In this paper, fabrication and characterization of bare and doped CdS nanoparticles as well as investigating the luminescence properties of these particles as an important II-VI semiconductor are presented. A novel Thermochemical method was used for synthesis of these quantum dots. Thiols were used as the capping agent to prevent further growth during fabrication process. The application of TGA as a capping agent instead of TG was studied as a novel idea in this paper and was used practically in the synthesis of semiconductor nanoparticles. Using this process resulted in particles with sizes between 3 - 7 nm. Several samples were synthesized and characterized under various Mn ions doping ratio from 1:10 to 1:180, different temperatures from 40℃ to 96℃ and different pH values from 6 to 10. Synthesis of CdS nanoparticles with high Mn ions concentration resulted in luminescence decrement, while luminescence of nanoparticles was increased by decreasing Mn/Cd doping ratio until Mn:Cd = 1:180. The best fabrication temperature was obtained at 96℃ and the highest luminescence was observed at the pH value of 9. A theoretical explanation for the behavior of fabricated high luminescent quantum dots is presented based on the principles of quantum mechanics.
The optoelectronic properties of semiconductor nanoparticles are strongly dependent on their size, due to the well-known quantum confinement effects [
Nanoparticles have been grown in a variety of ways, namely precipitation in the solid phase [
In this paper, a thermochemical method to fabricate the bare and Mn-doped CdS nanoparticles of small sizes is developed. The nanoparticles are synthesized by reaction of sodium thiosulfate Na2S2O3, cadmium sulfate 3CdSO4・8H2O as sulfur and cadmium sources, respectively. Manganese nitrate (Mn(NO3)2・4H2O) as Mn doping precursor and thioglycolic acid (TGA) C2H4O2S as capping agent were also used. One of contributions of this paper is the utilization of TGA to improve the capping of nanoparticles and reach better optical properties. The as-prepared particles were analyzed by X-ray diffraction (XRD), transmission electron microscope (TEM), UV-VIS spectrometer, Fourier transform Infra-red (FTIR) and dynamic light scattering (DLS). Besides, Mn/Cd doping ratio, temperature and pH value in fabrication processes were optimized to reach the highest possible luminescence.
This paper is structured as follow. A background of the previous works is presented in Section 2 emphasizing the state of art in the synthesis and characterization of CdS quantum dots. Section 3 draws the experimental procedures and results obtained under different conditions. The theoretical explanation of high luminescent CdS nanoparticles are discussed in Section 4 and finally the conclusion is drawn in Section 5.
Growth and fabrication of CdS nanoparticles was performed with some researchers [
The application of high luminescent bare and Mn-doped CdS nanoparticles is widely in optical devices such as optical limiting devices, display devices, optical switching and computing, phase conjugators and also optical correlators [
In this section the experiments for fabricating high luminescent CdS nanoparticles are presented. The CdS nanoparticles were grown by a chemical precipitation method at different doping concentrations, different temperatures and different pH values. The sodium thiosulfate and extra pure cadmium sulfate were used as the reactant materials and TGA was added as the capping agent. The concentrations of CdSO4 and Na2S2O3 were 5 mM and 50 mM, respectively. The concentration of TGA was 0.01 M. Mn doping concentration to cadmium concentration was variable in the range of Mn:Cd = 1:10 to Mn:Cd = 1:180. In a typical procedure, a 30 ml of Na2S2O3 solution was prepared and TGA drop-wise was added into the solution. Then, CdSO4 was added with pressure to make an agitation in the solution. Sounds that adding the cadmium thiosulfate with pressure into the solution results in a great effect on the size distribution of CdS nanoparticles. The pH value was adjusted by adding the appropriate amount of NH4OH while adding TGA. The pH was tuned on a desired value for process optimization. The reaction took place in a bath with a controlled temperature. The temperature was tuned from room temperature to 96˚C for different fabrication processes under different Mn doping concentrations. In all experiments, the samples were cleaned with a standard procedure in ultrasonic bath.
