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Abstract
This paper presents the results of studies of the luminescent properties for colloidal Ag2S quantum dots, coated with SiO2 shell, carried out by techniques of transmission electron microscopy, optical absorption and luminescence spectroscopy time correlated single photon counting, quantum yield of luminescence. Various approaches to the formation of SiO2 shell is analyzed. It is concluded that an increase in the quantum yield of Ag2S QDs luminescence in the condition of the formation of a SiO2 shell on the interfaces provides the passivation of dangling bonds and localization of charge carriers in the nucleus. It is shown that, under the considered conditions for the synthesis of Ag2S/SiO2 core/shell structures in ethylene glycol, the use of TEOS as a precursor for SiO2 shell provides the formation of a less defective shell, leading to an increase in the quantum yield of luminescence from 1.6% to 8%. On the contrary, the use of sodium metasilicate and high concentrations of MPTMS does not ensure the formation of a dense SiO2 shell of several monolayers thickness on Ag2S interfaces, coated with 2-mercaptopropionic acid.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The modification of interfaces of colloidal quantum dots (QDs), providing control of the quantum yield of size-dependent luminescence in the near-IR region is interesting for solving practical problems of modern photonics [1–6]. The unique optical properties of quantum dots, such as a wide excitation spectrum, tunable size-dependent luminescence spectrum, photostability, and high luminescence quantum yield make the quantum dots an extremely attractive object for use in various technical applications, such as miniature lasers, light-emitting diodes, biomedical markers, theranostics agents, solar sells [7–11]. In turn, quantum dots of silver sulfide are promising for applications of biomedicine and luminescence sensorics due to their non-toxicity, insolubility in water. Also, silver sulfide quantum dots have intense size-dependent luminescence in the near-IR region from 600 to 1300 nm [6,12–15]. However, the nonstoichiometry of Ag$_2$S has a significant negative effect on QDs luminescent properties [16]. In addition, silver sulfide has a remarkable photochemical activity. This fact affect the stability of the optical properties of colloidal Ag$_2$S QDs notably their luminescence under excitation [17–23].
Thus, the development of approaches to increasing the quantum yield of Ag$_2$S QD luminescence is an important task.
The condition of their interfaces plays a decisive role in optimizing the luminescence properties of QDs. When forming a core/shell QDs, the shell material is selected with aim to form a barrier for the tunneling of charge carriers into the matrix.For example, the relationship between the interfaces state and luminescence quantum yield of CdSe, ZnCdSe/ZnSe QDs is analyzed in [24,25] . It is concluded that the decrease in the quantum yield of the QDs luminescence is related to the presence of dangling bonds at the interfaces, which are centers of non-radiative recombination. Shell formation can passivate dangling bonds, removing the non-radiative recombination centers. At the same time, the shell material is chosen for the possibility to create a barrier that prevents the tunneling of charge carriers into the matrix. This leads to a change in the quantum yield of QDs luminescence. For Ag$_2$S QDs there are a number of papers that report the formation of Ag$_2$S/ZnS [26–31], Ag$_2$S/CdS [32], Ag$_2$S/SiO$_2$ [15,33], Ag$_2$S/Ag$_2$WO$_4$ [34], and Ag$_2$S/SnS$_2$ [35] core/shell quantum dots.
The shell formation in the listed cases is based on the introduction of the precursor of the shell material into the colloidal solution. If it is necessary than the solution subsequently processed (heat treatment,catalysts introduction, treatment with microwaves). In this case, a change in the quantum yield of QDs luminescence is observed.
In the case of Ag$_2$S/SiO$_2$ QDs, there are particular works that present data on the quantum yields of Ag$_2$S QDs, synthesized using silver dodecanethiol and hexadecylamine in Dowtherm environment and later coated with a SiO$_2$ shell using Tetraethyl orthosilicate (TEOS) and (3-Aminopropyl)trimethoxysilane (APTMS) as precursors [15,33].
These studies present information on the luminescence quantum yields of the obtained Ag$_2$S QDs and their dependence on the size of the core/shell complex. However, the analysis of the effect mechanism of SiO$_2$ shell on the luminescent properties of Ag$_2$S is not performed. There is no comparison of Ag$_2$S QDs luminescent properties before and after growing the shells. Also, there are no studies that would analyze the relationship between the interface state and quantum yield of QDs luminescence. Studies on the dependence of Ag$_2$S QDs luminescence properties on approach to shell growth are absent. Thus, the problem of controlling the quantum yield of Ag$_2$S QDs during the SiO$_2$ shell formation has not yet been solved.
