Herwig Döllefeld,a Kathrin Hoppe,a Joanna Kolny,a Kristian Schilling,b Horst Wellera and
Alexander Eychmüller*a
a
b
PCCP
Investigations on the stability of thiol stabilized semiconductor
nanoparticles
Institute of Physical Chemistry, University of Hamburg, Bundesstraße 45, D-20146, Hamburg,
Germany. E-mail: [email protected]
Nanolytics Gesellschaft für Kolloidanalytik mbH, Hauptstr. 20, D-14624, Dallgow b. Berlin,
Germany
Received 28th February 2002, Accepted 24th July 2002
First published as an Advance Article on the web 2nd September 2002
Various analytical methods like analytical ultracentrifugation, UV–vis absorption spectroscopy, NMR
spectroscopy, and powder X-ray diffraction have been applied to study the stability of thiol stabilized
semiconductor nanoparticles. Reduced particle sizes were found in solution at low concentrations. Most
probably the assumed covalently bound thiols desorb from the surface of the particles, leaving behind
vulnerable unstabilized particles that might even undergo continuous decay over time. Additionally, breaking of
the intra ligand S–C bond could be demonstrated, presenting a cadmium sulfide particle synthesis introducing
sulfur only by the ligands without any additional sulfide ions.
1
Introduction
Nanoparticles have been in the scientific focus for about 20
years with constantly increasing knowledge of the properties
and possible applications of these new materials (for reviews
see e.g. ref. 1–5 and references therein).
The amazing physical properties of nanostructured matter
are governed mainly by the particle cores. In addition, the capping agents also have an influence on the physical properties.
For example, the ligands can saturate dangling bonds from
the core atoms that determine defect states and thus provide
recombination channels for excited charge carriers reducing
the photoluminescence quantum yield.6,7 The chemical properties, for example the solubility in solvents of different polarities, the surface charges, or the functional behavior for
coupling the particles to other molecules, are dominated only
by the ligands.
Assemblies of nanoparticles have been formed by coupling
the particles via linker molecules.8,9 The formation of bioconjugates has been performed by connecting the nanoparticles to
biological material e.g. for use as fluorescent labels.10–17 The
build-up of complex nanoparticle superstructures has been
demonstrated with a very elaborate concept using highly specific interactions between bound biological functional groups as,
for instance, complementary DNA strands.18–22 For an extensive review of coupling reactions see Niemeyer.23
From the complex and rapidly growing amount of published
work on the synthesis of nanoparticles and their characterization, only four works on different wet chemical synthesis of
II–VI semiconductor particles shall be mentioned here: CdS particles stabilized with polyphosphates,24 CdS, CdSe, and CdTe
particles stabilized with trioctylphosphine/trioctylphosphine
oxide (TOP/TOPO),25 CdS particles stabilized with thiols,26
and CdTe particles stabilized with thiols.27,28 These preparations in the liquid phase have in common the presence of socalled ligands or stabilizing agents during the crystallization
process. These agents stick to the surface of the evolving particles and prevent them from growing into the macrocrystalline
DOI: 10.1039/b202101c
phase. Additionally, they inhibit agglomeration, yielding nanosized particles in solution that can be treated with common chemical procedures and behave like regular chemicals.
