Formation of group 12 [Zn, Cd] mixed-chalcogen nanoparticles from the reagent Me3Si-SeS-SiMe3

Article abstract of DOI:10.1139/V07-083

Mixed-chalcogen metal chalcogenide nanoparticles (MSe_xS _1-x; M = Zn, Cd) have been synthesized using Me₃Si-SeS- SiMe₃ as a delivery source of Se^2- and S^2- to the metal core. This method demonstrates the ease with which mixed-chalcogen particles can be fabricated at low temperature using colloidal techniques. Reaction with Me₃Si-SeS-SiMe₃ occurs via a redox pathway resulting in Se-S bond cleavage and ultimately contributing to the nonequivalent Se:S ratio observed in the isolated particles. Subsequent thermolysis of ZnSe_0.57S_0.43 and CdSe_0.28S_0.72 in hexadecylamine gives rise to controlled particle growth while maintaining the observed stoichiometry. Particles are characterized by EDX, TEM, and powder X-ray diffraction analysis in conjunction with UV-vis absorption and photoluminescence (PL) spectroscopy.

Full text of DOI:10.1139/V07-083

747
Formation of group 12 [Zn, Cd] mixed-chalcogen
1
nanoparticles from the reagent Me3Si-SeS-SiMe3
Elizabeth A. Turner, Harald Rösner, Yining Huang, and John F. Corrigan
Abstract: Mixed-chalcogen metal chalcogenide nanoparticles (MSexS1-x; M = Zn, Cd) have been synthesized using
Me3Si-SeS-SiMe3 as a delivery source of Se2– and S2– to the metal core. This method demonstrates the ease with
which mixed-chalcogen particles can be fabricated at low temperature using colloidal techniques. Reaction with
Me3Si-SeS-SiMe3 occurs via a redox pathway resulting in Se–S bond cleavage and ultimately contributing to the non-
equivalent Se:S ratio observed in the isolated particles. Subsequent thermolysis of ZnSe0.57S0.43 and CdSe0.28S0.72 in
hexadecylamine gives rise to controlled particle growth while maintaining the observed stoichiometry. Particles are
characterized by EDX, TEM, and powder X-ray diffraction analysis in conjunction with UV–vis absorption and
photoluminescence (PL) spectroscopy.
Key words: nanoparticles, semiconductors, mixed-chalcogen, quantum confinement, Group 12.
Résumé : On a effectué la synthèse de nanoparticules mixtes comportant un chalcogène et un chalcogénure métallique
(MeSexSl-x; M = Zn, Cd) en utilisant le Me3Si-SeS-SiMe3 comme source pouvant livrer les groupes Se2– et S2– au
métal central. On a fait une démonstration de cette méthode par la facilité avec laquelle des particules de chalcogènes
mixtes peuvent être fabriqués à basse température en faisant appel à des techniques colloïdales. La réaction avec le
Me3Si-SeS-SiMe3 se produit par une voie réactionnelle redox qui provoque un clivage de la liaison Se-S et qui con-
tribue éventuellement au rapport non équivalent de Se:S qui est observé dans les particules qui ont été isolées. Une
thermolyse subséquente du ZnSe0,57S0,43 et du CdSe0.28S0.72 dans l’hexadécylamine conduit à une croissance contrôlée
des particules tout en maintenant la stoechiométrie observée. On a caractérisé les particules par le biais de la diffrac-
tion des rayons X par des poudres et les techniques « EDX » et « TEM » en relation avec les spectres d’absorption
UV–vis et la spectroscopie de photoluminescence (PL).
Mots-clés : nanoparticules, semiconducteurs, chalcogènes mixtes, confinement quantique, groupe 12.
[Traduit par la Rédaction] Turner et al. 755
Introduction
band gap becomes tunable to a desired particle size. As a re-
sult, group 12–16 nanomaterials have found potential appli-
cations in the fabrication of new devices such as LEDs,
photovoltaic, and photoelectrochemical devices (5). The
control of particle size and the efficient delivery of
chalcogen (E2–) to the metal core, however, are two chal-
lenging aspects of nanomaterial synthesis. One means of
managing these two synthetic obstacles is through the use of
trimethylsilyl-chalcogenide reagents as an easily handled
chalcogen source in conjunction with passivating the metal
core with long alkyl chain amine (phosphine) ligands,
thereby preventing possible aggregation (6).
Metal-chalcogenides (ME; M = Zn, Cd, Hg; E = S, Se,
Te; or M2E; M = Cu, Ag) have been of tremendous interest
over the past several decades (1). In particular, an extensive
amount of research has surrounded both the synthesis
(2) and characterization (3) of group 12–16 nanomaterials.
