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In this work, PbS QDs were precipitated in the glass containing sodium strontium silicate (Na2SrSi2O6) nanocrystals. Tunable absorption and photoluminescence of PbS QDs in the range of 1700-2200 nm were achieved through thermal treatment. Increased heat-treatment duration and temperature would induce the formation of Na2SrSi2O6 nanocrystals. The results from absorption and photoluminescence spectra and X-ray diffraction patterns indicated that PbS QDs acted as nucleating agents for Na2SrSi2O6 nanocrystals and facilitate their formation. The high resolution transmission electron microscopy analysis confirmed that PbS/Na2SrSi2O6 core/shell nanocrystals were formed in the glasses.
© 2016 Optical Society of America
1. Introduction
Semiconductor quantum dots(QDs) have been extensively investigated over the past two decades and still attract tremendous attention from both fundamental and applications [1–4]. Because of quantum confinement effect, QDs provide size-dependent optical properties, and thus have potential applications in many fields ranging from chemical sensing and biomedicine through to light-emitting diodes and photovoltaics [5–8]. Compared to II–VI and III–V QDs, lead chalcogenide QDs have large Bohr exciton radii with a strong quantum confinement and narrow bandgap energies, thus resulting in a wide tunable spectral range from visible to even mid-infrared region by varying their size [9–12]. Although colloidal lead chalcogenide QDs have been successfully synthesized by solution-based approach [13,14], precipitation of QDs in inorganic glasses through heat-treatment method is continuously pursued [15–20], because of their promising potential in photonic devices [21,22].
The large surface-to-volume ratio makes QDs vulnerable to high trap state densities if unsaturated or dangling bonds on the surface of QDs are not passivated [23]. These surface defects act as traps for charge carriers, seriously reducing the luminescence efficiency through non-radiative recombination in surface traps or deep trap emissions on the low-energy side of the band-edge emissions [24–26]. For chemically synthesized colloidal QDs, surface defects can be easily passivated by organic ligands or by forming a compatible crystalline shell on the top of core QDs [27,28], whereas, formation of a crystalline shell on QDs might be the only way to passivate the surface defects of QDs in glasses.
Thus, in this work, we pursued the formation of a crystalline shell on PbS QDs in the glass through heat-treatment. Absorption and emission spectra, and X-ray diffraction (XRD) patterns indicated that PbS QDs act as nucleating agents and facilitate the formation of Na2SrSi2O6 nanocrystals in the glass, while, transmission electron microscopy directly showed core/shell nanostructures formed in the glass. Though photoluminescence (PL) properties of PbS QDs was not improved by formation of Na2SrSi2O6 shell, because of the mismatch of crystal structures and lattice constants, this result provides the potential that the surface defects of QDs in glasses could be passivated by formation of a compatible shell on the QDs core, thus inducing the increase of PL efficiency of QDs in glasses.
2. Experimental
A glass with a nominal composition (in mol%) of 48SiO2–19.2Na2O–28.8SrO–4ZnS doped with additional 0.2 mol% of PbO (abbreviated as SP glass, hereafter) was prepared using a conventional melt-quenching method. Extra sulfur was added to compensate the sulfur loss during melting. Starting powders (~60 g) was weighed from SiO2, Na2CO3, SrCO3, ZnS and PbO (analytical grade), and thoroughly mixed using a ball mill with ZrO2 balls. The mixtures were then melted in a covered alumina crucible at 1450 °C for 30 min under the ambient atmosphere. The melt was poured onto a brass mold and quenched by pressing with another brass plate. The glass thus obtained was annealed at 350 °C for 3 h to release the thermal stress, and then cut into pieces of ~1.0 cm × ~1.0 cm with a thickness of ~0.2 cm. After thermal analysis was performed, specimens were further heat-treated between temperatures of glass transition and crystallization for varied durations to induce the precipitation of PbS QDs and other crystals.
