CdS quantum dots were produced using the reaction of cadmium acetate (Cd(CH₃COO)₂·2H₂O) with thioacetamide (CH₃CSN₂) at a molar ratio of one. Cadmium acetate and thioacetamide were first dissolved with methanol in separate beakers. The molar ratios of cadmium acetate/methanol and thioacetamide/methanol were equal to 0.02. MPS (HS(CH₂)₃Si(OCH₃)₃) was mixed with cadmium acetate at a molar ratio of MPS/Cd = 0.3 12 . This ratio was decided after analyzing the absorption spectrum of the nano CdS at several other ratios (0.1, 0.2, 0.5) and after considering previous works 11 15 19 . The solutions in two beakers were then mixed and stirred for 10 min at 60°C in nitrogen medium.

MPS-capped CdS quantum dots were coated as thin film on a glass substrate (Corning 2947, Corning Incorporated, Tewksbury, MA, USA) by spin coating. The system was spun at a rate of 2,000 rev/min for 10 seconds. The substrate was heat-treated at 60°C for 10 min at each coating. The coated substrates were then heat-treated between 225°C and 325°C for various time intervals to investigate the growth kinetics. As explained in our previous work 12 , during the formation of thin films, we suppose that quantum dots were attached to the glass surface by Si-O-Si bonds created by the OH group of the glass surface and the Si-O group of MPS.

High-resolution transmission electron microscopy (HRTEM) images and energy-dispersive X-ray spectroscopy (EDS) spectra of the MPS-capped CdS quantum dots in powder form were recorded using a JEOL 2100 (JEOL Ltd., Akishima, Tokyo, Japan) HRTEM microscope operated at 200 kV. X-ray photoelectron spectroscopy (XPS) spectra measurements of the thin films were carried out by SPECS PHOIBOS-150 Electron Analyzer (SPECS GmbH, Berlin, Germany) with a monochromatic X-ray source (Al-Kα) of photon energy 1,486.6 eV. Absorbance of the thin films was recorded with a UV-visible spectrometer (Agilent 8453, Agilent Technologies, Inc., Santa Clara, CA, USA).

The HRTEM picture and the EDS pattern of MPS-capped CdS quantum dots are given in Figure 1a,b, respectively. In the EDS pattern, the presence of Cd and S is clear, and the atomic ratio of S/Cd was calculated as 1.07:1. In our work, cadmium acetate and thioacetamide were used as a Cd source and as an S source, respectively, in a molar ratio of 1 as explained in the ‘Methods’ section. On the other hand, MPS molecules contain sulfur due to their thiol group. Therefore, we think that S ratio is a little larger than that of Cd due to the contribution of the thiol group. Also, the oxygen and silicon peaks shown in the EDS pattern correspond to the capping agent of MPS.

XPS spectra of the films heat-treated at 250°C and 300°C for 15 min are shown in Figure 2. In Figure 2a, it can be observed that Cd 3d_5/2 peak at 405.73 eV and Cd 3d_3/2 peak at 412.53 eV for the film heat-treated at 250°C for 15 min, while the binding energy peaks of Cd 3d_5/2 and Cd 3d_3/2 are observed at 405.88 and 412.63 eV for the film heat-treated at 300°C for 15 min. In Figure 2b, the S 2p peaks appear at 161.85 and 161.80 eV for the films heat-treated at 250°C and 300°C, respectively, for 15 min. All of the observed binding energy values are in good agreement with the reported data 20 and confirm the chemical identity of the CdS quantum dots.

Absorbance values of films heat-treated at 225°C, 250°C, 275°C, 300°C, and 325°C are shown in Figures 3, 4, 5, 6, and 7, respectively. The second derivatives of the absorbance graphs are also shown in the inset of the graphs. The figures show that absorbance edges are shifted through a larger wavelength (smaller energy) with respect to higher heat treatment time or temperature due to quantum confinement effects. The first exciton peak positions (E _1s1s ) are given in Figure 8; they were calculated from the first minimum value of the second derivatives of the absorbance graphs. E _1s1s energy values change between 3.02 and 2.67 eV, and as the time and temperature of the heat treatment increased, it approached to 2.42 eV being the bandgap energy of bulk CdS. In addition, as shown in the graphs, it is seen that as time of heat treatment increased, the shifting in E _1s1s values also increased. Shifting of E _1s1s values of films heat-treated at 225°C and 325°C for 5 min was 180 meV, and then it became 244 meV when the films were heat-treated at 225°C and 325°C for 60 min.