The stabilizing agents are either adsorbed on the particle
surface (polyphosphates24 or TOP/TOPO25) or they are an
integral part of the surface as in the case of the thiols.29–31
Changing the chemical properties of the nanoparticles may
be done by regular derivative chemistry at the ligands or by
exchanging the complete ligand adsorbed to the particle surface. Ligand exchange on CdS particles was performed in
order to alter the solubility of the particles in solvents of different polarity.32 Similar work on CdS particles, CdSe particles,
and CdSe/ZnSe core–shell particles has been published by different groups. The altered surface properties were used for studies of charge carrier transfer,16,33 specific coating34,35 and in
order to conduct bioconjugation as mentioned above.10,11,14,20
The completeness of the ligand exchange is discussed contrarily. While Kuno et al. showed incomplete ligand exchange
at TOP/TOPO stabilized CdSe nanoparticles by means of
NMR spectroscopy36 Aldana et al. proved complete exchange,
likewise with NMR spectroscopy.37
Concentrating directly on the ligands, investigations have
been performed on particles synthesized inside micelles as
nanoreactors without any additional stabilizing agent. Following this preparation, different ligands were adsorbed to the
‘‘ naked ’’ surfaces and investigations directly concerning the
ligands were carried out with different methods, especially
NMR spectroscopy.38–41 Likewise with NMR spectroscopy,
Aldana et al. showed that photodegradation starts at the thiols
introduced by ligand exchange before the inorganic particle
core is attacked.37
Ligands covalently bound to the particle core instead of
being loosely adsorbed to the surface are supposed to be more
stable against exchange or loss of the ligand. Therefore, these
stabilizers might be more appropriate for connecting nanoparticles to other systems, like biological materials, or to build
up complex superstructures composed of nanoparticles.42
Unexpectedly, Løver et al. found ligand exchange on very
Phys. Chem. Chem. Phys., 2002, 4, 4747–4753
This journal is # The Owner Societies 2002
4747
small thiol stabilized CdS particles,43,44 putting into perspective the proposed stability of the covalently bound ligands.
The aim of this article is to present further investigations on
the stability of very small cadmium chalcogenide nanoparticles
stabilized with covalently bound thiols. We summarize our
experience concerning the stability of the covalent binding
between particle cores and their ligands. Following an experimental part in section 2 in section 3.1 we first discuss the particle sizes obtained. Subsequently, we show the measurements
referring to the breaking of the cadmium–sulfur bond performed with the analytical ultracentrifuge in section 3.2.1
and by UV–vis spectroscopy in section 3.2.2 followed by the
results obtained with NMR spectroscopy in section 3.2.3. Subsequently, we introduce the measurements referring to the
breaking of the sulfur–carbon bond in the thiol performed
with XRD and UV–vis absorption spectroscopy (section
3.3). We summarize our results in section 4.
2 Experimental
2.1 Preparation
Sample a.26,30. A solution of 1.97 g (4.70 mmol) of
Cd(ClO4)26 H2O and 1 mL (11.5 mmol) of the thiols (1-thioglycerol, 1-mercaptoethanol, and 1-mercaptoisopropanol) in
250 mL of water was adjusted to pH 11.2 with 1 M NaOH.
25 mL (1.12 mmol) H2S was added under vigorous stirring followed by further stirring for 2 h at room temperature. In order
to remove low molecular weight contaminants, dialysis was
carried out against water (using SERVAPOR dialysis tubings).
A similar preparation in an acidic medium was performed
as follows:45 2 g (4.77 mmol) Cd(ClO4)26 H2O and 3 g
(21.2 mmol) 2-(dimethylamino)-ethanethiol hydrochloride
were dissolved in 250 mL water. The pH was adjusted to 5.0
with 1 M NaOH. 50 mL (2.23 mmol) H2S were injected under
vigorous stirring. After stirring 2 days at room temperature,
the solution was dialysed against water.
Sample b.26,31. A solution of 1.97 g (4.70 mmol) of
Cd(ClO4)26 H2O and 1 mL (11.5 mmol) of the thiols (1-thioglycerol and 1-mercaptoisopropanol) in 250 mL of water was
adjusted to pH 11.2 with 1 M NaOH. 50 mL (2.23 mmol)
H2S was added under vigorous stirring followed by heating
to 100 C for 30 min and by refluxing for a further 30 min.
In order to remove low molecular weight contaminants dialysis
was carried out against water.