This work is aimed at solving the problem of controlling the trap state luminescence quantum yield of colloidal Ag$_2$S QDs passivated with 2-mercaptopropionic acid (2-MPA) in the condition of formation of Ag$_2$S/SiO$_2$ core/shell QDs using various SiO$_2$ precursors.
2. Materials and methods
2.1 Investigated samples
The initial samples of colloidal Ag$_2$S QDs were prepared using a photoinduced growth technique described in detail in [36]. Figure 1 shows the approaches to the samples synthesis schematically. 2-MPA molecules were both the passivating agent and sulfur precursor. The molar ratio [AgNO$_3$]: [2-MPA] was 1:2. In the synthesis, 2.4 mmol of AgNO$_3$ was dissolved in 30 ml of ethylene glycol under constant stirring with a magnetic stirrer at 200 rpm in a thermostatic glass flask at a temperature of 25$^\circ$ 4.8 mmol of 2-MPA was dissolved in ethylene glycol and added to the flask with the solution. After 30 minutes of stirring, the solution of colloidal QDs was kept at room temperature for 24 hours. After that, the colloidal solution was exposed to 405 nm laser radiation with an optical power of 100 mW under constant stirring during 20 hours. Then the solution was kept in the dark during 3 days and purified by dialysis through a semifipermeable membrane from regenerated cellulose. The pore size was 3 nm.
Figure 1 shows TEM images and histograms of the size distribution of the obtained QD samples.
The following chemicals were used for the synthesis of Ag$_2$S and Ag$_2$/ SiO$_2$ QDs: silver nitrate (AgNO$_3$), 2-mercaptopropionic acid (2-MPA), ethylene glycol, sodium metasilicate (Na$_2$SiO$_3$), 3-mercaptopropyltrimethoxysilane (MPTMS), and tetraethoxysilane (TEOS). All reagents were purchased from Sigma-Aldrich.
The shell was grown in two stages. First, the silanization of the Ag$_2$S QD surface was implemented with the ligand exchange (2-MPA to MPTMS) in ethylene glycol. For this, MPTMS (preliminarily hydrolyzed in ethanol) was added into a colloidal solution of Ag$_2$S QDs in the ratios of [MPTMS$]:[$Ag$_2$S$]=1:10$ (hereinafter designated as 0.1 MPTMS) and [MPTMS$]:[$Ag$_2$S$]=1:1$ (hereinafter designated as 1.0 MPTMS). The mixture was stirred in the dark for 24 hours.
In the second stage, the thickness of the SiO$_2$ shell was increased by introducing sulfur-free SiO$_2$ precursors (Na$_2$SiO$_3$ and TEOS), which were previously used to obtain SiO$_2$ shells [15,37–40]. Na$_2$SiO$_3$ was dissolved in ethylene glycol and added to the solution in the ratio of [Na$_2$SiO$_3]:[$Ag$_2$S$]=3:1$ (hereinafter designated as 3.0 EG Na$_2$SiO$_3$). Then, the mixture was stirred in the dark for 24 hours. In the second synthesis variant, Na$_2$SiO$_3$ was dissolved in water (hereinafter designated as 3.0 W Na$_2$SiO$_3$). The synthesis using TEOS was carried out in a similar way. TEOS was pre-hydrolyzed in ethanol for 20 minutes. Then, it was introduced into the QD solution in the ratio of [TEOS$]:[$Ag$_2$S$]=3:1$ (hereinafter designated as 3.0 TEOS), and the mixture was stirred in the dark for 24 hours.
2.2 Experimental techniques
Experimental data on structure and size distribution of Ag$_2$S/2MPA and Ag$_2$S/SiO$_2$ core/shell QD ensembles were performed by digital analysis of TEM images, obtained using a Libra 120 PLUS transmission electron microscope (CarlZeiss, Germany) with an accelerating voltage of 120 kV. The Altami Studio 3.5 software package was used to analyze TEM images. Determination of the boundaries of particles and counting their number was carried out using the "Configured find contours" automated function and "Adaptive Threshold" filter, which provide the selection of contours of nanoparticles and their automated counting.