UV-Vis absorption spectroscopy method was employed to monitor the growth of nanoparticles and measure the luminescence properties. An Ocean Optics HR4000 fiber-based spectrometer was employed for the UV-Vis spectroscopy. A UV-Violet laser diode at about 400 nm wavelength was used for the excitation of fabricated nanoparticles. For X-ray diffraction (XRD) measurements, the nanoparticles were extracted by centrifuging and spinned on a glass substrate. The measurements were carried out using a Bruker D4 X-ray diffractometer. The Cu Ka (1.54 Å) X-ray line was used as the probe beam. Also TEM was done using 100 kV PHILIPS EM2085 microscope. Besides, Fourier Transform Infrared Spectroscopy (FTIR) Analysis performed to obtain material bonds.
Following the experiments, the approximate size of nanoparticles was estimated and compared with DLS analysis results.
Following this section, different scenarios for fabrication of high luminescent CdS nanoparticles under different conditions are presented. These scenarios contain fabrication of bare and Mn-doped CdS nanoparticles with different Mn dpoing concentrations, different temperatures and different pH values.
The materials used in this section are the same as those mentioned earlier at the beginning of Section III. In this case, the temperature was adjusted to 96˚C, the heating time was 60 minutes and the pH value was kept at 8 using NH4OH solution. The main key point of experiment is to keep pH and temperature constant at their tuned values.
In order to investigate the photoluminescence properties of fabricated CdS:Mn nanoparticles with a thermochemical method, a 400 nm UV laser diode was used as an excitation source. The photoluminescence (PL) spectra of samples as a function of wavelength are shown in
In the synthesized bare CdS nanoparticles, there is a peak near 605 nm that is due to the recombination of a trapped electron in sulfur vacancies with a hole in CdS valence band. In this process, inadequate concentration of sulfur maybe results in sulfur vacancies. In this case, a photo excited electron will be trapped in sulfur vacancies and will produce a red shift in PL spectrum. While exciting the CdS nanoparticles with UV laser, the electrons of valence band will be excited into the conduction band and will remain holes in the valence band.
In
The sample with doping level of Mn:Cd = 1:40 has a peak in 548 nm that results in yellow emission caused by cadmium intermediates ( I C d + ) . The sample with doping Mn:Cd = 1:80 has two peaks near 500 nm and 550 nm that are due to the sulfur vacancies and Mn2+ ions (orange emission), respectively. The sample with doping Mn:Cd = 1:160 has a peak near 548 nm (yellow emission) due to the cadmium intermediates ( I C d + ) . In order to figure out the best PL intensity of Mn doped CdS nanoparticles, a set of CdS:Mn nanoparticles with doping levels of Mn:Cd = 1:180, 200, 240 and 320 were produced and their PL intensity was measured.
FTIR analysis was performed in order to find out the bonds of Mn doped CdS nanoparticles
The Transmission Electron Microscopy (TEM) image of this sample shown in
The crystalline properties of Mn-doped CdS semiconductor nanoparticles fabricated at 96˚C with doping level of Mn:Cd = 1:180 at pH = 9 was investigated via XRD analysis as shown in
with a = 3.8200 Ǻ and λX-Ray = 1.54 Ǻ, respectively. The Scherer’s formula depicted in (15) is used to calculate the size of nanoparticles where D is the crystalline size, λ is X-ray wavelength, β is full width at half maximum (FWHM) and θ is the diffraction angle.
D = 0.9 λ β cos θ (15)
In this section, the Mn-doped CdS nanoparticles were fabricated at different temperatures in order to understand the effect of temperature on the size distribution and photoluminescence properties of doped nanoparticles. The doped quantum dots were fabricated with doping level of Mn:Cd = 1:180 which achieved the best PL intensity in the former section. The temperature of nanoparticle growth was changed between 40˚C and 96˚C.
