A convenient synthesis of cadmium chalcogenide quantum dots from cadmium acetate and bis(diphenylphosphino)methane monosulfide and -selenide or 1,4-bis(diphenylphosphino)butane monoselenide

Article abstract of DOI:10.1080/10426507.2015.1100188

The synthesis of both cadmium selenide and cadmium sulfide quantum dots has been effected using a synthetic protocol that avoids the use of volatile, toxic organocadmium reagents and alkyl phosphines.

Full text of DOI:10.1080/10426507.2015.1100188

Phosphorus, Sulfur, and Silicon and the Related Elements
ISSN: 1042-6507 (Print) 1563-5325 (Online) Journal homepage: http://www.tandfonline.com/loi/gpss20
A Convenient Synthesis of Cadmium Chalcogenide
Quantum Dots from Cadmium Acetate and
Bis(diphenylphosphino)methane Monosulfide and
–selenide or 1,4-Bis(diphenylphosphino)butane
Monoselenide
J. W. Faller & Jonathan Parr
To cite this article: J. W. Faller & Jonathan Parr (2015): A Convenient Synthesis of Cadmium
Chalcogenide Quantum Dots from Cadmium Acetate and Bis(diphenylphosphino)methane
Monosulfide and –selenide or 1,4-Bis(diphenylphosphino)butane Monoselenide, Phosphorus,
Sulfur, and Silicon and the Related Elements, DOI: 10.1080/10426507.2015.1100188
To link to this article: http://dx.doi.org/10.1080/10426507.2015.1100188
Accepted author version posted online: 17
Nov 2015.
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A Convenient Synthesis of Cadmium Chalcogenide Quantum Dots from Cadmium Acetate and
Bis(diphenylphosphino)methane Monosulfide and --selenide or 1,4-
Bis(diphenylphosphino)butane Monoselenide
J. W. Faller* and Jonathan Parr
Department of Chemistry, Yale University, New Haven CT 06520-8107.
*Corresponding Author E-mail: Jack.Faller@yale.edu. Telephone: 203-432-3954.
ABSTRACT
The synthesis of both cadmium selenide and cadmium sulfide quantum dots has been effected
using a synthetic protocol that avoids the use of volatile, toxic organocadmium reagents and
alkyl phosphines.
KEYWORDS
Cadmium selenide quantum dot; cadmium sulfide quantum dot; quantum dot synthesis;
phosphine selenide; phosphine sulfide.
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Introduction
Nanoparticulate semiconductor materials, or quantum dots, have been shown to have
applications in a number of significant areas of research, including photovoltaic cells1,2
biological markers3,4,5 and light-emitting diodes.6,7,8 Amongst these materials, cadmium
chalcogenides have proven flexible and useful in a range of applications and, as a result, interest
in new synthetic routes is strong. Original syntheses of cadmium selenide quantum dots called
for the reaction of volatile and highly toxic organocadmium reagents with solutions of selenium
in tri-n-octylphosphine, the latter making tri-n-octylphosphine selenide in situ to serve as the
selenium source.9 In these reactions, the phosphine was both reagent and solvent which, looked
at from the point of view of a synthetic chemist, is an unusual choice as alkyl phosphines are
typically flammable or pyrophoric, toxic and have strong and unpleasant odors. This reaction
system calls for high temperatures, which, in combination with the volatile and highly toxic
organocadmium compounds and the toxicity and stench issues associated with the
organophosphine, makes the procedure very unattractive. As a results, a number of groups have
pursued synthetic approaches that avoid some or all of these issues, including the direct reaction
of cadmium(II) ions with hexamethyldisilaselenide10, the pyrolysis of single molecule
precursors11, molecular beam epitaxy12 and low-temperature reaction of cadmium(II) salts with
sodium selenide in ethylenediamine solution13 or in oleic acid and paraffin.14 Reactions using
reducing conditions have also been reported including the reaction of cadmium(II) ions with
selenium and hydrazine hydrate15 or cadmium(II) oxide with selenium and sodium borohydride
in oleylamine.16 Dialkyl and diaryl chalcogens have also been used a molecular precursor for
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chalcogenide nanocrystals.17 There is clearly interest in more amenable synthetic approaches to
cadmium chalcogenide quantum dots and this has prompted us to report our findings in this area.