The elemental analysis of the synthesized QDs is carried out in the framework of energy dispersive X-ray analysis (EDX) by TEM Libra 120 PLUS for the samples used to obtain of TEM images.
UV–Vis absorption spectra and luminescence spectra in the wavelength region of 400–1000 nm were acquired by a USB2000+ spectrometer (Ocean Optics, USA). The radiation source USB-DT (Ocean Optics) was used for the obtaining absorption spectra. In the IR region (800–1200 nm), PL spectra were recorded using an automated spectrofluorimeter based on MDR-4 diffraction monochromator (LOMO, Russia) coupled to a low-noise photodiode (Thorlabs Inc., USA) with an integrated amplifier. The samples were excited by an NDB7675 semiconductor laser (Nichia, Japan) emitting at 462 nm with an optical power of 500 mW.
The time correlated single photon counting method was used to study the luminescence decay at room temperature. Deconvolution with an experimentally measured instrument response function was employed in the fitting procedure for the experimental luminescence decay curves. The luminescence decay curves were measured using a TimeHarp 260 board (PicoQuant, Germany) and an PMT RMS-100-20 module (Becker & Hickl, Germany), working in the region of 300 - 900 nm and time resolution of 0.2 ns. An Alphalas PLDD-250 semiconductor UV laser (Alphalas, Germany) with the emitting wavelength of 375 nm, pulse duration of 60 ps, pulse repetition rate of 100 kHz and average optical power of about 3 µW was used as an excitation source.
The luminescence quantum yield for the samples was obtained by comparison with a reference standard. A dimethyl sulfoxide solution of the Indocyanine green dye with a quantum yield of 13% was used as the standard [41]. The measurement technique is described in detail in [42]. For each sample, the luminescence quantum yield was measured five times. The luminescence quantum yield (as well as the luminescence intensity at the same absorption and excitation intensity) increased from 1.6% to 8%, i.e. 5 times or 400%. The relative error of quantum yield does not exceed 15%; 1.6$\pm$ 0.24% and 8$\pm$1.2%. These values differ significantly more than the experimental error.
3. Results
3.1 Structural properties of colloidal Ag$_2$S/2-MPA and Ag$_2$S/SiO$_2$ core/shell QDs
Figure 2 shows TEM images of the obtained QD samples. The average size of Ag$_2$S/2-MPA QDs in the samples was 2.9 nm with a dispersion of 30 %.
Fig. 2. TEM images and size distribution histograms for the obtained samples. Ag$_2$S (a), 0.1 MPTMS (b), 1.0 MPTMS (c), 3.0 EG Na$_2$SiO$_3$ (d), 3.0 W Na$_2$SiO$_3$ (e), 3.0 TEOS (f). HR-TEM images of Ag$_2$S/2-MPA (a) and 3.0 TEOS (f) on inserts.
The average size of Ag$_2$S/2-MPA QDs in the samples was 2.9 nm with a dispersion of 30%. When the primary SiO$_2$ shell was grown on the surface of colloidal QDs using 0.1 MPTMS, a slight increase in the average size of the nanocrystals in the ensemble was revealed. With increasing the MPTMS concentration to 1.0 mole fraction, a further increase in the nanocrystal size was observed. In this case, the particles became larger, having an average size of 3.1–3.2 nm, and the dispersion increased slightly (to 40%). It was assumed that MPTMS shells were grown on the Ag$_2$S nanocrystals. This is indicated by the blurring of the TEM images at the nanoparticle borders.
When attempting to increase the shell thickness for the Ag$_2$S/2-MPA+0.1 MPTMS QD sample (0.1 MPTMS) using Na$_2$SiO$_3$ dissolved in ethylene glycol, almost no increase in the average QD size was observed. At the same time, the samples prepared using a Na$_2$SiO$_3$ aqueous solution showed an increase in the QD average size to 3.5 nm with a slight increase in the size dispersion (to 35%). There were practically no small particles in this sample, but the number of large particles with a size of 5 nm or greater increased markedly. Around the nanocrystals, a noticeable blurring is observed in the TEM images, which indicates the growth of the SiO$_2$ shell.
The use of TEOS as a precursor for extension of the SiO$_2$ shell led to an increase in the QD average size (to 3.3 nm) and size dispersion (to 35%).