The PL spectra of Mn-doped CdS nanoparticles at different growth temperatures is shown in
As shown in
The objective of this section is to investigate the effect of pH value of growth media on the size distribution and photoluminescence quality of fabricated Mn-doped CdS quantum dots. Mn-doped CdS semiconductor nanoparticles with doping level of Mn:Cd = 1:180 were fabricated at different pH values. Four samples including former fabrication conditions with pH values of 6, 8, 9 and 10 were prepared.
highest photoluminescence intensity that confirms the least size distribution for nanoparticles with two peaks around 495 nm and 535 nm that are referred to sulfur intermediates and orange emission due to Mn2+ ions, respectively. The sample fabricated at pH = 8 has less photoluminescence intensity due to spread size distribution of nanoparticles and has two peaks near 500 nm and 550 nm. The sample prepared at pH = 9 has less photoluminescence intensity compared to two former samples with a peak around 540 nm that is referred to orange emission caused by Mn2+ ions. This spectrum does not have any peak around 500 nm. This phenomenon is due to the compensation of sulfur intermediates with pH increase. In this case, at higher pH values the density of H+ ions is decreased, so the effective radius will be decreased. The reaction occurs around the surface of nanoparticles and will increase the size distribution of particles. This phenomenon will produce some sulfur intermediates. The sample with pH = 10 has the least photoluminescence intensity that confirms the wider size distribution of nanoparticles.
In this section, the theoretical aspect behind the fabrication of high luminescent CdS nanoparticles under different conditions is presented in order to justify the results obtained by experiments.
Since a large number of nanocrystals have a spherical structure (according to TEM image), so it is possible to consider the symmetric potential for this materials [
H = − ℏ 2 2 m ∇ 2 + U ( r ) (1)
where, m is the mass of nanoparticle and U ( r ) indicates the radical potential. So, the Laplace equation [
∇ 2 = − ∂ r 2 ∂ r ( r 2 ∂ ∂ r ) − 1 r 2 sin υ [ ∂ ∂ υ ( sin υ ∂ ∂ υ ) + ∂ 2 sin υ ∂ φ 2 ] (2)
The solution for Equation (2) is as shown in Equation (3).
Ψ n , l , m ( r , υ , φ ) = u n , l ( r ) r Y l m ( υ , φ ) (3)
In Equation (3), the wave function Ψ n , l , m is divided into two terms: the radical mode with spherical symmetric wave function u n , l , and Laguerre polynomials Y l m ( υ , ϕ ) [
[ − ℏ 2 2 m r 2 d 2 d r 2 + U ( r ) + ℏ 2 2 m l ( l + 1 ) ] u n , l ( r ) = E n , l u n , l ( r ) (4)
where, U ( r ) is the spherical potential. The solutions of Equation (4) are achieved with three quantum numbers: the fundamental quantum number n, the quantum number of angular momentum l, and the particle’s mass m which its magnitude is in z-axis direction. The potential function U ( r ) is considered as follow:
U ( r ) = { 0 r ≤ a ∞ r > a (5)
Also, the eigenvalues of energy are obtained from spherical Bessel functions X n l .
E n , l = ℏ 2 X n l 2 2 m a 2 (6)
The first value of spherical Bessel function ( X 10 ) equals to π. Since this function has an inverse square relation with radius, so, the large quantization energy will be obtained for the small nanocrystals. Different regimes of confinement for nanocrystals as the weak and strong confinement are considered by solving Equation (6).