Upon constricting the dimensions of a given material to the
nanometer-size regime, quantum confinement effects pre-
dominate such that electronic, physical, and optical proper-
ties become size specific (4). Ultimately, for
semiconductors, these phenomena give rise to an increase in
band gap energy from that of the bulk material, where the
One of the pioneering methods in controlling particle size
is what is commonly referred to as the controlled arrested
precipitation method from colloidal solutions where dilute
aqueous mixtures of M2+ and E2– species are combined to-
gether (7). This method was further extended into what is
considered to be one of the most notable advances in the for-
mation of discrete nanoparticles, the TOP or TOPO
(trioctylphosphine or trioctylphosphine oxide) pyrolysis
method. The rapid injection of sources of both M2+ and E2–
species into a hot coordinating solvent facilitates the devel-
opment of nanoparticles (8). An extension of this method
has been to inject metal clusters, where the metal-chalcogen
bond is already prepositioned, into a hot hexadecylamine
(HDA) solution (9). By increasing the reaction temperature,
Received 17 May 2007. Accepted 21 June 2007. Published on
the NRC Research Press Web site at canjchem.nrc.ca on
3 August 2007.
E.A. Turner, Y. Huang2, and J.F. Corrigan.3 Department
of Chemistry, The University of Western Ontario, London,
ON N6A 5B7, Canada.
H. Rösner. Institut für Nanotechnologie, Forschungszentrum
Karlsruhe GmbH, 76344 Eggenstein-Leopoldshafen, Germany.
1This article is part of a Special Issue dedicated to Professor
G. Michael Bancroft.
2Corresponding author (e-mail: yhuang@uwo.ca).
3Corresponding author (e-mail: corrigan@uwo.ca).
Can. J. Chem. 85: 747–755 (2007)
doi:10.1139/V07-083
© 2007 NRC Canada

748
Can. J. Chem. Vol. 85, 2007
particles are found to grow via Ostwald ripening (10) where
the more soluble smaller particles grow in an effort to form
larger particles (11). The large surface area to volume ratio
of the smaller particles promotes the number of energetically
less stable surface molecules, and larger particles having a
greater volume to surface area ratio are of a lower energy
state and thus preferentially form at the expense of smaller
particles (12).
sequent thermolysis of the zinc and cadmium mixed-
chalcogen nanoparticles in hot hexadecylamine is found to
provide controlled particle growth. The resulting
nanoparticles are analyzed by UV–vis absorption and PL
spectroscopy, in addition to TEM, powder X-ray diffraction,
and EDX analysis.
Experimental
Manipulating particle size is one means of dictating their
physical properties. An additional degree of freedom can be
gained by substituting some metal (or chalcogen) atoms with
another metal (or chalcogen), thus forming ternary MM′E (or
Materials
All synthetic procedures and manipulations were per-
formed under an inert nitrogen atmosphere using standard
Schlenk techniques and gloveboxes, unless otherwise noted.
Toluene, pentane, and hexanes (all from Caledon) were dried
and collected using an MBraun MB-SPS solvent purification
system with tandem activated alumina – activated copper re-
dox catalyst (22). Dichloromethane (EMD) was dried and
distilled over P2O5. Anhydrous methanol was purchased
from Caledon. Cadmium acetate dihydrate (98%), anhydrous
zinc acetate (99.99%), N,N,N′,N′-tetramethylethylenediamine
(TMEDA), n-butyl lithium in hexanes (1.6 mol/L), tri(n-
butyl)phosphine, and hexadecylamine (HDA) were pur-
chased from Sigma-Aldrich. Sulfur (99%) was purchased
from EM Science. Cadmium acetate dihydrate was dried un-
der vacuum (10–3 torr; 1 torr = 133.322 4 Pa) at 130 °C and
stored under inert conditions. TMEDA was dried and dis-
tilled over a lump of sodium and stored under inert condi-
tions. The reagent Se(SiMe3)2 was prepared according to
literature procedures (13c, 23).
′
MME ; M ≠ M′ and E ≠ E′) materials. These materials ex-
hibit physical properties whose energies are intermediate be-
tween those of ME and M′E (or ME′). Recent studies have
focused on the synthesis and characterization of group 12–
12′–16 nanoclusters and nanoparticles through the use of
trimethylsilyl-chalcogenide reagents (13). The formation of
12–16–16′ nanomaterials, however, remains relatively unex-
plored (vide infra).
Only a few known examples exist where discrete quantum
dots, rather than thin films, of mixed-chalcogen cadmium
materials have been synthesized. Quantum dots and tetrapod
nanocrystals of CdSexTe1-x (14) have been synthesized from
cadmium salts and stock solutions of selenium and tellurium
using either TOP, TOPO, or stearic acid as surface stabiliz-
ing ligands. Homogeneously alloyed zinc-blende CdSexS1-x
(15) nanocrystals have also been prepared from cadmium
oxide, oleic acid, and stock solutions of elemental sulfur and
selenium, added in varying proportions. Discrete particles
were observed using transmission electron microscopy
(TEM) and chemical homogeneity was demonstrated by
powder X-ray diffraction (PXRD). By changing the ratio
S:Se, the photophysical properties could be altered such that
an increasing selenium content leads to a shift to lower en-
ergy of the excitonic transition, as observed in the absorption
spectra. The majority of reports on mixed-chalcogen ternary
alloys, however, involve the preparation of CdSexS1-x thin
films typically on glass, quartz, GaAs, and silicon. A typical
procedure involves the mixture of CdS and CdSe powders
combined in a desired ratio (16). Other methods employ
CdCl2, thiourea and sodium selenosulfate as precursors (17),
while the elemental forms of cadmium, sulfur, and selenium
can also be combined with a desired stoichiometry in a glass
melt (18). An extension to this synthesis involves the use of
elemental sulfur and selenium with cadmium salts (19),
while other procedures use a combination of metal salts with
various chalcogen precursors (i.e., H2E and CS(NH2)2) (20).