The SiO$_2$ shell formation was confirmed by the analysis of dark-field TEM images (Fig. 3). In the dark-field images of Ag$_2$S crystals, intense electron diffraction was observed. The image of the amorphous dielectric SiO$_2$ had low contrast. Thus, the differences in the QD sizes obtained from the dark-field and bright-field images are due to the SiO$_2$ shell. Moreover, the increase in the average nanocrystal size upon introduction of the SiO$_2$ precursors also indicates the shell formation.
Fig. 3. Comparison of bright-field (a) and dark-field (b) TEM images of Ag$_2$S/SiO$_2$ nanocrystals for samples with 0.1 MPTMS (1), 1.0 MPTMS (2), 3.0 EG Na$_2$SiO$_3$ (3), 3.0 W Na$_2$SiO$_3$ (4), and 3.0 TEOS (5).
For the samples with 0.1 MPTMS and 1.0 MPTMS (Fig. 3, panels 1 and 2), the QD sizes obtained from the dark-field and bright-field images almost coincide. Therefore, we conclude that there is no noticeable extension of the SiO$_2$ shell when additional MPTMS is introduced. The increase in the number of larger particles seen in the histograms, especially for the sample with 1.0 MPTMS, probably corresponds to additional crystallization of Ag$_2$S since MPTMS can act as a sulfur precursor.
The sample obtained with the addition of Na$_2$SiO$_3$ dissolved in ethylene glycol (Fig. 3, panel 3) did not reveal the presence of a shell. For the core/shell samples formed using a Na$_2$SiO$_3$ aqueous solution and TEOS (Fig. 3, panels 4 and 5), there is a clear difference in the nanocrystal sizes in the dark-field and bright-field images, which indicates the SiO$_2$ formation.
Thus, a SiO$_2$ shell is successfully formed in synthesis approaches with use TEOS and aqueous solution of Na$_2$SiO$_3$. The synthesis approach when concentration of MPTMS Na$_2$SiO$_3$ solution in ethylene glycol increases does not allow the formation of noticeable SiO$_2$ shell.
Also, HR-TEM images were obtained for Ag$_2$S/2-MPA and 3.0 TEOS QDs. Images analysis of Ag$_2$S/2-MPA QDs sample (inset in Fig. 2(a)) showed diffraction from the crystallographic plane (012) of the monoclinic Ag$_2$S lattice (P21/c) with interplanar distance of 0.3306$\pm$0.005 nm. HR-TEM image of nanocrystals, coated with a SiO$_2$ shell (inset in Fig. 2(f)) shows in addition to diffraction from Ag$_2$S crystallographic plane with interplanar distance of 0.3404$\pm$0.005 nm of the monoclinic Ag$_2$S lattice a low-contrast halo. It is interpreted as electron scattering on the shell of amorphous silicon dioxide.
According to XRD analysis data (Fig. 4, curve 1), synthesized Ag$_2$S QDs have a monoclinic lattice P21/c. This fact confirms the HR-TEM data. The presence of a wide halo in the X-ray diffraction pattern is due to the abundance of reflections in the monoclinic lattice of Ag$_2$S. Decreasing QDs size leads to the broadening of each reflection and formation of a wide halo in the 2$\theta$ angle range from 10$^\circ$ to 50$^\circ$. In the case of adding 0.1 m.f. MPTMS we observed the change in the diffractogram structure in the range of 2$\theta$ angles from 15$^\circ$ to 40$^\circ$ (Fig. 4, curve 2). A wide structureless low-intensity band appears. Similar changes are observed in the case of the 1.0 m.f. sample. MPTMS and sample with 3.0 m.f. Na$_2$SiO$_3$, dissolved in ethylene glycol. In the case of a sample with the addition of 3.0 m.f. Na$_2$SiO$_3$ aqueous solution and sample with the addition of 3.0 m.f. TEOS there is an increase in the intensity of this band (Fig. 4, curves 5 and 6). According to the literature data [43–45], these changes in diffraction patterns are interpreted as the contribution of the amorphous SiO2 phase [43].
The results of EDX analysis are presented in Fig. 5. In the case of Ag$_2$S/2-MPA QDs, the emission lines of Ag and S atoms, as well as an intense peak of C atoms, present in the 2-MPA, ethylene glycol, and used amorphous carbon substrate was found. The presence of oxygen atoms lines corresponds to its presence in 2-MPA and ethylene glycol molecules.