Depending on the applied alkanethiol the powders readily
redissolve in either water (1-thioglycerol) or strong coordinating solvents (1-mercaptoethanol, 1-mercaptoisopropanol) such
as dimethylformamide (DMF) or dimethylsulfoxide (DMSO).
temperature of 25 C. The 113Cd NMR spectra were measured
at 44.4 MHz with a Varian Gemini 200 BB spectrometer.
Chemical shifts are referenced to external 0.1 M aqueous
Cd(ClO4)2 solution as zero. X-ray diffractometry was performed on a Bruker D 8 Advanced (Cu-Ka radiation, variable
entrance slit, Bragg–Brentano geometry, secondary monochromator).
3
Results and discussion
3.1 Particle sizes
Very small cadmium sulfide nanoparticles of various sizes were
prepared according to literature methods. Briefly, an aqueous
solution of a cadmium salt was reacted with H2S in the
presence of different alkanethiols (SR) as the stabilizing
agents.26,30,31,45 The size of the resulting particles was controlled by variation of the concentrations of the precursors,
the temperature, and the duration of the reaction. Intense dialysis yielded the purified particles. With the neutralization of
the pH during dialysis, the particles precipitated as white crystalline powders.
When using 1-mercaptoethanol or 1-mercaptoisopropanol
as stabilizing agents, crystalline superstructures can be
obtained and it is possible to determine exact atom positions
and thus the accurate shape and size of the particles from
single crystal X-ray diffraction measurements (SC-XRD).
Thus, formula units of Cd17S4(SCH2CH2OH)26 and
Cd32S14(SCH2CH(CH3)OH)36(solvent)4 are evaluated for the
samples a and b, respectively.30,31 These nanoparticles, possessing tetrahedrally shaped inorganic cores with edge lengths of
1.4 nm and 1.8 nm, respectively, redissolve in DMF or DMSO.
Exemplarily, the structure of Cd32S14(SCH2CH(CH3) OH)36(solvent)4 is presented in Fig. 1.
Other particles synthesized using 1-thioglycerol as a stabilizing agent are soluble in water and do not yield single crystal
superstructures. The sizes of these particles were determined
by other techniques such as powder XRD.26 The particle sizes
evaluated for the samples a and b were 1.4 nm and 1.8 nm on
average, respectively.
Particles of the same size but different solubility are synthesized in a similar procedure. Different alkanethiols were used
as stabilizing agents. These particles exhibit comparable
EXAFS data and identical transition energies for the first
absorption feature in UV–vis spectroscopy. Thus, the inner
Sample c. In addition, a preparation of CdS nanocrystals
without the use of H2S was performed. Briefly, a N2-saturated
solution of Cd(ClO4)26 H2O in the presence of 1-thioglycerol
(molar ratio 1 : 2.4) was adjusted to the required pH value by
NaOH (1 M) or HClO4 (0.8 M), respectively. The solution was
heated in the dark and the evolution of the reaction mixture
was monitored by UV–vis absorption spectroscopy (see section
3.3).
All particles were characterized by different methods such as
UV–vis spectroscopy, powder XRD, and SC-XRD.26,30,31,45
2.2 Apparatus
UV–vis spectroscopy was performed with a Varian Cary 500
Scan spectrometer. For measurements of the sedimentation
velocity, a Beckman Optima XL Analytical Ultracentrifuge
was used with absorbance detection at 290 nm and 330 nm,
respectively, at an angular velocity of 50 000 rpm and a
4748
Phys. Chem. Chem. Phys., 2002, 4, 4747–4753
Fig. 1 Schematic plot of a CdS nanoparticle with the formula unit
Cd32S14(SCH2CH(CH3) OH)36(solvent)4 . Cadmium atoms are represented by black spheres, sulfur atoms inside the lattice by gray spheres
and sulfur atoms in the thiols by white spheres. Solvent molecules at
the tips of the tetrahedron are represented by hatched spheres.
structures of the CdS cores are assumed to be identical.26,46
Therefore the CdS nanoparticles having a size of 1.4 nm and
1.8 nm are referred to as samples a and b, respectively, without
any respect to the actual stabilizing agent used.