Regardless of the choice of reagents, the methods involve
specialized techniques such as chemical bath deposition,
molecular beam epitaxy, or pulsed laser ablation, all of
which require extremely high temperatures. The deposited
materials typically have nanometer dimensions, where exam-
ples of nanowires and nanoribbons have been formed on a
Si[111] substrate (21).
Synthesis of TMEDA Li+ –SeSiMe3
In a Schlenk tube with no stir bar, (SiMe3)2Se (0.15 mL,
0.7 mmol) and TMEDA (0.1 mL, 0.7 mmol) were added to
tetrahydrofuran (10 mL) and cooled to 0 °C, producing a
light brown solution. n-Butyl lithium (0.44 mL, 0.7 mmol)
was added to the reaction, upon which the solution immedi-
ately became colourless, and the reaction was kept at 0 °C
for 30 min with periodic mixing via agitation. A stir bar was
then added to the solution and the reaction was warmed to
RT with stirring. The solvent was removed under vacuum
(10–3 torr for one hour) producing a colourless oil. The an-
ion was used immediately in the subsequent chalcogen inser-
1
tion as the product has limited stability at RT. H NMR
(C7D8, 203K) δ: 0.72 (s, Si(CH3)3), 1.64 (br s, CH2), 2.11 (s,
CH3). 13C{1H} (C7D8, 203 K) δ: 8.41 (Si(CH3)3), 47.08
(CH2), 56.11 (CH3). 29Si NMR (C7D8, 203 K) δ: 1.1. 77Se
NMR (C7D8, 203 K) δ: –520.
Synthesis of TMEDA Li+ –SSeSiMe3
In a Schlenk tube, sulfur (0.023 g, 0.7 mmol) was sus-
pended in pentane (10 mL) and cooled to -78 °C. Toluene
(5 mL) was added to the lithio trimethylsilyl selenolate an-
ion (0.7 mmol) and the solution was cooled to –78 °C. The
anion solution was rapidly added to the sulfur suspension at
–78 °C upon which a slightly yellow solution developed.
The reaction was removed from the cold bath until a bright
yellow cloudy solution resulted. The solution was then im-
mediately cooled again to –78 °C and allowed to stir for an
additional 20 min. The product was used in situ as removal
of the solvent leads to product decomposition; in addition,
the lithio trimethylsilyl selenothiolate anion must be used
Herein we report the synthesis of MSexS1-x (M = Zn, Cd)
nanoparticles formed from the reagent Me3Si-SeS-SiMe3.
The highly reactive nature of this reagent allows for the fac-
ile low temperature delivery of both sulfur and selenium to
the corresponding metal acetate. The resulting nanoparticles
are easily formed in solution at room temperature (RT). Sub-
© 2007 NRC Canada

Turner et al.
749
immediately as it quite reactive to both air and elevated tem-
peratures. 1H NMR (C7D8, 203 K) δ: 0.31 (s, Si(CH3)3), 1.58
(br s, CH2), 2.12 (s, CH3). 13C{1H} (C7D8, 203 K) δ: 1.22
(Si(CH3)3), 48.55 (CH2), 56.00 (CH3). 29Si NMR (C7D8,
203K) δ: 15.3. 77Se NMR (C7D8, 203 K) δ: –96.
Characterization
1
Solution H, 13C{1H}, and 77Se NMR spectra were re-
corded on a Varian Inova 400 spectrometer with an operat-
ing frequency of 399.76, 100.53, and 76.24 MHz
respectively. 29Si NMR spectra were obtained indirectly as
gradient heteronuclear multibond coupling (g-HMBC) ex-
periments to proton (JSi–H = 6 Hz). H and 13C{1H} NMR
1
Synthesis of Me3Si-SeS-SiMe3
spectra were referenced internally to the respective
deuterated solvent as well as externally to TMS, 29Si NMR
spectra were referenced to silicon grease (–20 ppm), and
77Se NMR spectra were referenced externally to PhSeSiMe3
(+85.6 ppm) (24). Powder X-ray diffraction (PXRD) pat-
terns were obtained using a Rigaku diffractometer with a Co
Kα radiation source (λ = 1.799260 Å). UV–vis absorption
spectra for ZnSexS1-x nanoparticles were recorded on an
Ocean Optics SD2000 UV–vis fiber optic spectrometer
equipped with a Mini-D2T light source and a UV2/OFLV-4
detector. The spectra were obtained as a mineral oil mull be-
tween two quartz plates. UV–vis absorption spectra of the
CdSexS1-x nanoparticle series were recorded on a Varian
Cary 100 UV–vis absorption spectrometer. The spectra were
obtained in a CH2Cl2 solution. Luminescence measurements
were obtained using a Photo Technology International
fluorimeter equipped with an LPS 220B lamp power supply
and an 814 photomultiplier detection system. Energy
dispersive X-ray (EDX) analyses were carried out by Dr.
