Kinetics and Nucleation of CdSe Nanocrystals
A R T I C L E S
Scheme 1. Lewis Acid Activation and Phosphine Selenide
However, NCs display temperature-dependent optical properties,
making it a challenge to monitor CdSe NCs growing in situ.
As a result, most researchers have undertaken such studies by
timed aliquoting of the reaction mixture and room-temperature
analysis of the NC optical properties. Obtaining precise kinetics
data with these methods is challenging, given the air sensitivity
of the reaction mixture, the high temperatures of these crystal-
lizations, and the labor-intensive nature of quantitative aliquot-
ing. In order to address these challenges, we have employed an
automated, high-throughput reactor to measure reaction kinetics,
which was previously reported by Chan et al. to greatly aid in
synthetic reproducibility.31 This instrument provides precise
control over the reaction temperature as well as the timing of
the injection and aliquots and allows precise dilution of aliquots
to a known concentration for quantitative absorption spectros-
copy, all within an inert atmosphere glovebox.
In the present study we explore the relationship between the
precursor reaction mechanism, the kinetics of precursor conver-
sion, and the final number of NCs. Our observations indicate
that phosphine selenides and anhydrous cadmium octade-
cylphosphonate undergo a slow chemical reaction that continu-
ously liberates CdSe to the reaction medium, causing supersat-
uration, nucleation, and NC growth. The reliability of our study
is ensured by the well-defined composition of our reagents, the
purity of which has an important influence on colloidal
crystallizations.40-42 Our results highlight the importance of the
precursor conversion to crystal nucleation and suggest that
reproducible NC syntheses can be designed using well-defined
starting materials and carefully tuned reactivity.
Cleavagea
a CdX2 refers to alkylcarboxylate and alkylphosphonate complexes of
cadmium, e.g., Cd-ODPA in this study. (CdSe)i refers to a solute form of
CdSe.
between oleic acid and the starting cadmium ion or the surfaces
of nuclei was proposed as the origin of its inhibitory effect.
Less is known about the kinetics of reaction between
precursor molecules and its influence on NC formation.25-27
Only recently have detailed studies of the microscopic steps
that control the precursor reaction begun to appear. For example,
cadmium-alkylphosphonate and -alkylcarboxylate complexes
have been shown to bind tri-n-alkylphosphine chalcogenides,
activating them toward nucleophilic attack and PdE bond
cleavage (Scheme 1).28 Related Lewis-acid activation mecha-
nisms have also been shown for II-IV29 and III-V30 NCs,
while other syntheses appear to follow multistep pathways via
heterogeneous intermediates.31,32 The relationship between these
reaction pathways and crystallization is unclear for semiconduc-
tor NCs, although it has been a subject of study in transition
metal NCs.33
In many colloidal crystallizations,34 the precursor reaction is
an integral part of monodisperse particle formation since it
provides an internal reservoir of solute that is continually
released during the crystallization. Under the right conditions,
a balance between solute production and consumption during
growth can maintain the supersaturation at a level that causes
size distribution focusing.35 In these cases, precursor reaction
kinetics can play an important role in the resultant crystal size
and size distribution. While the kinetics of solute supply is a
well-developed avenue of study in canonical colloidal crystal-
lizations, it is a relatively unstudied aspect of NC syntheses.
High-temperature colloidal crystallizations have been studied
using flow cells,36 microfluidic chips,37,38 in situ absorption
spectroscopy,39 and timed aliquoting of the reaction mixture.21-25
Results
Reaction of Phosphine Selenides with Cadmium
Octadecylphosphonate. Cadmium selenide NCs were prepared
using carefully purified reagents shown to eliminate inconsisten-
cies across different sources of starting material.40,41,43 Anhy-
drous cadmium octadecylphosphonate (Cd-ODPA) was prepared
by dropwise addition of a 2:1 tri-n-octylphosphine:CdMe2
solution to molten tri-n-octylphosphine oxide (TOPO) and
octadecylphosphonic acid (ODPA, 1.67-2.0 equiv) at 275 °C.44
At low ODPA:Cd ratios, the Cd-ODPA coordination polymer
becomes insoluble at this temperature and dissolves only upon
heating.45,46 At 1.67 equiv of ODPA and above, the reaction
mixture is clear and colorless at 325 °C, while at lower ODPA:
Cd ratios, the precipitate does not completely dissolve, resulting
in irregularly branched NCs. All reactions were run with at least
(25) Yordanov, G. G.; Dushkin, C. D.; Adachi, E. Colloids Surf. A 2008,
316, 37–45.
