Luminescent CdS nanocrystals embedded in a polystyrene matrix were successfully prepared. The
In the last years nanocomposite materials have attracted much attention due to the synergistic combination of the quantum dots (QDs) and polymers characteristics, leading not only to the sum of the properties of organics and inorganics [
Significant attention has been paid to the use of QDs belonging to II–VI groups (CdS, CdSe, CdTe), because their electrooptical properties could be perfectly combined with the charge transporting properties of conductive polymers. However, the mixture of these two materials is not obvious in terms of QDs distribution inside the polymer and charge transfer optimisation between the polymer and the QDs. Therefore, a possible approach to overcome these drawbacks is to grow the QDs directly inside the polymer [
However, the exploitation of the semiconductor nanocomposites on a new devices requests the possibility to tune the QDs’ size and to optimize the interaction of nanoparticles with the polymeric matrix. Both of these goals are possible using the thermolytical QDs generation, because this methodology allows the QDs size modulation by varying the time and temperature of annealing and permits the growth of the QDs directly inside the polymer (
The effect of reaction conditions such as temperature of decomposition, precursor concentration, and time of annealing on physical-chemical properties of CdS QDs was already investigated [
The time course of the CdS growth was studied by means of UV-Vis absorption, photoluminescence (PL), and X-ray diffraction analysis (XRD). The Brus and Scherrer equations were used to calculate the particle size from UV-Vis and XRD spectra, respectively. Additional information about the QDs structure and their state in solution were obtained through high-resolution transmission electron microscopy (HRTEM) observation and dynamic light scattering (DLS) analysis.
All the reagents were purchased from Sigma-Aldrich and used without further purification unless indicated. The thermal properties of polystyrene (
The synthesis of the precursor from dodecanethiol [HS
The preparation of the precursor/polymer foil was carried out dissolving 900 mg of standard polystyrene in hot toluene under reflux and further addition of 300 mg of Cd bis(thiolate) single-source precursor (25% w/w). The mixture was stirred until becomes clear. The solution is then poured in a Petri dish (100 mm diameter) and left to dry overnight.
Half of the obtained precursor/polystyrene disk was heated under vacuum at specific temperatures and times of annealing, whereas the resulted nanocomposite was dissolved in chloroform. This solution was split in two aliquots. The first half was used to obtain a new disk pouring it on a small Petri dish (40 mm diameter) and leaving it to dry. To allow an optimal disk formation, an additional amount of standard polystyrene (150 mg) was added to the solution. The second aliquot was further purified to obtain QDs without degradation byproducts and polystyrene as reported following. For that, the solution was centrifuged at 4000 rpm for 10 minutes in order to recover possible precipitated CdS crystals. This precipitate, if any, will be added to the final purified QDs. The supernatant was dried and resuspended in 5 mL of THF. After that, a volume of 40 mL of octane was added slowly, and the final solution was centrifuged for 10 minutes at 4000 rpm to remove the precipitate. The organic solution was evaporated using a rotavapor, and the pellet was dissolved in 2 mL of chloroform.
In order to precipitate the QDs and to remove the degradation products, another 8 mL of acetone was added. The resulting precipitate was centrifuged for 10 minutes at 4000 rpm and washed two times more with acetone. Finally, the precipitate was dissolved in chloroform and mixed with the recovered crystals (if any) separated at the beginning of this procedure.
These solutions were utilized for the optical, structural, and microscopic measurements.
The XRD analysis was performed with a Philips conventional Bragg-Brentano vertical diffractometer with CuK
The absorption spectra were measured with a UV-Vis spectrophotometer (Perkin Elmer Lambda 40) in the range of 350–600 nm. The sample solutions dissolved in chloroform were dispersed on a quartz slide to obtain a thin film. The calculation of the QDs size was carried out using the Brus equation [
The samples for photoluminescence (PL) spectra were prepared diluting 100
The transmission electron microscopy (TEM) measurements were carried out with a FEI Tecnai F20, operated at 200 kV.