Brian Hart at Surface Science Western (UWO). A Quartz
Xone EDX analysis system coupled to a Leo 440 SEM
equipped with a Gresham light element detector was used to
obtain semiquantitative analysis of Zn, Cd, S, and Se. Anal-
yses were carried out using a 20 kV electron beam rastered
over 100 µm × 100 µm areas and repeated to ensure
reproducibility. Samples for high-resolution transmission
electron microscopy (HR-TEM) were obtained by dispersing
the solid in CH2Cl2 and dropping these dilute suspensions on
gold-plated grids, allowing the excess solvent to evaporate.
TEM investigations were performed in a Philips Tecnai F20
ST operated at 200 kV. The TEM was equipped with an
EDAX Energy-Dispersive X-ray SiLi detector with a S-
UTW (super ultra-thin window). EDX analyses were carried
out in the STEM mode with a HAADF (high angle annular
dark field) detector using a nanometer-sized probe (1 nm
spot size for the presented measurements). Use of an ana-
lytic double tilt holder (Philips) was made. High-resolution
micrographs were taken with a 1K × 1K CCD camera and
analyzed with the software package Digital Micrographs
(Version 3.7.4, Gatan Company) to perform fast Fourier
Transformations (FFT).
An excess of chlorotrimethylsilane (0.2 mL, 1.4 mmol)
was added to lithio trimethylsilyl selenothiolate anion
(0.7 mmol) at –78 °C. The reaction was allowed to gradually
warm to RT upon which the cloudy solution went through a
clear bright yellow stage followed by the development of a
cloudy yellow suspension. Upon achieving this stage the re-
action was immediately cooled to –78 °C and kept cold until
further use. The product cannot be isolated as removal of the
solvent leads to rapid decomposition. Furthermore, the gen-
erated lithium chloride must be added to subsequent reac-
tions as filtration of the solution also leads to decomposition.
The product is exceptionally reactive to air and has only
1
short-term stability at RT. H NMR (C7D8, C5D12, 218 K) δ:
0.26 (SSi(CH3)3), 0.34 (SeSi(CH3)3). 13C{1H} NMR (C7D8,
C5D12, 218 K) δ: 1.27 (SeSi(CH3)3), 3.86 (Si(CH3)3). 29Si
NMR (C7D8, C5D12, 218 K) δ: 15.9 (SeSi(CH3)3), 16.4
(SSi(CH3)3). 77Se NMR (C7D8, C5D12, 218 K) δ: –100.
Synthesis of CdSexS1-x nanoparticles
Anhydrous cadmium acetate (0.320 g, 1.4 mol) was
solubilized with 2.2 equiv. of tri(n-butyl)phosphine
(0.76 mL, 3.05 mol) in toluene (30 mL), producing a turbid
solution. After stirring at RT for 1 h, a solution of Me3Si-
SeS-SiMe3 (1.4 mol) was added and the reaction was stirred
at RT overnight, resulting in the precipitation of a pale yel-
low solid. The solid was isolated via removal of the mother
liquor, followed by washing (3 × 10 mL methanol), sonica-
tion for 15 min at 45 °C, and centrifugation. The resulting
solid was dried under vacuum (10–3 torr) for 1 h.
Synthesis of ZnSexS1-x nanoparticles
Anhydrous zinc acetate (0.255 g, 1.4 mol) was solubilized
with 2.2 equiv. of tri(n-butyl)phosphine (0.76 mL, 3.05 mol)
in CH2Cl2 (10 mL). After stirring at RT for one hour, Me3Si-
SeS-SiMe3 (1.4 mol) was added and the reaction was stirred
at RT overnight. A resulting white solid precipitated from
solution and was isolated via removal of the mother liquor
and washing with hexanes (3 × 15 mL), followed by drying
under vacuum (10–3 torr) for 1 h.
Thermolysis of MSexS1-x in hexadecylamine
To the obtained MSexS1-x [M = Zn, Cd] nanoparticles,
hexadecylamine (HDA; 20 g, 82.8 mmol) was added to the
flask and heated to 60 °C, until all of the HDA had melted.
The solution was then degassed under vacuum (10–3 torr) for
20 min. The mixture was then heated to 240 °C, where at
specified temperature intervals an aliquot was taken. At each
temperature the reaction was annealed for 15 min, an aliquot
was taken, washed with anhydrous methanol (15 mL) thus
precipitating the resulting particles, centrifuged at 45 °C,
and washed again with methanol. The obtained solids were
dried under vacuum (10–3 torr) for 15 min and stored in a
glovebox.