Temperatures of glass transition and crystallization of as-prepared glass were determined using a simultaneous differential scanning calorimetry (DSC) (STA449c/3/G, NETZSCH, Germany). Crystalline phases formed during heat treatment were identified with X-ray diffractometer (D8 Advance, Bruker, Germany). Cu-Kα radiation (λ = 1.5406 Å) with a scanning rate of 2 °/min was used for the diffraction measurement with a resolution of 0.02°. Structure of nanocrystals precipitated in glasses was analyzed with a transmission electron microscope (TEM, JEOL, JEM-2200FS, Japan) under an accelerating voltage of 200 kV. TEM specimens were prepared through standard dimple grinding and ion-milling method. Absorption spectra of optically polished glasses were recorded using an UV/Vis/NIR spectrophotometer (UV3600, Shimadzu, Japan). PL spectra were recorded at room temperature using an 800-nm excitation beam from a Ti-Sapphire laser (3900S, Spectra Physics, USA), which was modulated by a mechanical chopper at a frequency of 30 Hz. The excitation laser beam was focused into the specimen by a silica lens with a focal length of 5 cm. Power of the excitation laser was maintained at 200 mW in order to avoid the photo-darkening effect [29]. PL was collected at the direction perpendicular to the excitation beam and recorded using a combination of 0.25 m monochromator, InGaAs detector and lock-in amplifier.
3. Results
Figure 1 shows the specific-heat-flow curve for the as-prepared (AP) glass heated at 10 °C/min. The glass transition temperature was found at 503 °C, and three crystallization peaks appeared at 583, 627 and 668 °C, respectively. Accordingly, specimens were heat-treated at temperatures between 540 °C, higher than glass transition temperature, and 560 °C, lower than the first crystallization peak of 583°C for different durations (10, 20, 30 and 40 h) to monitor the growth behavior of PbS QDs in the glass.
Under various heat-treatment schedules, either absorption peaks or absorption shoulders induced by the formation of PbS QDs appeared in the near-infrared region (Fig. 2). Absorption peaks occurred at 1740, 1940, 2050 and 2070 nm when glasses were heat-treated at 540 °C for 10, 20, 30 and 40 h, respectively (Fig. 2(a)). The shift of absorption peak towards the longer wavelength indicated the continuous growth of PbS QDs. Average sizes of PbS QDs formed in the glasses were calculated from their first excitonic absorption peak using empirical equation [30], and the calculated average diameters were listed in Table 1. The average diameters of PbS QDs increased from 7.2 nm to 10.0 nm (Table 1), whereas absorption coefficient at the peak wavelengths of those absorption bands decreased from 1.90 cm−1 to 0.77 cm−1 (Fig. 2(a)), when the heat-treatment duration increased from 10 h to 40 h (Table 1). These results suggested that Ostwald ripening might occur as heat-treatment duration increased at 540 °C.
Fig. 2 Absorption spectra of SP glasses heat-treated at (a) 540 °C, (b) 550 °C and (c) 560 °C for different durations.
Table 1. Center wavelength of absorption (λabs) and PL (λPL) bands, calculated average diameter (D) of PbS QDs, FWHM of the PL bands, and the estimated diameter (D) of the Na2SrSi2O6 nanocrystals.
When glasses were heat-treated at 550 °C, the absorption peak appeared at 1980 nm for 10-h heat treatment, whereas it turned to be absorption shoulders for 20-h and 30-h heat treatment (Fig. 2(b)). Those two shoulders nearly overlapped and displayed the center wavelengths of the shoulders at ~2040 nm through multiple Gaussian fitting (Fig. 2(b)). The calculated average diameters from the first excitonic absorption peak were also summarized in Table 1.When the glass was heat-treated at 550 °C for 40 h, the absorption of the glass significantly increased and the glass lost its transparency in the measured spectral range, which was likely due to the formation of other crystals in addition to PbS QDs. When the glass was heat-treated at 560 °C for 10 h, the center wavelength of the shoulder occurred at ~2200 nm, while glasses heat-treated for longer durations (>10 h) lost transparency in the recorded spectral range (Fig. 2(c)).