3.2 Experiments on the stability of the cadmium–sulfur bond
3.2.1 Analytical ultracentrifuge (AUC) measurements. Samples a and b stabilized with 1-mercaptoisopropanol were characterized with respect to their particle sizes by measuring the
sedimentation velocity in the AUC. Measurements were performed in DMF at two different particle concentrations (150
mmol L1 and 10 mmol L1). The sedimenting particles were
monitored by means of absorbance scans in the rotating cuvette during the progress of sedimentation. Thus, by acquiring
concentration profiles at various experimental runtimes repetitive information on the sedimentation velocity is obtained.
Assuming a spherical shape (a reasonable approximation for
a tetrahedron) and a density of 3.1 g mL1 (an approximation
explained below), the sedimentation coefficient distributions
were converted into the particle size distributions shown in
Fig. 2 for the higher concentration (150 mmol L1). The density used is somewhat lower than the bulk density (4.8 g
mL1) due to the ligand groups that considerably decrease
the density of the overall object sedimenting. Calculations
show, however, that this model yields a molecular mass of
Fig. 2 Measurement of the particle size distribution for samples a
and b in DMF at the higher concentration (150 mmol L1) in the
AUC. The distributions are broadened by diffusion along the sedimentation boundary during the course of the experiment. The diffusion
corrected distributions (dotted line) are provided to show that the particles exhibit discrete molecular masses and particle sizes. Particle sizes
of 1.7 nm and 2.4 nm, respectively, were obtained.
4200 g mol1 for sample a, which is in accordance with its stoichiometry thus approving the assumed density.
The sedimentation coefficients in DMF were found to be 2.1
Sv (Svedberg) for the sample a and 3.82 Sv for the sample b,
yielding average particle sizes of 1.7 nm and 2.4 nm, respectively.
Compared to the particle sizes of the samples a and b
obtained by single crystal and powder XRD the AUC yields
slightly larger values because in the AUC, the hydrodynamic
diameter is obtained, including the swollen organic ligand
sphere, rather than the geometric diameter of the inorganic
cluster core. Thus, particle sizes determined with the AUC
are in good agreement with results already published.
Particle size measurements by means of AUC were also performed at the lower concentration of 10 mmol L1. Fig. 3 shows
the results for sample a overlaid with the results obtained at the
higher concentration (150 mmol L1) for better comparison.
Particle sizes of 1.74 nm for the high concentration (150
mmol L1) and 1.35 nm for the low concentration (10 mmol
L1) were obtained. At low concentrations the particle sizes
are reduced dramatically in comparison to the measurements
at high concentrations. This indicates that ligand molecules
desorb from the particle surface. For better comparison, the
distribution at 10 mmol L1 was calculated using the same density for the sedimenting particle. However, the detachment of
ligand groups implied by the decrease in diameter would
demand a higher density, yielding even smaller particle sizes.
Fig. 3 Measurement of the particle size distribution of sample a in
DMF at two different concentrations (150 mmol L1 and 10 mmol
L1) in the AUC. Particle sizes of 1.74 nm and 1.35 nm, respectively,
were obtained.
Phys. Chem. Chem. Phys., 2002, 4, 4747–4753
4749
Fig. 4 Size distribution of sample a in DMF at a very low concentration (1 mmol L1) in the AUC. Continuous decay of the particles can
be seen during the time period of the sedimentation process.
It is obvious that the particle size is dependent on the concentration of the particles in solution. This must be a thermodynamic process because once adjusted, the new particle size is
stable for at least 2 h, i.e. the duration of the measurement.
Analogous results were obtained for the sample b. While at
high concentration (150 mmol L1) a particle size of 2.4 nm is
obtained (see above) at low concentration (10 mmol L1) a
reduced particle size of 1.5 nm (not shown) is observed.