Samples’ size distributions were evaluated by a Dynamic Light Scattering Analyzer (Zetasizer Nano-S, Malvern); measurements were performed using chloroform-diluted samples.
The synthesis of the Cd bis(thiolate) single-source precursor and CdS/polystyrene nanocomposite formation was done using quite standard procedures [
However, the
This practice is necessary especially for XRD and TEM analysis, because the presence of the polymer gives rise to important undesired effects. In fact in the XRD measurements, the polystyrene broad peak is found around 20°, and it covers the main peak of the CdS at 26° (111 plane reflection) and does not permit the use of this reflection to calculate the CdS size. Moreover, this reflection is the most intense reflection of the CdS nanoparticles and is useful for size determination of the QDs especially in the first stages of QDs formation when the other reflections are very weak.
The presence of the polymer chain is also self-defeating for the TEM analysis because the nonconductive polymer causes the charging of the samples resulting in a poor image resolution. On the contrary, when the polymer is removed, a small amount of the sample is enough to observe well-resolved images.
That’s way, a second purification step was also carried out with acetone in order to remove the impurities present in the samples after the thermal annealing. These impurities come out even if the annealing procedure is carried out under vacuum, and they are probably due to the precursor degradation byproducts dissolved in the nanocomposite matrix.
To study the time and temperature dependence of the CdS QDs growth, a set of experiments covering two temperatures and different times of CdS precursor decomposition were analyzed. In particular, the precursor was annealed at 240°C for 30, 120, 180, 240, 300, 360, and 480 minutes and at 300°C (Figures
CdS nanoparticles (a) inside the polystyrene matrix and (b) in chloroform after the polymer removal. The reported samples were obtained heating a polymer/precursor foil at 300°C for 2, 5, 10, 15, 20, and 30 min (from left to right) when different colors of the samples indicate QDs with different sizes.
A detailed understanding of the CdS QDs growth process and kinetics is quite impossible to be done. Almost certain is the fact that the preparation of nanocrystals is strongly affected by the synthesis parameters (temperature of decomposition, precursor concentration, and time of annealing) with direct influence over the size and optical characteristics of the final particles.
In terms of kinetics, the growth of crystallite depends on the alteration of the surface energy with size. In our case, two major reaction mechanism for the CdS nanoparticles formation were taken in consideration: Ostwald’s ripening and particles’ aggregation.
Generally, the Ostwald ripening is believed to be the main path of crystal growth during the
So, the UV-Vis spectra of the CdS QDs samples after polymer removal at 240°C (Figure
UV-Vis spectra of CdS nanoparticles at (a) 240°C and (b) 300°C after polymer removal. The difference of the absolute value of the absorbance is due to the different film thickness and QDs concentration.
The size of CdS nanoparticles has been determined by choosing the absorption wavelength corresponding to the peak of the first derivative of these spectra (not shown) and using these values in the Brus equation. As can be seen from this time dependence isotherm curves (Figures
Time dependence of size for the CdS QDs sample annealed at (a) 240°C and (b) 300°C.
Another effect of the low-temperature QDs formation is the contemporary presence of two absorption shoulders clearly observable in the UV-Vis spectra at 240°C, especially at intermediate annealing times (between 120 and 300 minutes, Figure
The PL spectra too (Figures
PL spectra of CdS samples at (a) 240°C and (b) 300°C. The PL emissions of the nanoparticles were obtained with an excitation wavelength of 337 nm.
Correlation between the sizes obtained from UV-Vis spectra and the emission wavelength (WL) of the QDs synthesized in the nanocomposites at (a) 240°C (broken line) and (b) 300°C (solid line).
Another important consideration of the PL spectra is the band broadening which depends from the QDs size distribution [
Figure
XRD spectra of CdS nanoparticles annealed at (a) 240°C and (b) 300°C. The vertical lines indicate the angular position of the reflections and in parenthesis are reported the Miller indices.
The average size of the nanoparticles was calculated using the Scherer equation, based on 111 reflection (
QDs sizes from Scherrer equation as a function of time for sample at (a) 240°C and (b) 300°C. The logistic curve was used to fit the data.