In the case of 0.1 MPTMS, the spectrum shows a weak peak at 1.76 keV. It corresponds to the group of emission lines of Si atoms. This band is associated with the superposition of the K$_{\alpha _1}$ (1.739 keV), K$_{\alpha _2}$ (1.739 keV), and K$_{\beta 1}$ (1.835 keV) lines. An increase in the intensity of the oxygen emission line K$_{\alpha _1}$ (0.525 keV) is also noted. It is a consequence of an increase in its concentration under condition of the SiO$_2$ shell formation. An increase in the intensity of the emission line of sulfur atoms and carbon atoms is observed in the 1.0 MPTMS sample. In this case, no significant increase in the intensity of the band, corresponded to Si atoms is observed. The observed changes are probably a consequence of an increase in the Ag$_2$S QDs core size in the absence of changes in the SiO$_2$ thickness. This regularity is in agreement with the conclusions, based on the analysis of dark-field TEM data. For 3.0 W Na$_2$SiO$_3$ sample, an increase in the intensity of the silicon and oxygen emission lines is observed. At the same time, according to the TEM data, an increase in the SiO$_2$ shell thickness was found. The addition of a similar amount of Na$_2$SiO$_3$ solution in ethylene glycol (sample 3.0 EG Na$_2$SiO$_3$) does not lead to any significant changes in the spectrum structure. In both samples using sodium metasilicate, a weak peak is observed in the region of 1.05 kEv, formed by the superposition of the sodium K$_{\alpha _1}$ (1.041 keV) and K$_{\beta 1}$ (1.067 kEv) lines. For 3.0 TEOS sample, an increase in the intensity of the emission lines of silicon and oxygen atoms is observed. It corresponds to an increase in the SiO$_2$ layer thickness in the TEM image.
3.2 Spectral properties of colloidal Ag$_2$S/2-MPA and Ag$_2$S/SiO$_2$ core/shell QDs
3.2.1 Absorption properties
In the UV–Vis absorption spectra for all studied samples (shown in Fig. 6, curves 1a–6a), broad spectra characteristic of semiconductor nanocrystals are observed. The spectra have features near the long-wavelength absorption edge, which transform into resolved peak for some samples (1a, 2a, 4a, 6a in Fig. 6). These maxima are within 710-725 nm (1.71–1.63 eV) and correspond to the ground state exciton absorption. In all cases, the peak positions of the ground state exciton absorption bands are higher in energy than the band gap of bulk Ag$_2$S crystals with a monoclinic crystal structure (1.0 eV) [46]. It is explained by the quantum confinement in UV-Vis absorption spectra of QDs.
Fig. 6. UV–Vis absorption (a) and luminescence (b) spectra for colloidal Ag$_2$S/2-MPA and Ag$_2$S/SiO$_2$ quantum dots.
The theoretical estimate of QDs size was realized based on the effective mass approximation using the equation published for the first time by Y. Kayanuma in [47]. The data on the effective masses of the electron and hole and dielectric permittivity were taken from [46].
The results of estimating the QD size by the Kayanuma equation and TEM data are given in Table 1.
Table 1. Positions of the ground state exciton absorption band and luminescence peaks, theoretical estimates of QD sizes ($d_{\textit {QD theor}}$), and estimates from TEM data ($d_{\textit {QD TEM}}$)
The discrepancy between the theoretical sizes and sizes, obtained from the analysis of TEM images is due to the roughness of the effective mass approximation. It does not take into account the parabolicity of Ag$_2$S bands [46]. However, a comparison of the change pattern in sizes shows that for the majority of samples in the theoretical estimation practically there are no noticeable changes in size. Only for sample 1.0 MPTMS we observe an increase in size by about 10%.
At the same time, according to the TEM data for the 3.0 W Na$_2$SiO$_3$ and 3.0 TEOS samples, an increase in the average QD size by 15% and 10%, respectively, was observed. Thus, the analysis of UV-Vis absorption spectra indirectly indicates the formation of SiO$_2$ shells on these samples. The reason for the change in the size of the 1.0 MPTMS sample is the growth of the QD core with an increase in the concentration of sulfur ions due to the large amount of the sulfur-containing precursor MPTMS.