Results and discussion
Synthesis of mixed-chalcogen nanoparticles
Trimethylsilyl-chalcogen reagents (RESiMe3, E(SiMe3)2;
R = alkyl, aryl, ferrocenyl) are valuable in the formation of
structurally characterized nanoclusters (25) and nano-
particles (26). The reactive pendant trimethylsilyl moiety al-
lows for the facile and homogeneous delivery of either
chalcogenolate (RE–) or chalcogenide (E2–) bonding interac-
tions to a metal salt via the generation and elimination of the
corresponding trimethylsilane (27). Although the synthesis
of binary and ternary mercury chalcogenides has been a syn-
© 2007 NRC Canada

750
Can. J. Chem. Vol. 85, 2007
thetically challenging area (28), one recent example involves
the formation of HgSexS1-x particles from both S(SiMe3)2
and Se(SiMe3)2 and mercury acetate, using TOP in a phase
separation method, to control particle growth (29). Particles
varying in size from 2 to 3 nm were formed.
by calculating the average based on the atomic percentages
for the corresponding elements. Table 1 clearly demonstrates
the nondeviating stoichiometry beginning with the as-
synthesized particles and those obtained through thermolysis
in HDA. The ratio between Se and S in the isolated particles
is not 1:1 although a stoichiometrically precise reagent is
used in the synthesis. The reactivity of Me3Si-SeS-SiMe3
however, is expected to occur with Se–S bond cleavage as
has previously been established with the synthesis of a
[Cu84Se27S15(PEt2Ph)24] cluster (30). It is necessary to note
that the stoichiometry is reproducible and is a reflection of
the isolated particles.
The growth in particle size is surveyed using HR-TEM
microscopy. Shown in Fig. 1 are the HR-TEM images for as-
synthesized, 180 °C, 210 °C, and 240 °C lyothermally
grown particles. At 180 °C the particles are found to be
~3.5 nm in diameter, while an increase in thermolysis tem-
perature to 240 °C leads to particles of ~9 nm in diameter.
The d spacing, as determined by TEM (3.54 Å), is relatively
consistent with the d spacing associated with the (1 1 1) re-
flection in the powder X-ray diffraction pattern (3.40 Å).
The photophysical properties of the obtained
nanoparticles were analyzed using UV–vis absorption and
photoluminescence (PL) spectroscopy. The UV–vis absorp-
tion spectra of the thermolysed samples (Fig. 2) distinctly
display a red shift in absorption maximum upon increasing
thermolysis temperature. At a thermolysis temperature of
160 °C, the absorption maximum is observed at 445 nm
(2.79 eV) and shifts by over 100 nm to lower energy upon
increasing the thermolysis temperature to 240 °C (560 nm;
2.21 eV). This shift in absorption maximum is consistent
with an increase in particle size upon increasing thermolysis
temperature. The observed absorption maxima are consistent
with what has previously been reported for CdSexS1-x having
a similar stoichiometry and particle size (15).
The low- and high-energy PL spectra for this series is
shown in Fig. 3, where the spectra were obtained by irradiat-
ing samples at 468 nm. As is illustrated in Fig. 3a, the emis-
sion maximum shifts from 488 nm (2.54 eV) to 590 nm
(2.10 eV) upon increasing thermolysis temperature and is as-
sociated with band-edge emission. At lower energy, two
types of emission are observed (Fig. 3b) that, although nor-
malized in the figure, are found to increase in intensity (rela-
tive to the high energy emission) as the thermolysis
temperature is raised. The emission observed at lower en-
ergy is much broader in comparison to the sharp band-edge
emission and results from deep-trap emission coinciding
with nanoparticle defects. The appearance of both band-edge
and deep-trap emission is not uncommon for CdSexS1-x sys-
tems, although several reported examples have developed
techniques thereby eliminating deep-trap emission (16–21).
One method is to prolong the thermal annealing time, thus
forcing defects within the nanoparticle to move to the sur-
face creating a defect-free nanoparticle core. This was inves-
tigated for the sample prepared at 230 °C, where, rather than
annealing for 15 min, the thermolysis temperature was held
for 4 h. Based on the PL spectrum (not shown) after 4 h,
Traditional trimethylsilyl-chalcogenide reagents (i.e.,
E(SiMe3)2; E = S, Se, Te) are well-known and extensively
used in synthesis, however, there is recent interest in synthe-
sizing silylated-chalcogen reagents containing two
chalcogen elements, whereby both E2– and E′2– are supplied
to the metal core. Recently we have communicated the syn-
thesis and characterization of the first mixed-chalcogen
silylated reagent Me3Si-SeS-SiMe3 and demonstrated its use
in the formation of a [Cu84Se15S27(PEt2Ph)24] cluster (30).
Bis(trimethylsilyl)selenosulfide [Me3Si-SeS-SiMe3] is a
highly reactive reagent, for which both exposure to atmo-
sphere and elevated temperatures leads to rapid decomposi-
tion. In addition, the reagent cannot be isolated and can only
be synthesized in dilute solutions. Lithium chloride gener-
ated in the formation of Me3Si-SeS-SiMe3 cannot be re-
moved via filtration and must be carried forward in
subsequent syntheses. Although Me3Si-SeS-SiMe3 is not a
storable precursor, its low temperature stability is ideal for
the generation of MSexS1-x (M = Zn, Cd) nanoparticles at RT
vs. the typically explored high temperature methods.