PL spectra were also recorded to examine the growth of PbS QDs with respect to heat-treatment schedules (Fig. 3). For glasses heat-treated at 540 °C, center wavelengths of PL bands shifted from ~1740 nm to ~1920 nm, ~1960 nm and ~1990 nm when the duration increased from 10 h to 20 h, 30 h, and 40 h, respectively (Fig. 3(a)). PL bands did not shift largely when the heat-treatment duration increased from 30 h to 40 h, which was similar to that observed in the absorption spectra (Fig. 2(a)). In addition, full width at the half maximum (FWHM) of PL bands became wider from ~200 nm to 270 nm when the heat-treatment duration increased from 10 h to 40 h. The anti-Stokes shifts of PL from absorption bands were probably due to the large size distribution of PbS QDs that caused the wide FWHM. However, we cannot rule out the mechanism of a two-photon absoption in two separate steps through a surface state leading to anti-stokes PL [31], though the size of PbS QDs in this work is large.
Fig. 3 PL spectra of SP glasses heat-treated at (a) 540 °C, (b) 550 °C and (c) 560 °C for different durations.
At 550 °C, the center wavelengths of PL band shifted from ~1800 nm to 1970 nm as heat-treatment duration increased from 10 h to 40 h, and FWHM became even wider (~250 nm to 500 nm), compared to that at 540 °C heat-treatment (Fig. 3(b) and Table 1). Interestingly, the glass heat treated at 550 °C for 40 h also shows smooth PL emission, though it lost the transparency in the measured spectral range (Fig. 3(b)). At 560 °C, the center wavelength of PL band with the widest FWHM (~500 nm) occurred at ~1890 nm for 10-h heat-treatment, whereas PL bands did not move as heat-treatment time increased from 20 h to 40 h, but only showed a small reduction in FWHM (Fig. 3(c) and Table 1). One important feature in the PL spectra of glasses heat-treated at 560 °C was the fluctuation in the luminescence intensities, especially for the glass heat-treated for 40 h (Fig. 3(c)). Such fluctuation could be attributed to the scattering of photoluminescence from PbS QDs by other crystals with a relative large size or phase separation in the glass.
Figure 4 shows XRD patterns of glasses heat-treated at 540 °C, 550 °C and 560 °C for different durations. As shown in Fig. 4(a), only broad halo was recorded for the as-prepared glass, indicating the non-crystalline nature of the specimen. For glasses heat-treated at 540 °C for 10 h, 20 h and 30 h, the diffraction patterns were similar to that of the AP specimen, indicating that no detectable crystals were formed in these glasses. Similar to previous works 17-19,28], diffraction peaks from PbS QDs cannot be recorded, due to small volume fraction of PbS QDs in the glass. However, when the heat-treatment time extended to 40 h, several small diffraction peaks appeared, which closely matched with those from Na2SrSi2O6 crystal (JCPDS No. 32-1159). Average diameter of Na2SrSi2O6 nanocrystals was estimated to ~31 nm using the Scherrer equation. At 550 °C and 560 °C, diffraction peaks of the Na2SrSi2O6 nanocrystals were observed from all glasses except those heat-treated for 10 h (Figs. 4(b) and 4(c)). Average diameter of Na2SrSi2O6 nanocrystals formed in the glasses was estimated using Scherrer equation and summarized in Table 1.
Fig. 4 XRD patterns of SP glasses heat-treated at (a) 540 °C, (b) 550 °C and (c) 560 °C for different durations.