According to the calculated values of X n l in
l | n = 1 | n = 2 | n = 3 |
---|---|---|---|
0 | 3.14 | 6.28 | 9.42 |
1 | 4.49 | 7.73 | -- |
2 | 5.72 | 9.10 | -- |
3 | 6.99 | 10.42 | -- |
4 | 8.18 | -- | -- |
5 | 9.32 | -- | -- |
l | n = 1 | n = 2 | n = 3 |
---|---|---|---|
0 | 8.576E−14 | 3.431E−13 | 7.719E−13 |
1 | 1.753E−13 | 5.198E−13 | -- |
2 | 2.846E−13 | 7.203E−13 | -- |
3 | 4.250E−13 | 9.445E−13 | -- |
4 | 5.820E−13 | -- | -- |
5 | 7.556E−13 | -- | -- |
The bandgap of CdS semiconductor nanoparticles can be assumed as the following:
α = A ( h υ − E g ) n / h υ (7)
where, α is the absorption coefficient, E g is the absorption bandgap, A is a scalar constant, and n depends on transition type and could be considered as 1/2, 2, 3/2 or 3 that corresponds to direct allowed transitions, indirect allowed transitions, direct forbidden transitions and indirect forbidden transitions, respectively. Since the CdS semiconductor nanoparticles have direct allowed transitions, so n = 1 / 2 in Equation (7).
In this section, a new equation is provided in order to estimate the average radius for CdS semiconductor nanoparticles. This equation is based on absorption spectrum characteristics as shown in Equation (8).
R = h ( 8 μ Δ E g ) − 1 / 2 (8)
where, h is Plank’s constant, μ = ( m e ∗ ) − 1 + ( m h ∗ ) − 1 which m e ∗ = 0.2 m e and m h ∗ = 0.81 m e for CdS nanoparticles, and Δ E g is the difference between bandgap energy of nanoparticles E g 0 and the bulk semiconductor E g . For CdS nanoparticles, E g = 2.4 eV and E g 0 are obtained from absorption spectrum as shown in Equation (9).
E g 0 ( eV ) = 12397.8 / λ ( Å ) (9)
In photoluminescence phenomenon, the solid crystal is exited with photonic absorption. The absorption and transmission wavelengths are different and the transmission energy is smaller than the absorption energy [
The energy band diagram for CdS nanoparticles was utilized in order to understand the effect of TGA on the nanoparticles’ growing process [
traps. If the surface is free from absorbed impurities, the photo excited electrons of conduction band will be trapped in Cd2+ states and the holes of valence band will be trapped in S2− states. In the solution phase, sulfur atoms are saturated with hydroxyl bonds and hence, the photoluminescence of nanoparticles will be dominated due to the recombination of electrons in conduction band and also the recombination of Cd2+ surface states with holes in the valence band.
Most of the molecules join together after a few moments. this removes the surface states of Cd2+ ions which decreases the surface states-to-band emission and increases band-to-band emission [
Our results showed wide wavelength emission near 500 - 600 nm which is due to the surface states created by sulfur vacancies. Using an organic agent material like TGA, capping of Cd2+ ions instead of S2− ions results in a wide wavelength emission.
The luminescence mechanism in the bare and Mn-doped CdS semiconductor nanoparticles was studies in order to verify the luminescence quality of CdS and CdS:Mn nanoparticles [
Also, it was found that when the CdS nanocrystals are not capped with capping molecules, the red band will be dominant and determines the PL spectrum for Nanocrystals. The band in the range of 700 - 800 nm addresses the complex defects including cadmium or sulfur vacancies.
In CdS nanoparticles, the cadmium vacancies and the sulfur intermediates act as acceptor, while the sulfur vacancies and cadmium intermediates will be the donors [
The energy diagram shown in
1) the band-to-band transmission.
2) the free exciton recombination.
3) the exciton mutation for donor neutralization.
4) the exciton mutation for acceptor neutralization.
5) the donor-acceptor pairs.
6) the excitation from sulfur intermediates I S − to the conduction band, results in green emission.
7) the yellow emission due to the cadmium intermediates I Cd + .
8) the red emission due to the sulfur vacancies.
9) the excitation from cadmium vacancies to the valance band.