CdSexS1-x nanoparticles
Given the sensitivity of Me3Si-SeS-SiMe3, cadmium ace-
tate [Cd(OAc)2] can only be introduced in a select number
of solvents. The addition of Me3Si-SeS-SiMe3 at RT to a to-
luene solution of (PnBu3)2Cd(OAc)2 results in the immediate
consumption of Me3Si-SeS-SiMe3, as noted by the disap-
pearance of the yellow colour associated with the
selenosulfide, in conjunction with the complete
solubilization of Cd(OAc)2. This reactivity is then followed
by the precipitation of a colourless solid from solution,
which gradually develops into a pale yellow colour after stir-
ring overnight. The resulting solid is easily isolated where
washing with anhydrous methanol effectively removes any
LiCl. Isolated particles were characterized by HR-TEM,
however, given their poor solubility a UV–vis absorption
spectrum could not be obtained.
To investigate the growth of the resulting CdSexS1-x
nanoparticles in a coordinating solvent, the solid was added
to a hot (60 °C) solution of hexadecylamine (HDA) and
heated to 240 °C. Upon increasing the temperature, the sam-
ple colour changes from pale yellow to an intense red (Sup-
plementary data, Fig. S1).4 At specified temperature
intervals the sample is annealed (where upon there is no cor-
responding change in colour) and an aliquot is taken. The re-
sulting nanoparticles are isolated by precipitating from
solution using anhydrous methanol.
Lyothermal growth of the nanoparticles results in an in-
crease in particle size without altering the chemical compo-
sition (i.e., the ratio between Cd:Se:S). EDX (energy
dispersive X-ray) analysis was performed on three separate
areas of each sample, where the overall ratio was determined
4 Supplementary data for this article are available on the journal Web site (canjchem.nrc.ca) or may be purchased from the Depository of Un-
published Data, Document Delivery, CISTI, National Research Council Canada, Ottawa, ON K1A 0R6, Canada. DUD 5193. For more in-
formation on obtaining material refer to cisti-icist.nrc-cnrc.gc.ca/irm/unpub_e.shtml
© 2007 NRC Canada

Turner et al.
751
Table 1. Cd:Se:S ratios as determined by EDX analysis. Ratios
Fig. 2. UV–vis absorption spectra of thermolysed CdSexS1-x
were calculated using atomic percentages.
nanoparticle series. The spectra have been normalized for clarity.
Ratio as determined by EDX analysis
Cadmium
Selenium
Sulfur
As-synthesized
160 °C
180 °C
200 °C
210 °C
220 °C
230 °C
240 °C
1
1
1
1
1
1
1
1
0.31
0.34
0.32
0.31
0.30
0.31
0.31
0.30
0.57
0.57
0.57
0.56
0.56
0.56
0.56
0.54
Fig. 1. HR-TEM images of (a) as-synthesized CdSexS1-x and
HDA capped particles thermolysed at (b) 180 °C, (c) 210 °C,
and (d) 240 °C. Box outlines individual particles.
Fig. 3. Photoluminescence spectra of the CdSexS1-x thermolysis
series at (a) high energy and (b) low energy. Excitation at
468 nm. The spectra have been normalized for clarity.
prolonged annealing times minimized the low energy emis-
sion but did not all together eliminate it.
Spectroscopically, the two types of deep-trap emission be-
have differently. The higher energy emission maximum is
red-shifted upon increasing thermolysis temperature from
620 nm (2.00 eV) at 160 °C to 770 nm (1.61 eV) at 240 °C.
The lower energy emission however only begins to appear at
200 °C and remains relatively unaltered in peak position
upon increasing thermolysis temperature (~790 nm;
1.57 eV). Conversely, the intensity of this low energy emis-
sion greatly increases upon raising the thermolysis tempera-
ture, where at 240 °C this is the predominant emission
observed. Photoluminescence excitation (PLE) measure-
ments confirm the single species nature of the observed
emission (Fig. S2).4 Previous work with CdS and CdSe
nanocrystals have described a similar spectroscopic behav-
iour where the higher energy emission is associated with
shallow traps, while the lower energy emission results from
deep traps (7c, 31).
peak positions remain unchanged with increasing
thermolysis temperature, which is consistent with the chemi-
cal composition remaining unaltered. Second, the diffraction
peaks become sharper as the thermolysis temperature is
raised, consistent with the observed particle growth. The ob-
served PXRD pattern is characteristic of the cubic phases of
CdS and CdSe (32). Thus using the (1 1 1) reflection, the
lattice parameter a can be calculated using eq. [1]. From
The powder X-ray diffraction (PXRD) patterns for
CdSexS1-x (Fig. 4) thermolysed at 200 °C, 220 °C, and
240 °C display two important features. First, the relative
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Can. J. Chem. Vol. 85, 2007
Fig. 4. Powder X-ray diffraction patterns of CdSexS1-x samples
thermolysed at (a) 200 °C, (b) 220 °C, and (c) 240 °C. Asterisk
(*) Denotes (1 1 1) reflection.