A long-wavelength shift of the ground state exciton absorption by 5–50 nm is observed with the shell growth. One of the possible explanations for this effect can be partial penetration of nonequilibrium carriers into the shell. However, the band gap of a-SiO$_2$ is 9 eV [48]. Therefore, noticeable carrier penetration and deterioration in confinement are unlikely to provide such spectral shift. The second possible reason for the shift is the increase in the average size of Ag$_2$S nanocrystals due to the use of MPTMS, which acts as an additional sulfur precursor. The latter conclusion is supported by the analysis of TEM images, where an increase in the nanocrystal core size is observed for the samples with 1.0 MPTMS. In addition, the analysis of the dark-field images does not provide any evidence of the SiO$_2$ shell for these samples. Thus, the long-wavelength shift of the ground state exciton absorption band is caused by the increased size of Ag$_2$S core nanocrystals.
3.2.2 Photoluminescent properties
For the studied Ag$_2$S/2-MPA QD samples, IR luminescence is observed with the peak at 810 nm, band half-width of about 100 nm (0.19 eV), and the Stokes shift of 0.21 eV relative to the position of the ground state exciton absorption band (Fig. 6, curve 1b). The luminescence quantum yield is 1.6%.
Earlier, the size dependence of the luminescence spectra in colloidal Ag$_2$S QDs was studied by us in detail [49]. It was shown that the observed luminescence spectra are due to recombination on the trap state. It was also found that the spectral properties of the radiative trap states strongly depend on the interfaces conditions.
In the sample with 0.1 MPTMS, the luminescence peak is slightly shifted and appears at 805 nm (Fig. 6, curve 2b). In this case, there is an increase in the luminescence quantum yield up to 4.5%. A probable cause of these phenomena is the passivation of QD interface defects at adsorption of MPTMS on Ag$_2$S/2MPA QDs.
In the sample with 1.0 MPTMS, the luminescence peak is shifted to 860 nm with no changes in the Stokes shift (Fig. 6, curve 3b). The luminescence quantum yield in this case is decreased by a factor of 3 (from 4.5% to 1.4%). A similar tendency was noted in [50–53]. The fast increase in the concentration of sulfur ions in the solution leads to decrease in luminescence intensity. Based on the data obtained in the analysis of the dark-field TEM images, we can conclude that here the Ag$_2$S core size increases due to the excess sulfur. In this case, the long-wavelength shift of the luminescence peak is a manifestation of the size effect in the luminescence.
The use of Na$_2$SiO$_3$ dissolved in ethylene glycol for increasing the shell thickness does not lead to significant changes in the peak position in the luminescence spectrum (Fig. 6, curve 4b) relative to the sample with 0.1 MPTMS. A slight increase in the luminescence quantum yield (to 5%) is obtained. The use of a Na$_2$SiO$_3$ aqueous solution for growing the SiO$_2$ shell on Ag$_2$S QDs leads to a decrease in the luminescence quantum yield from 4.5% to 3.8% for the band peaked at 805 nm (Fig. 6, curve 5b). A probable reason for the decrease in the luminescence quantum yield is the alkaline medium associated with the Na$_2$SiO$_3$ aqueous solution, which can result in the formation of a defective shell. This fact was noted in [37]. The highest increase in the luminescence quantum yield (up to 8%) has been achieved for the sample with TEOS as the SiO$_2$ precursor (Fig. 6, curve 6b). In this case, a long-wavelength shift of the luminescence peak to 820 nm is observed. For this sample, the formation of the SiO$_2$ shell of a significant thickness was mentioned above.
3.2.3 Luminescence decays and recombination rates
The changes in the luminescence intensity and quantum yield revealed in the photoluminescence measurements are accompanied by a change in the average luminescence lifetime (Fig. 7, curves 1c–6c). The luminescence decay curves for all samples in the time range up to 1000 ns are not single-exponential.
Fig. 7. Luminescence decay and luminescence life time of the studied sample Ag$_2$S/2-MPA (a), 0.1 MPTMS (b), 1.0 MPTMS (c), 3.0 EG Na$_2$SiO$_3$ (d), 3.0 W Na$_2$SiO$_3$ (e), 3.0 TEOS (f).