(26) Viswanatha, R.; Amenitsch, H.; Sarma, D. D. J. Am. Chem. Soc. 2007,
129, 4470–4475.
(27) Chen, O.; Chen, X.; Yang, Y. A.; Lynch, J.; Wu, H. M.; Zhuang,
J. Q.; Cao, Y. C. Angew. Chem., Int. Ed. 2008, 47, 8638–8641.
(28) Liu, H. T.; Owen, J. S.; Alivisatos, A. P. J. Am. Chem. Soc. 2007,
129, 305–312.
(29) Steckel, J. S.; Yen, B. K. H.; Oertel, D. C.; Bawendi, M. G. J. Am.
Chem. Soc. 2006, 128, 13032–13033.
(40) Wang, F. D.; Tang, R.; Buhro, W. E. Nano Lett. 2008, 8, 3521–3524.
(41) Wang, F.; Tang, R.; Kao, J. L. F.; Dingman, S. D.; Buhro, W. E.
J. Am. Chem. Soc. 2009, 131, 4983–4994.
(30) Allen, P. M.; Walker, B. J.; Bawendi, M. G. Angew. Chem., Int. Ed.
2010, 49, 760–762.
(42) Kopping, J. T.; Patten, T. E. J. Am. Chem. Soc. 2008, 130, 5689–
5698.
(31) Chan, E. M.; Xu, C.; Mao, A. W.; Han, G.; Owen, J. S.; Cohen, B. E.;
Milliron, D. J. Nano Lett. 2010, 10, 1874–1885.
(32) Joo, J.; Pietryga, J. M.; McGuire, J. A.; Jeon, S. H.; Williams, D. J.;
Wang, H. L.; Klimov, V. I. J. Am. Chem. Soc. 2009, 131, 10620–
10628.
(43) Owen, J. S.; Park, J.; Trudeau, P. E.; Alivisatos, A. P. J. Am. Chem.
Soc. 2008, 130, 12279–12281.
(44) Cd-ODPA refers to the mixture of octadecylphosphonic acid and
cadmium octadecylphosphonate that results from the protonolysis of
dimethylcadmium with octadecylphosphonic acid. Our observations
suggest this starting material is a coordination polymer, even at the
growth temperature (vide infra). Thus, Cd-ODPA generally refers to
a polymeric species of cadmium ions bridged by octadecylphosphonic
acid and octadecylphosphonate moieties.
(33) Finney, E. E.; Finke, R. G. J. Colloid Interface Sci. 2008, 317, 351–
374.
(34) Sugimoto, T. Monodispersed Particles; Elsevier: Amsterdam, 2001;
see especially Chapter 7.
(35) Sugimoto, T. AdV. Colloid Interface 1987, 28, 65–108.
(36) Yen, B. K. H.; Stott, N. E.; Jensen, K. F.; Bawendi, M. G. AdV. Mater.
2003, 15, 1858–1862.
(45) Cao, G.; Lynch, V. M.; Yacullo, L. N. Chem. Mater. 1993, 5, 1000–
1006.
(37) Yen, B. K. H.; Gunther, A.; Schmidt, M. A.; Jensen, K. F.; Bawendi,
M. G. Angew. Chem., Int. Ed. 2005, 44, 5447–5451.
(38) Chan, E. M.; Mathies, R. A.; Alivisatos, A. P. Nano Lett. 2003, 3,
199–201.
(46) Methane is evolved during this addition, and at the same time the
surface tension of the solution increases due to the polymer formation.
At low ODPA:Cd ratios, a foam often forms at the surface of the
reaction mixture that can cause the last few drops of CdMe2 to
decompose thermally rather than by reaction with the phosphonic acid.
(39) Qu, L. H.; Yu, W. W.; Peng, X. P. Nano Lett. 2004, 4, 465–469.
9
J. AM. CHEM. SOC. VOL. 132, NO. 51, 2010 18207