For this study, we prepared colloidal ZnS quantum dots using 3-mercaptopropyltrimethoxysilane (MPS) as the capping agent. Colloidal ZnS quantum dots were directly deposited on glass substrates by a spin coating process. Therefore, self-assembled films made of ZnS quantum dots in a SiO2 network were obtained using only one production step. The films were heat-treated at 100°, 125°, 150°, 175° and 200°C in an N2 atmosphere. The results showed that the dimension of quantum dots changed from 2.8 nm to 3.2 nm by heat treatment. The refractive index, extinction coefficient, thickness, and dielectric coefficient values of the films were calculated. The present study showed that size and the refractive indices of films can be controlled by the heat treatment. Therefore, such films can be a good candidate in optical filter applications.
Nanoparticles have been extensively investigated due to their interesting size-dependent, optoelectronic, and physicochemical properties. Semiconductor nanoparticles belonging to II–VI group elements show significant quantum confinement effects [
One of the capping agents used in colloidal quantum dot production is 3-mercaptopropyltrimethoxysilane (MPS). Addition of MPS in the solvent will serve as a precursor for the growth of silica shell around the particle. Such a shell is chemically inert and optically transparent [
The MPS molecule has 2 functional groups. One of these is the thiol (–SH) group, which is able to make covalent bonds with different metals [
Silica is an ideal shell material for ZnS nanoparticles since its refractive index is significantly lower than the refractive index of ZnS [
ZnS nanoparticles were produced using zinc acetate (Zn(CH3COO)2·2H2O) : thioacetamide (CH3CSNH2) = 1 molar ratio. Zinc acetate and thioacetamide were solved in methanol in two different beakers using zinc acetate : methanol = 0.02 and thioacetamide : methanol = 0.02 molar ratios. MPS (HS(CH2)3Si(OCH3)3), as the surface capping agent, was added to the beaker, including zinc acetate and methanol at MPS : Zn = 0.3 molar ratio. Finally, the two solutions in the beakers were mixed in another beaker at 60°C in an N2 atmosphere for 10 minutes. The solution with MPS-capped ZnS quantum dots was coated on Corning 2947 glass substrates using the spin coating method at 2000 rpm rotation speed for 10 seconds. The coated films were dried at 60°C for 10 minutes. Then they were heat-treated at 100, 125, 150, 175, and 200°C in an N2 atmosphere for 15 minutes. Another part of the solution was kept in laboratory conditions for 2 weeks to produce it in a gel form. Then, powdered MPS-capped ZnS quantum dots were obtained by grinding the gel. The powdered samples are used for the X-ray diffraction (XRD), high resolution transmission electron microscope (HRTEM), and Fourier transform-infrared (FT-IR) measurements.
XRD measurements were performed using a diffractometer (GBC-MMA) operated at 35 kV and 28 mA using
Figure
XRD patterns of the MPS-capped ZnS quantum dots in powder form.
The average particle size can be found from the XRD measurements using the Scherrer equation [
The HRTEM picture of MPS-capped ZnS quantum dots is given in Figure
HRTEM picture of MPS-capped ZnS quantum dots in powder form.
The FT-IR curve of the powder form of the ZnS quantum dots is given in Figure
FT-IR graph of MPS-capped ZnS quantum dots in powder form.
XRD and HRTEM measurements showed that the average radii of the nanoparticles in this study are below the excitonic Bohr radius of bulk ZnS (~2.5 nm). It is reasonable to assume a strong confinement for electrons and holes in the ZnS nanoparticles and assuming nanoparticle in a spherical box model. Absorbances of the films and their second derivative curves in the range between 250 and 350 nm are shown in Figure
Absorbance graph of thin films forms of MPS-capped ZnS quantum dots heat-treated at different temperatures and second derivatives of absorbance spectra for the films.
Particle in a spherical box model, the first exciton peak position is given by [
Calculated value of particle size (
Heat treatment |
|
|
---|---|---|
100 | 4.75 | 2.84 |
125 | 4.70 | 2.90 |
150 | 4.63 | 2.98 |
175 | 4.54 | 3.10 |
200 | 4.46 | 3.24 |
Figure
Thickness and optical parameters (at
Heat treatment temperature (°C) |
|
|
Transmittance in p polarization | Transmittance in s polarization |
|
|
|
|
---|---|---|---|---|---|---|---|---|
100 | 100 | 97 | 0.90 | 0.81 | 1.82 | 0.007 | 3.30 | 0.024 |
150 | 84 | 79 | 0.89 | 0.81 | 1.76 | 0.014 | 3.09 | 0.048 |
200 | 74 | 72 | 0.88 | 0.80 | 1.68 | 0.023 | 2.80 | 0.078 |
Transmittances of thin films forms of MPS-capped ZnS quantum dots in s and p polarizations.
Figure
Variation of refractive index and extinction coefficient with wavelength for thin films forms of MPS-capped ZnS quantum dots heat-treated at different temperatures.
ProOptix optical data analysis software was used to calculate the thickness, and the results were checked with a profilometer (Table
The complex dielectric constant is given by
Variation of real (
The AFM images of the ZnS thin films deposited on Corning glass substrates and heat-treated at 150°C and 200°C are shown in Figure
AFM images (scan area: 1.50
The present study aimed at preparation and characterization of self-assembled MPS-capped ZnS thin film of quantum dots for optical applications. Quantum dots were self-assembled directly on a glass substrate using spin coating method without introducing any matrix. Therefore, self-assembled films made of ZnS quantum dots in an SiO2 network were obtained using only one production step. This study showed that size and the refractive indices of MPS capsulated films can be controlled by heat treatment. Therefore, such type of thin films can be a good candidate in optical filter applications [
The authors would like to thank Dr. Bahadir Keskin for the FT-IR measurement. This work is supported by the Research Fund of Istanbul Technical University.