As previously reported (30), the use of Me3Si-SeS-SiMe3
imposes limitations with regards to solvent choice and reac-
tion conditions that must be considered when designing a
chemical reaction. Employing similar conditions used in the
formation of CdSexS1-x, zinc acetate [Zn(OAc)2] was
solubilized in toluene with 2.2 equiv. of tri(n-butyl)phos-
phine. Toluene, however, does not effectively solubilize
Zn(OAc)2 and Pn-Bu3. As opposed to using phosphine-based
ligands to coordinate to Zn(OAc)2 in toluene, an amine
ligand (triethylamine) was alternatively employed. Although
this leads to the dissolution of Zn(OAc)2, the addition of
Me3Si-SeS-SiMe3 results in the formation of a brightly col-
oured red solid after a few hours of stirring, suggesting de-
composition and the possible precipitation of elemental
selenium. In contrast, tri(n-butyl)phosphine oxide is able to
solubilize Zn(OAc)2 in toluene, however, the addition of
Me3Si-SeS-SiMe3 leads to the immediate formation of a red
solid. This reactivity suggests an additional limitation of
Me3Si-SeS-SiMe3 in which hard bases such as amines and
oxides should be avoided, while soft bases such as
phosphines are less reactive toward Me3Si-SeS-SiMe3.
An alternative solvent to toluene is to use dichloro-
methane to solubilize Zn(OAc)2 and Pn-Bu3. The addition of
Me3Si-SeS-SiMe3 causes the immediate consumption of the
selenosulfide. Although a precipitate does not form initially,
continued stirring overnight results in the formation of a
white solid. The solid, however, is quite soluble in anhy-
drous methanol and thus LiCl cannot be removed at this
stage and must be carried through to the thermolysis.
The obtained solid is added to a hot (60 °C) HDA solution
and gradually heated to 240 °C. At specified temperature in-
tervals the sample is annealed for 15 min, an aliquot is
taken, precipitated out with anhydrous methanol, and iso-
lated via centrifugation. By washing with methanol, LiCl is
effectively removed from the sample as determined by EDX
analysis. The increase in thermolysis temperature does not
result in an associated colour change.
each PXRD pattern the d spacing of (1 1 1) is found to be
3.40 Å and a is calculated to be 5.89 Å, where the corre-
sponding lattice parameters for CdS and CdSe are 5.818 and
6.077 Å, respectively (33).
1
d2
h2 + k2 + l2
3
a2
[1]
=
=
a2
Using Vegard’s rule (34), which assumes a linear depend-
ence of composition on lattice parameter, the stoichiometry
of the obtained CdSexS1-x nanoparticle system can be deter-
mined using eq. [2],
To ensure that the chemical composition remains unal-
tered during the thermolysis, the atomic ratio between zinc,
selenium, and sulfur was determined from EDX analysis
(Table 2). As is shown in Table 2, the ratio between zinc, se-
lenium, and sulfur remains relatively unaltered throughout
thermolysis, however, at 240 °C the quantity of both sulfur
and selenium increases, although the ratio between the two
elements is similar to what is observed at the lower
thermolysis temperatures. The same ratio is not observed in
the as-synthesized ZnSexS1-x largely because of the presence
of chloride within the sample (from LiCl), which conse-
quently affects the overall ratio calculated for the respective
elements. Again, the ratio between selenium and sulfur is
not 1:1, likely due to the expected Se–S bond cleavage and
the presence of both phosphine sulfides and phosphine
selenides in the reaction mixture. Phosphine sulfides and
selenides are known to form from Pn-Bu3 with various
chalcogen sources (39). Both sulfur and selenium are how-
ever, incorporated into the nanoparticles through the use of
the Se–S reagent.
[2]
a(x) = x ⋅aCdSe + (1 − x)aCdS
Based on eq. [2], x is calculated to be 0.28 (i.e.,
CdSe0.28S0.72), which is similar to what is observed via EDX
analysis.
ZnSexS1-x nanoparticles
A handful of published examples have investigated the
synthesis of ZnSexS1-x nanomaterials. ZnSexS1-x (where x =
0.05–0.08) is almost perfectly lattice-matched to GaAs and
thus improves the blue-green response over the convention-
ally used ZnSe. Consequently, ZnSexS1-x is well-suited for
fabricating optoelectronic devices (35). Much like its cad-
mium counterpart, ZnSexS1-x is formed as thin films on ei-
ther glass or GaAs substrates. The synthesis typically
involves the mixing of ZnS and ZnSe powders in a specified
ratio (35, 36), although some procedures use dimethylzinc in
the presence of gaseous H2Se and H2S (37) as well as ZnCl2
with thiourea and selenourea (38). A variety of specialized
techniques such as spray pyrolysis, atomic layer epitaxy, and
close-space evaporation are used for thin film formation and
thus require high operating temperatures. Using Me3Si-SeS-
SiMe3, discrete nanoparticles of ZnSexS1-x can be produced.
Particle size measurements were calculated based on HR-
TEM images (Fig. 5). As can be seen, the particle size in-
creases upon increasing thermolysis temperature. At 200 °C
(Fig. 5a), individual particles of ZnSexS1-x are observed and
© 2007 NRC Canada

Turner et al.