It was found that the bare CdS nanoparticles usually have lower PL intensity due to the created surface states in their unsaturated bands [
The four-state photoluminescence mechanism for Mn-doped CdS nanoparticles is shown in
Step 3 is followed by a radiative decay from 4 T 1 excited state to A 6 1 ground state of Mn2+ ion 5) that results in orange emission due to the dominance of doping ions. The shallow traps just below the conduction band will be thermally removed while increasing the temperature results in decreasing of defect related CdS emission and also decrement of Mn2+ emission. If the excited state 4 T 1 of Mn2+ ions was directly filled with electrons of conduction band 6), so maybe the emission due to CdS and the emission due to Mn2+ ions have different quenching temperatures [
The PL spectrum confirms that the Mn-doped CdS nanocrystals contain a large number of Mn2+ ions that produce a non-uniform distribution of Mn2+ transition energy states in the nanocrystal structure [
The effect of pH on the photoluminescence intensity of CdS nanoparticles was also studied. Increasing pH value will cause a red shift in the absorption edge. To explain this phenomenon, we focused on the H+ concentration in the chemical reaction [
It seems that there is a local source of H+ ions near the surface of nanoparticles in the presence of TGA that speeds up the reaction process when pH > 8. This replaced source of H+ ions can be the thiols’ molecules that propagate H+ ions while sticking to Cd+ ions. So, the pH is increased near the surface of nanoparticles and as a result increases the size distribution [
Since, all of the conditions are constant during the fabrication process; we can attribute the particle’s mass to the process time. With the assumption of spherical shape for nanoparticles with diameter d, the mass m is denoted as density, the number of nanoparticles n, and the volume of each particle. Using the EMA method, the following formula is obtained.
m = ρ n π 6 d 3 = ρ n π 6 [ 2 π 2 ℏ 2 ( 1 m e ∗ + 1 m h ∗ ) 1 Δ E g ] 3 2 ∝ t (13)
where, m e ∗ and m h ∗ are respectively the electron and hole’s effective mass in CdS nanocrystal. By assuming the constant number of particles during the process, we have Equation (14).
Δ E g − 3 2 ∝ t (14)
We presented a model to explain this phenomenon as shown in
There are two sources for H+ ions: 1) the background H+ ions that control the macroscopic pH value of the solution, and the local H+ ions that are released by sticking to the TGA molecules as shown in
Released H+ ions on the surface of nanoparticles are propagated toward the outside and will produce a gradient for concentration of H+ ions. Since the density of local H+ ions is decreased by increasing the particle’s radius, we can assume a spherical region with an effective radius around the particle. When pH value is low, the density of H+ ions is high and will quench the effect of local H+ ions.
By increasing pH value, the amount of background H+ ions will be decreased and thus the effect of local ions would be dominant. In this case, we can define an effective radius that is decreased by pH increasing. The effective radius is the radius that has adequate H+ ions for chemical reaction called threshold concentration. Also, while releasing an H+ ion, there is most probable to be joined to S 2 O 3 2 − or OH− ions. While increasing pH value, since with 50 mM of Na2S2O3, the amount of S 2 O 3 2 − ions is much more than the amount of OH− ions, so the released H+ ions will join to the S 2 O 3 2 − ions and do not be neutralized by H+ ions.
This paper presents results supporting the synthesis of Mn-doped CdS semiconductor nanoparticles in the range between 3 - 7 nm. The luminescence of the nanoparticles showed an increase by decreasing Mn-to-Cd doping ratio. As a conclusion, the growth temperature of 96˚C, Mn:Cd doping ratio of 1:180 and pH = 8 were the optimized parameters for the synthesis of high luminescent Mn-doped CdS nanoparticles. Also a theoretical explanation based on quantum mechanics presented to justification the behavior of fabricated high luminescent Mn-doped CdS quantum dots.
Darvishi, M. and Nikfarjam, A. (2018) A Novel Thermochemical Method for Fabrication and Theoretical Explanation of High Luminescent Mn-Doped CdS Nanoparticles. Journal of Materials Science and Chemical Engineering, 6, 1-20. https://doi.org/10.4236/msce.2018.63001