Bisphosphines that have been selectively monoxidized by the addition of a single chalcogen
have been known for many years18 and have a rich and well-developed coordination chemistry
with a range of metals.19 Recently, research in this group has resulted in a number of publications
on the use of chiral bisphosphine monosulfides and --monoselenides as supporting ligands in the
preparation of catalytically active coordination and organometallic compounds.20,21 It is apparent
that such bisphosphine monochalcogenides have some analogy to the mixture of tri-n-
octylphosphine selenide and tri-n-octylphosphine used in the original solution phase synthesis of
cadmium selenide quantum dots. They comprise a phosphine and a phosphine chalcogenide and
therefore a synthesis of cadmium selenide quantum dots was attempted using in the first instance
bisphosphine monoselenides as the selenium donors.
Many bisphosphines are available commercially with, correspondingly, a wide range of
bisphosphine monoselenides potentially available, though the number of these monoxidized
species reported is at present somewhat smaller. As a beginning point for this study,
bis(diphenylphosphino)methane monoselenide (DPPMSe) was taken as the prototypical
bisphosphine monoselenide because it is a well-known compound with an excellent synthesis
available in the literature18,22 and because both the selenide and the precursor bisphosphine are
stable, crystalline solids that are easy to manipulate and store. In addition, the precursor
bisphosphine is commercially available, inexpensive and has none of the toxicity, flammability
or stench issues associated with alkyl phosphines, such as tri-n-octylphosphine. Equally, the
corresponding monosulfide (DPPMS) is also readily prepared. The other phosphine selenides
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studied were 1,4-bis(diphenylphosphino)butane monoselenide (DPPBSe) and triphenylphosphine
selenide (TPPSe), the former can be prepared by an analogous route to that used for the
preparation of DPPMSe and the latter is commercially available.
Synthesis and Characterization of Quantum Dots
The synthetic protocol that emerged in this study used elements taken from two literature
syntheses, one used for the preparation of lead selenide quantum dots23 and the other for
cadmium selenide quantum dots. The initial step in the reaction called for the preparation of
cadmium oleate as a diphenyl ether (0.50 mL) solution by heating cadmium acetate hydrate
(0.100mmol) with oleic acid (0.400 mmol) at 170 C under a stream of nitrogen for thirty
minutes to remove the acetic acid and water produced. This is in direct analogy to the procedure
used to prepare lead oleate reported by Gaschler.23 The solution was then cooled to 70 C before
the addition of a solution of DPPMSe (0.250 mmol) and hexadecylamine (0.041 mmol)
dissolved in a further 1.5 mL of diphenyl ether. This mixture was then heated to 180 C until the
color evolution, from colorless to deep red, was completed (60 min.). The reaction mixture was
cooled to 70 C, methanol (1.00 mL) was added and the solid red product formed was collected
by centrifugation (Method A). This material showed an absorbance feature at 487 nm and an
emission peak at 517 nm when dispersed in hexanes, Figure 1, appearing red-orange. Electron
microscopy shows particles with diameters ~ 40-50 Å, which is consistent with the wavelength
In order to track the evolution of the product, the reaction was sampled at two-minute intervals
and the emission spectra of the samples recorded, Figure 2. Initially the emission spectrum
shows what appear to be two maxima but this quickly disappears and the maximum drifts
modestly to longer wavelengths as the reaction time continues.
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In order to investigate the nature of the surface decoration a chloroform suspension of the
fluorescent material obtained from these reactions was treated with an excess of 4-
dimethylaminopyridine. This leads to the release of a number of phosphorus-containing species
identified by 31P NMR, including DPPM, DPPMO, DPPMSe and DPPMSeO. This observation
suggests that the phosphine has a number of roles in the stabilization of the nanoparticles formed
in addition to acting as the vector delivering the selenium.25 The redistribution of oxygen and
selenium between the phosphine, the phosphine selenide and the carboxylate is strongly
analogous to that reported for lead chalcogenide systems.26
Repeating the experiment (Method B) using DPPMS in place of DPPMSe resulted in a
reaction that is similar in all respects except that the reaction requires the higher temperature of
200 C. The ultimate color of the reaction mixture is yellow-orange rather than red and under
fluorescence, the sample emits yellow-green with the observed emission maximum at 532 nm.
