AQUEOUS PHASE 383
Fig. 5. Typical CdTe QDs samples with different annealing times from from 30
min, 60 min, 90 min, 150 min, 240 min, and 360 min from left to right, respectively.
Fig. 6. XRD patterns of CdTe QDs annealed in 540 minutes (a) and TEM image
of CdTe QDs annealed in 360 minutes (b)
IV. CONCLUSIONS
In conclusion, we have studied the influences of the pH value and molar ratio between
the Cd and the S precursors on the quality of CdTe and CdTe/CdS QDs. By doing
systematically the experiments we have determined the optimal parameters for making
high quality CdTe and CdTe/CdS QDs. These QDs synthesized in aqueous phase showed
average sizes in the range of 2-3 nm, correspondingly emitted luminescence in the range
between 520 nm and 650 nm with high QY of 40%.
ACKNOWLEDGEMENTS
The authors thank L. Q. Phuong for taking the TEM images and UTD Thuy for
her help in checking the manuscript. The Basic Research Programme in Natural Science
(MOST of Vietnam) and the Materials Science Direction (VAST) are gratefully acknowl-
edged for financial supports.
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Received 22 June 2009.


Large-scale Synthesis of CdTe Quantum Dots  in Aqueous Phase

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Large-scale Synthesis of CdTe Quantum Dots in Aqueous Phase

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