Further reduction of the concentration of the nanoparticles
in solution leads to a loss of stability. Fig. 4 shows data for
sample a in the AUC at 1 mmol L1 indicating that the
diameter does not remain at a particular particle size but exhibits continuous decay over the whole measuring period.
3.2.2 UV–vis absorption spectroscopy. According to the
quantum size effect, smaller particle sizes as determined in
lower concentration solutions would result in a larger bandgap. Therefore, UV–vis spectra were recorded for the two differently sized nanoparticle samples at different concentrations.
Fig. 5 shows the energy of the first electronic transition as a
function of the particle concentration. For both particle sizes,
a shift to higher energies of the first electronic transition can be
seen with decreasing particle concentration.
Most probably, the stabilizing agents desorb from the surface at lower concentrations. This desorption is accompanied
with the breaking of the Cd–S bond inside the inorganic
core. Because of the contribution of the sulfur from the
thiols to the size of the inorganic core, the loss of sulfur
reduces the size of the optically active core (i.e. the ‘‘ electronic size ’’) and, in accordance with the quantum size effect,
leads to a shift of the first electronic transition to higher
energies.
Thus, in agreement with the results from the AUC measurements, it could be shown with UV–vis spectroscopy that
reduced concentrations result in smaller particle sizes for both
samples, a and b.
The continuous decay of the particles at very low concentrations was also observed with UV–vis spectroscopy. Fig. 6
shows sample a at two different concentrations over a time period of about 20 min.
The spectra of sample a at the concentration of 1.0 mmol L1
show a continuous decay of the major absorption band at 290
nm originating from the continuous decay of the particles. At
the same time intensity evolves at 320 nm and 360 nm, indicating growth of the next larger nanoparticle homologues from
the decomposed particles. The particles are no longer effectively stabilized in the very small dimension and for thermodynamic reasons larger units are generated following the
discontinuous growth of the particles. The continuous decay
at a concentration of 1 mmol L1 can be suppressed by an
increased concentration (2 mmol L1, see Fig. 6) yielding stable
absorption features and, thus, stable particles.
Fig. 5 Dependence of the energy of the first electronic transition on the concentration of the particles in DMF. Results are shown for two particle
sizes (samples a and b). For both particle sizes higher transition energies are observed for smaller particle concentrations.
Fig. 6 Absorption spectra of sample a at different particle concentrations in water, recorded over a time period of about 20 minutes. At a concentration of 2.4 mmol L1 the absorption features remain unchanged while at 1.0 mmol L1 continuous decay of the major absorption band at
about 290 nm can be seen and new bands at 330 nm and 360 nm emerge.
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Phys. Chem. Chem. Phys., 2002, 4, 4747–4753
Fig. 7 Absorption spectra of sample b over a time period of about 40 min. The data were recorded in pure water and in an aqueous solution of
NaCl (1.6 mol L1). An excess of NaCl in solution yields continuous decay of the particles.
The measurements shown in Fig. 6 were performed with
sample a in water. Measurements performed on equally sized
particles dissolved in DMF yield identical results (not shown).
Also not shown is the influence of an excess of stabilizing
agent. Addition of extra stabilizing agent to the nanoparticle
solution at the concentration of 1.0 mmol L1 yields stable
absorption features equal to the nanoparticle solutions at
higher particle concentrations (not shown).
Further experiments were conducted with respect to the stability in aqueous solutions of NaCl. They were performed on
positively charged particles of sample b stabilized with 2(dimethylamino)-ethanethiol.
Fig. 7 shows the decay of sample b in a 1.6 mol L1 NaCl
solution over a time period of 40 min, while the same sample
in pure water with the same concentration remains stable. This
experiment was repeated with NaClO4 instead of NaCl (not
shown). In contrast to the results obtained in NaCl solution
the particles remain stable, indicating that the decay in the presence of NaCl does not originate from the ionic strength of the
solution. Consequently, a kinetic salt effect can be excluded.