Approximation of the experimental curves with a sum of two exponentials provides a means to determine the average luminescence lifetimes:
here $a_{i}$ and $\tau _{i}$ are the amplitude and lifetime for the $i$-th exponential component. Data on the components parameters are presented in Table 2.Table 2. Components parameters of the luminescence decay curves for colloidal Ag$_2$S/2-MPA and Ag$_2$S/SiO$_2$ QDs
The satisfactory analysis of the luminescence decay of exciton luminescence is present in [24]. It is supposed that the hole capture to trap state during the initial stage corresponds to the fast component. In the case of the studied samples, we discus the luminescence as result of trap state recombination (Fig. 6). In this case the complex dynamics of luminescence decay is due to several processes. They are the capture of an electron to the luminescence center, radiative recombination. Since the quantum yield is some less than unity for studied Ag$_2$S QDs there are also channels of non-radiative recombination, about which nothing is known. These facts complicate the luminescence decay law and do not allow to compare the decay stages and real physical processes as in the case of [24]. The problem of trap state luminescence decay was considered in detail by us earlier [49]. It was shown that the distribution of the recombination constant leads to a non-exponential decay law.
The recombination constants were estimated using the approach employed previously in [36]. It implies that the charge carriers (in the case of Ag$_2$S, the luminescence occurs at recombination of a localized electron and a free hole, with the electron being trapped at the luminescence center) are quickly localized at the luminescence center and are not involved in the non-radiative recombination. The scheme explaining the luminescence processes is present in Fig. 8.
Fig. 8. Scheme of trap state luminescence process (a); relations between average lifetime vs QY (b) and radiative constant rate vs QY (c).
In this case, we can use the expression for a simple two-level system
or where $k_{r}$ and $K_{nr}$ are radiative and non-radiative recombination constants, ${<\tau >}$ is the average luminescence lifetime. ${<\tau >}=\frac {1}{k_{r}+k_{nr}}$. Consequently, the non-radiative recombination rate is The data on the average luminescence lifetimes and recombination rates are given in Table 3.Table 3. Average luminescence lifetimes, luminescence quantum yields, as well as radiative and non-radiative recombination rates for colloidal Ag$_2$S/2-MPA and Ag$_2$S/SiO$_2$ QDs
Figure 8(b) shows the correlations between the data in Table 3. It can be seen that the average luminescence lifetime $\tau$ and QY, as well as the radiative constant rate and QY are linear. This indirectly confirms the inconsistency of the proposed primitive model of the luminescence process in Ag$_2$S QDs, as well as core/shell systems studied in this work.
For the studied Ag$_2$S/2-MPA samples, the measured average lifetime was 58 ns, with the luminescence quantum yield being 1.6%. The recombination rates estimated using relations (4) and (5) were $2.8\cdot 10^5$ s$^{-1}$ (radiative) and $1.7\cdot 10^7$ s$^{-1}$ (non-radiative). The addition of 0.1 MPTMS led to an increase in the average luminescence lifetime to 78.5 ns. A twofold increase in the radiative recombination rate (to $5.7\cdot 10^5$ s$^{-1}$) and a decrease in the non-radiative recombination rate (to $1.2\cdot 10^7$ s$^{-1}$) were obtained. These changes are probably the result of passivation of QD surface defects, which are quenchers of luminescence by the MPTMS molecules. It leads to a decrease in the concentration of defects that act as non-radiative recombination centers.
A further increase in the concentration of MPTMS led to a decrease in the average luminescence lifetime by 33.5 ns and a decrease in the radiative recombination rate by a factor of 1.8 (to $3.1\cdot 10^5$ s$^{-1}$), while the non-radiative recombination rate increased by a factor of 1.8 (to $2.2\cdot 10^7$ s$^{-1}$). This fact correlates with the obtained structural and spectral-luminescent data. It also indicates an increase in the concentration of non-radiative recombination centers. Consequently, the increase in the Ag$_2$S nanocrystal size is attributed to the rise in the concentration of sulfur ions in the solution associated with MPTMS. The surface defects resulting from the fast growth of the nanocrystals are effective centers of non-radiative recombination.
When using sodium metasilicate dissolved in ethylene glycol (3.0 EG Na$_2$SiO$_3$) for the SiO$_2$ shell growth, there was practically no change in the average luminescence lifetime. A slight increase in the luminescence quantum yield was observed. Also, the radiative recombination rate subtly increased, with almost no changes in the non-radiative rate. Taking into account the TEM data discussed above, we conclude that the addition of sodium metasilicate dissolved in ethylene glycol does not have any effect on the quantum dots. It appears that the shell is not formed. Therefore, no significant changes in the QD luminescent properties are observed.