753
Table 2. Zn:Se:S ratio as determined by EDX analysis. Ratios
Fig. 6. UV–vis absorption spectra of thermolysed ZnSexS1-x
were calculated using atomic percentages.
nanoparticle series. The spectra have been normalized for clarity.
Ratio as determined by EDX analysis
Zinc
Selenium
Sulfur
As-synthesized
190 °C
210 °C
220 °C
240 °C
1
1
1
1
1
0.47
0.43
0.40
0.40
0.52
0.36
0.49
0.44
0.48
0.59
Fig. 5. HR-TEM images of ZnSexS1-x particles thermolysed at
(a) 200 °C and (b) 240 °C. Box outlines individual particles.
Fig. 7. PL spectra of the ZnSexS1-x thermolysis series. Excitation
between 310 and 320 nm. The spectra have been normalized for
clarity.
are ~4 nm in diameter. Upon increasing the thermolysis tem-
perature to 240 °C, the particles grow to ~8 nm in diameter,
however, it appears as though homogeneity is lost at ele-
vated temperatures.
The spectroscopic properties of the obtained nanoparticles
were analyzed using UV–vis absorption and PL spectros-
copy. The UV–vis absorption spectra of the thermolysis se-
ries (Fig. 6) distinctly display a red-shift in absorption
maximum with increasing the thermolysis temperature. At a
thermolysis temperature of 190 °C, the absorption maximum
is found at 305 nm (4.07 eV) and shifts 30 nm to lower en-
ergy with increasing the thermolysis temperature to 240 °C
(335 nm, 3.70 eV). This shift in absorption maximum is
consistent with an increase in particle size.
The PL spectra (obtained by irradiating between 310–
320 nm) for the series ZnSexS1-x (Fig. 7) differs significantly
from the PL spectra of the CdSexS1-x series in that one type
of emission is observed, corresponding to band-edge emis-
sion. The emission maximum shifts from 342 nm (3.63 eV)
at 200 °C to lower energy (362 nm, 3.42 eV) with increasing
the thermolysis temperature to 240 °C. This red shift in
emission maximum upon increasing thermolysis temperature
confirms an increase in particle size. PLE measurements
confirm the nature of the emitting species (Fig. S3).4
Much like the PXRD patterns for CdSexS1-x, the PXRD
patterns for ZnSexS1-x thermolysed at 190 °C, 210 °C, and
220 °C (Fig. 8) indicate a sharpening of diffraction peaks
with increasing thermolysis temperature, in addition to con-
sistent peak positions. Conversely, the obtained ZnSexS1-x
nanoparticles display a characteristic hexagonal phase pat-
tern as observed for ZnS and ZnSe (40). Thus, using the
(0 0 2) reflection, the lattice parameter c can be calculated
(double the associated d spacing, d = 3.2 Å). The obtained c
parameter (6.4 Å) is found to be intermediate between the c
parameters for ZnS (6.234 Å) and ZnSe (6.53 Å) (37), and
using Vegard’s rule (eq. [2], substitute a for c), x is found to
be an intermediate value.
It is noteworthy to mention that the observed
stoichiometry in the zinc system varies significantly from
the stoichiometry of the cadmium system, thus reflecting a
differing reactivity between the two elements.
Conclusions
It has been demonstrated that Me3Si-SeS-SiMe3 is an ef-
fective delivery agent of both sulfur and selenium. As such,
the reagent has found utility in the synthesis of mixed-
chalcogen zinc and cadmium nanoparticles. Me3Si-SeS-
SiMe3 serves as a reagent for the distribution of both
chalcogen elements at low temperature vs. the conventional
methods used for the formation of MSexS1-x materials, which
typically require two chalcogen reagents and relatively high
temperatures. The resulting nanoparticles display
a
nonstoichiometric amount of both sulfur and selenium as the
ensuing chemistry with the corresponding metal acetate
leads to Se–S bond cleavage. Thermolysis of both zinc and
cadmium nanoparticle species in hexadecylamine leads to
controlled particle growth with increasing thermolysis tem-
© 2007 NRC Canada

754
Can. J. Chem. Vol. 85, 2007
Fig. 8. Powder X-ray diffraction patterns of ZnSexS1-x samples
thermolysed at (a) 190 °C, (b) 210 °C and (c) 220 °C. * De-
notes (0 0 2) reflection.
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perature, as has been noted by the red-shift in both the ab-
sorption and emission excitonic transition. An extension of
this research will be to investigate the effectiveness of
Me3Si-SeS-SiMe3 in the synthesis of HgSexS1-x
nanoparticles.
Acknowledgements
We gratefully acknowledge the Natural Sciences and En-
gineering Research Council (NSERC) of Canada for finan-
cial support of this research, equipment funding, and for a
postgraduate scholarship (EAT). The Government of Ontario
Premier’s Research Excellence Awards (PREA) program is
also acknowledged for financial support and The University
of Western Ontario and the Canada Foundation for Innova-
tion (CFI) are thanked for equipment funding.
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