Most probably, in comparison to perchlorate the chloride
binds stronger to released cadmium ions, thus favoring the
decay of the particles.
3.2.3 NMR studies. Direct investigations of the interaction
between the thiol ligands and the nanocrystalline surface have
been performed by means of NMR measurements. Due to a
prolonged relaxation time caused by restricted motion of the
ligands on the nanocrystal surface the NMR signals are broadened, complicating the interpretation, especially for 1H-NMR
spectra.47 Apart from this influence of the dipole interaction
mechanism, a ligand exchange on the nanocrystalline surface
would also cause signal broadening.48 For this reason 1HNMR measurements were disregarded.
Comparing the 13C-NMR spectra of a size series of CdS
nanocrystals stabilized by thiol ligands in D2O and DMSO-d6
Fig. 8
an increased broadening of the signals in DMSO-d6 is striking
(not shown). This leads to the assumption that ligand exchange
is more pronounced in DMSO-d6 than in D2O. In both solvents, no free ligands could be detected, presumably because
of superposition of the NMR signals and too small concentrations in the solution.
Fig. 8 shows the 113Cd-NMR spectrum of sample b stabilized by thioglycerol in DMSO-d6 . For the structure of this
particular CdS nanocrystal please refer to Fig. 1.
Following the empirical observations of the 113Cd-NMR
chemical shifts of Cd(SAr)2 species in DMF by Dance et al.
and Cd(SR)4 in different solvents by Carson et al. the NMR
signals are construed as follows.49,50 Signals in the low field
region of the NMR spectrum (700–600 ppm) belong to the
Cd atoms inside the nanocrystal core that are tetrahedrally
surrounded by S atoms (sulfide and thiol). The three signals
in the higher field region (550–350 ppm) represent the Cd
atoms that are coordinated with three S atoms and solvent
molecules at the corners, the edges, and the faces of the particle
core tetrahedron, respectively.
This solvent coordination demonstrates the exchange process of thiol ligands versus solvent molecules on the surface
of the nanocrystals and the S–Cd bond breaking. This is the
initial state of a degradation of the nanocrystal core evidenced
by AUC measurements (Fig. 3 and 4) and UV–vis absorption
spectroscopy (Fig. 5 and 6). Since 113Cd-NMR spectra of the
same nanocrystal in D2O reveal no high field signals we conclude that the exchange rate is dependent on the coordination
strength of the solvent.
3.3 Experiments on the stability of the sulfur–carbon bond
in the stabilizer performed with XRD and UV–vis absorption
spectroscopy
Swayambunathan et al. have reported on the growth of CdS
colloidal particles initiated by pulse radiolytic release of sulfide
from a thiol in the presence of Cd2+ ions.51 Breaking of S–C
113
Cd-NMR spectrum of sample b stabilized by thioglycerol in DMSO-d6 .
Phys. Chem. Chem. Phys., 2002, 4, 4747–4753
4751
Fig. 9
ple c.
Temporal evolution of the UV–vis absorption spectra of sam-
bonds in the thiols after prolonged heat treatment was
also observed for thiol stabilized CdTe nanocrystals, as
reported by Rogach et al.52 The formation of CdTe(S) mixed
crystal structures was confirmed by XRD, where reflexes
belonging to the CdTe phase were observed at early stages of
the synthesis, whereas an increasing contribution of reflexes
referring to the CdS phase was obtained during the heat
treatment.
In consequence, these results inspired investigation of a preparation of CdS nanocrystals with thiols as the sulfide source.
Here we present the formation of CdS nanocrystals (sample c)
by hydrolysis of thiols under analogous preparation conditions
to the CdTe nanocrystal synthesis mentioned above without
further addition of H2S. The UV–vis spectra recorded during
the preparation within 32 h at pH 9 are presented in Fig. 9.