With the particle in a spherical box model, the first exciton peak position is given as follows 21 22 :

E 1 s 1 s = E g + ħ 2 π 2 2 r 2 1 m e * + 1 m h * − 1.786 e 2 ε r − 0.248 e 4 2 ε 2 ħ 2 1 m e * + 1 m h * − 1 ,

where E _g is the bandgap energy of the bulk material (2.42 eV for CdS), r is the radius of the nanoparticle, m _e ^* and m _h ^* are the effective mass values of electrons and holes, respectively, (m _e ^* = 0.19m ₀ and m _h ^* = 0.8m ₀ for CdS), and ε is the dielectric constant (5.7 for CdS) 21 . The average particle size of these films is calculated by inserting E _1s1s values into Equation 1. The results of the calculations showed that the average E _1s1s values decreased from 3.3 eV (for films dried at 60°C for 10 min) to 2.7 eV (for films heat-treated at 325°C for 60 min), and sizes of the CdS quantum dots increased from 2.9 to 4.6 nm. The results show that the redshift of E _1s1s values with increasing heat treatment temperature is due to the increasing nanocrystal size (quantum size effect).

The growth of nanocrystals can be described in terms of three stages: nucleation, normal growth stage, and Ostwald ripening. Size of nanocrystals is determined by the monomer concentration, capping ligand, and temperature at the nucleation and growth stages. At a fixed monomer concentration, Ostwald ripening dominates between nanoparticles. At this stage, the coarsening effects are characterized by diffusive mass transfer from smaller particles to larger ones, and the final size of nanocrystals is thus determined by temperature and time period of the Ostwald ripening 6 . We are going to look at the growth process of the films at this stage.

This diffusion-limited Ostwald ripening process can be described by the Lifshitz-Slyozov-Wagner (LSW) theory. The LSW theory for coarsening kinetics is given as follows 17 :

r ¯ 3 − r ¯ 0 3 = K t ,

where r̄ ₀ is the average initial radius of nanocrystals, r̄ is the average radius of nanocrystals after ripening occurs, t is the ripening time, and K is the ripening rate coefficient. K is given as follows 17 :

K = 8 γ D V m 2 C ∞ 9 R T ,

where D is the diffusion coefficient, γ is the interfacial energy, V _m is the molar volume, C _∞ is the equilibrium concentration at a flat surface, ℜ is the gas constant, and T is the temperature.

The cubic term of r̄ versus time of heat treatment are given in Figure 9. In this figure, for each temperature value, the linearly fitted curves are also given. It is seen that the results conform with the LSW theory given by Equation 2. For each heating temperature, K values were obtained from the slope of the linear curves. K values were calculated as 0.023, 0.039, 0.060, 0.090, and 0.106 nm³/min for the films heat-treated at 225°C, 250°C, 275°C, 300°C, and 325°C, respectively. As expected, these results show that for the Ostwald ripening process, the K values increased as the temperature increased. This shows that while the heat treatment temperature increased, the average radius of the quantum dots grew faster.

The relation between diffusion coefficient, activation energy, and temperature at constant pressure is given through the Arrhenius relation:

D = D 0 e − Q d R T ,

where D ₀ is a coefficient independent of temperature, ℜ is the gas constant, and T is the heat treatment temperature. Q _d is the (activation) energy of 1 mol of atom to make a diffusion movement.

Using Equations 3 and 4, the following equation for the activation energy is obtained:

ln K T = ln K 0 − Q d R T ,

where K ₀ is a coefficient. In Figure 10, to find the activation energy from the slope of the fitted line, experimental points are ploted on the axis of ln(KT) with respect to 1,000/T. The activation energy for the Ostwald ripening process is found to be approximately 44 kJ/mol.