The crystal structure of the CdS nanoparticles was investigated using electron microscopy (TEM measurements). For example, Figure
HRTEM image of the sample annealed at 300°C for 10 minutes. Several small crystalline nanoparticles of CdS are visible embedded in the amorphous polymer (one is highlighted with a red circle). For one CdS particle, of cubic phase, the orientation and interplanar distance corresponding to
The TEM images were used to estimate the average size of the clusters for the different annealing conditions. Figure
Summary of CdS QDs sizes obtained from XRD, UV-Vis, HRTEM, and DLS measurements for sample annealed at 240°C (120, 240, 480 minutes) and at 300°C (5, 10, 20 minutes).
Annealing temperature | 240°C | 300°C | ||||
Annealing time | 120 min | 240 min | 480 min | 5 min | 10 min | 20 min |
XRD size | 1.0 nm | 1.2 nm | 1.8 nm | 1.2 nm | 1.5 nm | 1.8 nm |
UV-Vis size | 3.3 nm | 3.4 nm | 4.2 nm | 3.4 nm | 3.8 nm | 4.5 nm |
TEM size | 2.6 nm | 2.9 nm | 3.3 nm | 3.2 nm | 3.7 nm | 4.6 nm |
(a) Histograms of the particles sizes as a function of the annealing time and temperature with Gaussian fit functions, (b) plot of the variation of the dimension of the CdS nanoparticles as a function of the annealing time.
It is interesting to note that the grain size estimated by XRD data (using the Scherrer’s formula) is smaller as compared to the size observed by TEM micrographs (Table
As already reported in the literature, TEM investigation is based on the difference between the visible grain boundaries, while the XRD analysis provide information of the average crystallite size (coherently diffracting domains), which is usually much smaller as compared to that observed by any direct method (including TEM). Furthermore, the grain size evaluation provided by XRD method is accompanied by a peak broadening (full width at half maximum of the diffraction peaks) which influence considerably the final size number. This peak enlargement is a result of the various contributions regarding the instrumental broadening effect, lattice strain, or other lattice defects of the particle. Even if several correction methods of the XRD profiles are already available in the literature, none of them can take care of all these contributions because the structure of the CdS nanoparticles is quite complex with direct influence on the XRD profiles and important effects on the final CdS QDs size (indirect approximation of the particle sizes via the full width at half maximum of the most intense peak).
In order to quantify the characteristic microscopic features of the QDs samples, dynamic light scattering (DLS) analysis was also carried out. The DLS measurements allow the determination of the particle size in solution, involving translational diffusion coefficient by using the Stokes-Einstein equation [
DLS analysis of CdS nanoparticles treated at (a) 240°C and (b) 300°C for different times of annealing.
The DLS data seems to indicate the same trend for the growth process too (increase of the particles size as a function of the annealing time) but with absolute values higher than those obtained from other structural characterization methods (Table
Another important aspect affecting the enhanced measured size is correlated with the fact that the DLS analysis reflects the diffusion of a particle within a fluid so refers to a
Luminescent CdS nanoparticles have been successfully grown
The XRD data revealed that the synthesized CdS nanoparticles have the cubic structure of the bulk CdS. Additionally, the UV-Vis spectra evidence a steady increase in particles size as a function of time when the typical CdS QDs absorption shifts to higher wavelength as a consequence of the increase of the annealing time. The size of the QDs calculated with the Brus equation is compared with that obtained making use of Scherrer’s formula even if the two values do not agree closely. Instead, optical data are in good agreement with electron microscopy (HRTEM) and represent an optimal way of size evaluation. The wavelength shift of the PL spectra as a function of particle size was also observed.
Furthermore, the DLS measurements of the CdS nanoparticles behaviour in solution illustrated that, for longer times of annealing, the QDs tend to aggregate into microcrystals, with diameters biased towards larger values.
The presented work has received funding from the European Community's Seventh Framework Programme (FP7/2007–2013) under Grant Agreement number 247928. The authors gratefully thank the Dr. Stefania Albonetti for her support in measurements’ organization.