Initially, the spectrum shows only the absorption at
approximately 248 nm, representing the cadmium thiol coordination compounds in the solution. In contrast to the other
spectra presented this spectrum is recorded with dilution for
better comparison. During the heat treatment, an absorption
band at approximately 330 nm evolves, indicating the formation of the known CdS nanoparticles (referred to as sample
b in this article). This absorption band increases with time.
Upon further heating a second band develops at approximately 360 nm indicating the formation of larger nanocrystals
with a diameter of 2.3 nm.26 This reaction appears to be
dependent on pH. In the acid pH range no CdS nanocrystals
are detected. With increasing basicity of the solution the formation of nanocrystals is enhanced. The resulting CdS nanocrystals (after 30 h) are investigated by X-ray diffractometry
(Fig. 10).
The XRD pattern and the absorption spectra show CdS
nanocrystals analogous to those known from Vossmeyer et al.
(sample b).26 These nanocrystals possess a cubic lattice morphology, a diameter of 1.8 nm and a maximum in the absorption spectrum at 334 nm. It is known that the oxidation of
thiols in aqueous solution is promoted photocatalytically
at the nanocrystal surface.37 On the one hand adsorbed
thiols can be reduced by an electron excited by light
(e + RSH ! R + HS)51 and on the other hand the formation
of disulfides from surface adsorbed thiols can likewise lead to a
breaking of the S–C bond.40 Since these investigations have
been carried out in the dark, photocatalytic processes can be
excluded here as an explanation for the breaking of the sulfur–carbon bond. Most probably, hydrolysis takes place in a
nucleophilic substitution of the mercapto group through a
hydroxide ion, explaining the evidenced pH dependence mentioned above.
4752
Phys. Chem. Chem. Phys., 2002, 4, 4747–4753
Fig. 10 Powder X-ray diffraction pattern of sample c after 30 h. The
bar patterns show the bulk cubic CdS reflections with their relative
intensities.
4
Summary and conclusion
Investigations on the stability of thiol stabilized nanoparticles
have been performed with different methods. Even though the
thiols are assumed to be an integral part of the inorganic core
and covalently bound, it could be shown that they desorb from
the surface at low particle concentrations. This was observed
directly by AUC measurements and in UV–vis absorption
spectroscopy. Both methods yield results for the particle sizes
and absorption spectra, respectively, in good agreement with
the literature as long as low particle concentrations are
avoided. At low concentrations absorption features shift to
higher transition energies and thus, in accordance with the
quantum size effect, reduced particle sizes are observed. The
reduced particle size could also be shown directly by the
AUC at lower particle concentrations. At very low particle
concentrations continuous decay of the particles has been
observed both, in AUC and UV–vis absorption spectroscopy
measurements. Analogous results were obtained with NMR
spectroscopy. Ligand exchange could be shown in 113CdNMR, affirming the weakness of the binding of the ligands.
Earlier, breaking of the intra ligand S–C bond could be proven during the formation of CdTe(S) mixed crystal particles.
As a consequence it was possible to build up CdS nanoparticles only by adding thiols and the cadmium precursor to the
reaction mixture without any additional sulfide ions.
In conclusion, thiol stabilized II–VI-semiconductor nanoparticles can be treated as stable particles only at sufficiently
high particle concentrations. Otherwise, desorption of the
ligands or even continuous decay of the particles takes place.
Furthermore, heat treatment during preparation of these
nanoparticles gives the danger of decomposition of the stabilizing agent, yielding free sulfide ions that interfere with the preparation process.
Acknowledgements
We gratefully acknowledge Helmut Cölfen and Antje Völkel,
Max Planck Institut (MPI) in Golm, Germany, for some of
the AUC measurements and data evaluation and Erhard
Haupt and his staff, University of Hamburg, for the NMR
measurements, and Dirk Dorfs for some of the UV–vis measurements.
This work was supported by the German science foundation
(DFG) within the framework of the SFB-508.
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