A R T I C L E S
Liu et al.
tion), in which case peak integrals were obtained by deconvolution of
the two resonances using MestReC (Mestrelab Research). A detailed
discussion of the accuracy of the NMR method used in this study can
be found in the Supporting Information.
The kinetics of growth and shape evolution, especially that
of CdSe, has been discussed extensively. Many important
concepts such as size distribution focusing,24 selective adhesion
shape control,2,3,25 and branching3,4,12 have been developed.
Recent kinetic studies by Peng,26,27 as well as by Mulvaney,28,29
have examined the nucleation and growth of CdSe quantum
dots and rods by monitoring the time-dependent evolution of
the nanocrystals. In spite of the important conclusions from these
studies, they have yet to be linked to a detailed understanding
of how precursor molecules are converted to group II-VI
materials.
Using a combination of NMR (1H, 13C, and 31P) spectroscopy
and mass spectrometry (MS), we have investigated the synthesis
of group II-VI semiconductor nanocrystals in TOPO and ODE
by following the disappearance and appearance of molecular
precursors and products. Our results show that surfactant
molecules are reactants that convert precursor molecules to
II-VI semiconductor materials. In particular, our experiments
suggest that trialkylphosphine chalcogenides deoxygenate the
alkylphosphonate or alkylcarboxylate surfactants, liberating the
chalcogen atom. A mechanism is proposed where nucleophiles
such as alkylphosphonate and alkylcarboxylate attack a Lewis
acid activated (TOPdE)sM (E ) S, Se, Te; M ) Zn, Cd)
complex, breaking the PdE double bond.
Synthesis and Characterization of TOPSe, Tri-n-octylphosphine
Sulfide (TOPS), Tri-n-butylphosphine Sulfide (TBPS), TBPSe, Tri-
n-butylphosphine Telluride (TBPTe), and Triisopropylphosphine
Selenide (i-TPPSe). Traditional syntheses of group II-VI nanocrystals
use a mixture of TOP and TOPE (or a mixture of TBP and TBPE) as
the injection solution.27 To simplify the analysis, we used the pure
phosphine chalcogenide instead of a mixture with its parent phosphine.
Phosphine chalcogenides were prepared by stirring the appropriate
phosphine with a stoichiometric or excess amount of elemental S/Se/
Te in a glovebox at room temperature. The supernatant was separated
from the excess solid chalcogenide and was found to be pure by NMR
(1H, 13C, and 31P) and elemental analysis (Supporting Information).
Synthesis of Cadmium and Zinc Oleic Acid Complexes (M-OA;
M ) Zn, Cd).32 To a 25 mL flask was added H-OA (4.55 g, 16 mmol)
and CdO (0.518 g, 4.0 mmol). The mixture was degassed at 100 °C
and 250 mTorr for 30 min. The flask was then filled with Ar and heated
to 190 °C to dissolve CdO. After the dissolution of CdO, the mixture
was cooled to 110 °C and degassed again at 300 mTorr for 20 min.
The solution was then cooled to room temperature and stored in a
freezer under N2. A stock solution was prepared by dissolving 2.40 g
of this complex in 1.60 g of n-nonane-d20 and was used in the synthesis
of CdS, CdSe, and CdTe. Zn-OA was prepared similarly by dissolving
ZnO (0.167 g, 2.05 mmol) in a mixture of H-OA (2.319 g, 8.02 mmol)
and ODE (2.50 g) at 300 °C under Ar followed by degassing at 100
°C. The neat reaction mixture was used in the synthesis of ZnS, ZnSe,
and ZnTe.
Experimental Section
Tri-n-octylphosphine (TOP; 97%, Strem), tri-n-butylphosphine (TBP;
99%, Strem), triisopropylphosphine (Aldrich), TOPO (Aldrich, 99%,
lot numbers 24801MB and 04017PC),30 n-octadecylphosphonic acid
(H2-ODPA; Polycarbon), CdO (Aldrich, 99.99+%), ZnO (Aldrich,
99.99+%), oleic acid (H-OA; Aldrich, 99%), n-nonane-d20 (Aldrich,
98 atom % D), n-decane-d22 (Arcos, 99 atom % D), and ODE (Aldrich,
90%) were used as received. Standard air-sensitive techniques were
used to handle air- and moisture-sensitive compounds.