When using an aqueous solution of sodium metasilicate (3.0 W Na$_2$SiO$_3$) for the SiO$_2$ shell growth, the average luminescence lifetime decreased to 63.7 ns and the quantum yield – to 3.8%. There was almost no change in the radiative recombination rate, but the non-radiative recombination rate increased (to $1.5\cdot 10^7$ s$^{-1}$). This result supports the hypothesis that there is an excess of defects, which are non-radiative recombination centers.
The use of TEOS provides the formation of SiO$_2$ shell. A change in the average luminescence lifetime from 78.5 to 87.6 ns and an increase in the luminescence quantum yield by a factor of 1.7 (up to 8%) are observed. In this case, there is a significant increase in the radiative recombination rate (by a factor of 1.6) and a slight decrease in the non-radiative recombination rate. These changes can be explained by the formation of a relatively perfect SiO$_2$ shell with a low concentration of defects (non-radiative recombination centers). This shell provides effective localization of charge carriers in the core, thus giving a significant increase in the luminescence quantum yield.
So, the estimation of the radiative and non-radiative recombination rates leads to an unexpected result. The non-radiative recombination rate does not change significantly with the extension of the shell relative to the sample with 0.1 MPTMS. The exception is the sample with 1.0 MPTMS, where, apparently, structural defects are formed in a significant concentration with an increase in the average QD size. On the contrary, the radiative recombination rate changes noticeably. This generally leads to the change in the quantum yield. These contradictions are easy to explain if we take into account the strong dependence of the luminescent properties of Ag$_2$S QDs on the environment (for different passivators) [49,50,52,54]. In this case, the growth of the SiO$_2$ shell can also lead to changes in the recombination rate for the luminescence center. At the same time, when using relations (4) and (5), we have assumed that the electron trapping is fast and the exciton does not recombine non-radiatively before localization. Taking into account the non-radiative recombination of the exciton requires the employment of the three-level model or even more complex models [55]. In this case, the quantum yield of recombination luminescence will depend not only on the radiative and non-radiative rates of recombination at the luminescence center (their relationship is established by (4) and (5)) but also will be governed by the ratio of radiative and non-radiative exciton recombination rates, as well as by the rate of electron trapping at the luminescence center. To establish the role of the non-radiative exciton decay in the investigated samples, an additional study is required, which will be the subject of future research.
4. Conclusions
In this paper, we considered the effect of SiO$_2$ shell formed within the framework of various approaches on luminescent properties of Ag$_2$S/SiO$_2$ core/shell structures. It has been found that the introduction of small concentrations (0.1 molar fractions) of MPTMS leads to effective passivation of surface defects and a significant increase in the quantum yield of QDs trap state luminescence. On the contrary, the use of high concentrations of MPTMS leads to the increase in the concentration of sulfur ions in the solution and undesirable growth of nanocrystalline nuclei and formation of additional interface defects. In this case, the concentration of non-radiative recombination centers increases, and the luminescent properties of QDs deteriorate. The found regularities are compared with the data for Ag$_2$S/SiO$_2$ QDs obtained using sulfur-free silicon dioxide precursors. It is shown that the use of an aqueous solution of sodium metasilicate leads to the formation of a shell. But the resulting surface is defective, probably, due to the alkaline reaction of aqueous solutions of sodium metasilicate. This leads to the increase in the non-radiative recombination constant and decrease in the quantum yield of luminescence. The employment of Na$_2$SiO$_3$ in ethylene glycol as a precursor does not facilitate the growth of the SiO$_2$ shell and provides no noticeable changes in the QD luminescent properties. The approach involving TEOS as a precursor provides the shell growth with a small number of defects. In this case, the effective localization of charge carriers in the core gives rise to a strong increase in the luminescence quantum yield for Ag$_2$S QDs (up to 8%) together with a significant increase in the radiative recombination rate. The Ag$_2$/SiO$_2$ quantum dots synthesized in the framework of this approach are promising for efficient luminescent sensors in the near-IR region.
Funding
Russian Science Foundation (19-12-00266).
Acknowledgments
This study was supported by the Russian Science Foundation under project no. 19-12-00266.
Disclosures
The authors declare no conflicts of interest.
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