A series of experiments were conducted using n-tetradecylphosphonic acid in place of the
hexadecylamine. Using 0.034 mmol of the phosphonic acid and using the same cadmium oleate
solution and DPPMSe leads to the formation of a comparable red color in the reaction mixture
upon heating to the higher temperature of 300C for sixty minutes (Method C). This material
absorbs at 507 nm and fluoresces at 515 nm.
Repeating this experiment (Method C) heating for ninety minutes leads to a material that
absorbs at 500 nm and emits at 506 nm. The relatively minor change in size with time indicated
by the observed maxima indicates a general lack of Ostwald ripening in both the selenide and
sulfide preparations, as has been observed with relatively lower temperature (240 C)
preparations using cadmium bis(diphenyldithiophosphinate).27 Reducing the quantity of n-
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tetradecylphosphonic acid to 0.017 mmol resulted in an intensely-colored brown product which
showed a very broad feature in the absorbance at 532 nm and no discernible emission spectrum.
The reaction also works well with other phosphine selenides as the selenium source. Repetition
of the experiment using DPPBSe in place of DPPMSe according to Method C results in a sample
that shows an absorbance at 521 nm and an emission maximum at 535 nm. Repeating Method C
again with a mixture of triphenylphosphine (TPP) (0.30 mmol) and TPPSe (0.30 mmol) leads to
no observed fluorescent product and neither changing the heating regime nor the molar ratio
TPP/TPPSe yields any such product. Using only TPPSe (0.30 mmol) in Method C in place of the
DPPMSe, however, results in the rapid formation of a product with an absorbance maximum of
474 nm and an emission maximum at 486 nm. This suggests a smaller size of particle perhaps
due to the presence of a different phosphine and phosphine selenide set. This result and the
DPPBSe result both indicate an important role for the phosphine, perhaps in conjunction with the
phosphine selenide, in controlling the product formation. This observation consistent with recent
results that link the size of dots formed to the rate of formation which varies with different
phosphines used in the synthesis28 The composition and morphology have also been linked to
phosphine-chalcogenide reactivity.29,30
These preliminary results show that cadmium selenide and sulfide quantum dots can be
prepared using the corresponding bisphosphine monochalcogenides and cadmium oleate
prepared in situ at a comparatively low temperature. The properties of the quantum dots vary
somewhat with the bisphosphine used which suggests, given that there is a wide range of
bisphosphines available, that there may be equally a range of quantum dots with different
characteristics available from this synthesis.
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ABBREVIATIONS
DPPM(Se)
DPPM(S)
DPPB(Se)
bis(diphenylphosphino)methane selenide
bis(diphenylphosphino)methane sulfide
1,4-bis(diphenylphosphino)butane selenide
TPP triphenylphosphine
TPP(Se)
triphenylphosphine selenide.
Acknowledgements
The authors thank Dr. Fairland Amos for the electron micrographs. Financial support was
provided by the Yale Initiative for Quantum Engineering (YINQUE) program.
AUTHOR INFORMATION
Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval
to the final version of the manuscript.
Funding Sources: Financial support was provided by the Yale Initiative for Quantum
Engineering (YINQUE) program.
ACKNOWLEDGMENT
The authors thank Dr. Fairlane Amos for assistance with the electron micrographs.
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Figure 1. The absorption (blue line) and emission spectrum (red line) for the cadmium selenide
quantum dots prepared following of the emission maximum.24 method A. The spectra were
recorded on samples dispersed in hexanes and the intensities normalized for ease of
visualization.
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Figure 2. The emission spectra for the cadmium selenide quantum dots prepared following
method A as a function of reaction time. The spectra were recorded on samples dispersed in
hexanes and the intensities again normalized for ease of visualization.
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