NMR Methods. All NMR (1H, 13C, and 31P) spectra were collected
on a 400 MHz Bruker Advance spectrometer. 31P NMR spectra were
acquired either without proton decoupling or with inverse gated
decoupling, and care was taken to ensure adequate relaxation (g5T1)
between pulses. In situ kinetics experiments were conducted under
vacuum in flame-sealed NMR tubes. These samples were inserted into
a preheated NMR probe that was calibrated using ethylene glycol as a
standard according to an established procedure.31 CDCl3 solutions of
aliquots from the TOPO-based reaction were prepared in air. Control
experiments showed that CDCl3 solutions of TOPE and TBPE (E )
Se, S) stored in air at room temperature are air stable for several weeks,
with only <2% conversion to TOPO by 31P NMR spectroscopy (Figure
S2, Supporting Information).
CdSe Synthesis in TOPO/H2-ODPA at 260 °C: “Doubly De-
gassed” Protocol. To a 25 mL three-neck flask equipped with a
condenser and a thermocouple adapter were added TOPO (2.73 g, 7.06
mmol), H2-ODPA (1.07 g, 3.20 mmol), and CdO (0.204 g, 1.60 mmol).
The mixture was degassed at 120 °C and 200-400 mTorr pressure for
60 min. The flask was then filled with Ar, and the temperature was
raised to 320 °C to dissolve CdO. After CdO was dissolved, the
temperature was lowered to 150-180 °C and the pressure was reduced
to ∼300 mTorr for 60 min (this step will be referred to in the text as
the “second degassing”). The flask was then filled with Ar, and the
temperature was raised to 270 °C. TOPSe (0.70 g, 1.6 mmol) was
injected, and the temperature was allowed to stabilize at 260 ( 2 °C.
The amount of TOPSe injected (1.4 ( 0.1 mmol) was measured as the
difference between the masses of the syringe before and after the
injection. Aliquots taken after the injection of TOPSe were dissolved
in CDCl3 and transferred to NMR tubes in air. NMR spectra of the
aliquots were collected within 24 h of sampling.
In Situ Monitoring of the CdSe Synthesis in n-Nonane-d20. To a
5 mm NMR tube were added TOPSe (0.0809 g, 0.18 mmol), the Cd-
OA stock solution (0.375 g, 0.18 mmol of Cd2+ and 0.72 mmol of
H-OA/OA), and n-nonane-d20 (0.0478 g). The mixture was degassed
by four freeze-pump-thaw cycles before the NMR tube was flame
sealed under vacuum. The NMR probe was then preheated to the desired
reaction temperature, and the sample was inserted into the probe and
allowed to temperature equilibrate for 4 min before NMR spectra (1H,
13C, and 31P) were collected. NMR spectra were collected at room
temperature before and after the reaction to analyze the reaction
products.
The concentration of phosphine chalcogenide was obtained by
comparing the integral of its 31P NMR peak with that of the phosphine
oxide peak and assuming the total concentration of the two species
was constant during the reaction. This assumption was verified to be
valid (<(2% error) by using an internal standard (ethylphosphonic
acid diethyl ester) in the in situ kinetics runs. The 31P NMR resonances
of TOPO and TOPS partially overlap (Figure S3, Supporting Informa-
(24) Peng, X. G.; Wickham, J.; Alivisatos, A. P. J. Am. Chem. Soc. 1998, 120,
5343-5344.
(25) Peng, Z. A.; Peng, X. G. J. Am. Chem. Soc. 2001, 123, 1389-1395.
(26) Qu, L. H.; Yu, W. W.; Peng, X. P. Nano Lett. 2004, 4, 465-469.
(27) Peng, Z. A.; Peng, X. G. J. Am. Chem. Soc. 2002, 124, 3343-3353.
(28) Bullen, C. R.; Mulvaney, P. Nano Lett. 2004, 4, 2303-2307.
(29) van Embden, J.; Mulvaney, P. Langmuir 2005, 21, 10226-10233.
Syntheses of other group II-VI materials were carried out similarly
using TBPE and M-OA (E ) S, Se, Te; M ) Zn, Cd) in n-nonane-
d20, n-decane-d22, or ODE (Supporting Information).
(30) We found that the purity of TOPO is less than 99% on the basis of its 31
P
NMR spectrum (Supporting Information Figure S1). The same batch of
TOPO was used whenever two experimental results were to be compared.
(31) Ammann, C.; Meier, P.; Merbach, A. E. J. Magn. Reson. 1982, 46, 319-
321.
(32) Yu, W. W.; Wang, Y. A.; Peng, X. G. Chem. Mater. 2003, 15, 4300-
4308.
9
306 J. AM. CHEM. SOC. VOL. 129, NO. 2, 2007