4. Discussion
In order to clarify the effect of PbS QDs on the growth of Na2SrSi2O6 nanocrystals, glasses with nominal compositions (in mol%) of 48SiO2–19.2Na2O– 28.8SrO–4ZnS (SS glass, without PbO) and 48SiO2–19.2Na2O–28.8SrO–4ZnO (SZ glass, without PbO and ZnS) were prepared for comparison. Both glasses were melted under the same condition as specified in the experimental section for the SP glass. Afterwards, the obtained glasses were heat-treated at 560 °C for different durations. Absorption spectra of as-prepared and heat-treated SS glasses were shown in Fig. 5(a). In these glasses, one absorption band located at ~400 nm was observed, which is similar to that observed in glasses containing sulfur and is attributed to S2+ interstitial molecular ions [32]. With the increase in the heat-treatment duration, absorption coefficient of this band decreased gradually, because of thermal bleaching [32]. However, no crystalline phase was detected, as XRD patterns of those heat-treated glasses showed only broad halos (Fig. 5(b)). Similar to SS glasses, SZ glasses also showed an absorption band at ~365 nm, which did not show temperature dependent shift (Fig. 6(a)). This absorption band was probably due to the presence of impurity color centers [33]. XRD patterns of SZ glass also showed that it still remained in amorphous state after heat-treatment at 560 °C (Fig. 6(b)).
Both Fig. 5 and Fig. 6 showed that Na2SrSi2O6 nanocrystals cannot be formed in the absence of PbS QDs, even at 560 °C for 40 h. This result indirectly indicated that PbS QDs acted as the nucleating agent and promoted the formation of Na2SrSi2O6 nanocrystals in SP glasses. In order to confirm this hypothesis, two-step heat-treatment experiments were performed in the SP glasses. It has been shown that only PbS QDs but no Na2SrSi2O6 nanocrystals were formed, when SP glasses were heat-treated at 540 °C for 10 h, 20 h and 30 h, or 560 °C for 10 h. Therefore, one could expect that PbS QDs were formed in the first step at 540 °C for 10 h, 20 h or 30 h, and Na2SrSi2O6 nanocrystals would precipitated out during the second step at 560 °C for 10 h, if thus formed PbS QDs could act as the nucleating centers for Na2SrSi2O6 nanocrystals.
Figure 7(a) shows the absorption spectra of SP glasses with two-step heat-treatment, compared with glasses heat-treated at 540 °C for 30 h and 560 °C for 10 h. For SP glass heat-treated at 560 °C for 10 h, the first absorption shoulder appeared at ~2200 nm (Fig. 2(c)).
Fig. 7 (a) Absorption and (b) XRD patterns of two-step heat-treated SP glasses. Absorption and XRD patterned recorded from glass heat-treated at 540 °C for 30 h and glass heat-treated at 560 °C for 10 h were included for comparison.
However, when the SP glasses were heat-treated at 540 °C for 10 h, 20 h or 30 h, and subsequently heat-treated at 560 °C for 10 h, the first absorption peak (or shoulders) appeared at ~2025 nm, ~2080 nm and ~2085 nm, respectively, which is shorter than that observed in the glass heat-treated at 560 °C for 10 h. Average size of PbS QDs formed after two-step heat-treatment was estimated from the first absorption bands and listed in Table 1. The two-step heat-treatment yielded a comparable size with that from one-step heat-treatment at 560 °C for 10 h, indicating the further heat-treatment could not result in the size increase. For all glasses with two-step heat-treatment, diffraction peaks corresponding to Na2SrSi2O6 nanocrystals were observed. However, no any diffraction peaks were recorded, when glasses singly heat-treated at 540 °C for 10 h, 20 h and 30 h, or at 560 °C for 10 h. This also indicated that PbS QDs formed in the SP glasses acted as the nucleating agent for Na2SrSi2O6 nanocrystals.
To further confirm the formation of PbS QDs and its effect on the formation of Na2SrSi2O6 nanocrystals, TEM images of SP glasses subjected to one-step (540 °C for 30 h) and two-step heat-treatment (540 °C for 30 h and 560 °C for 10 h) are shown in Figs. 8 and 9, respectively. Nanocrystals with the size of approximately 10 nm formed in the glass heat-treated at 540 °C for 30 (Fig. 8(a)). Figure 8(b) shows the high resolution TEM image of a single PbS QD, and Fig. 8(c) is the corresponding diffraction pattern of the nanocrystal obtained through the fast Fourier transformation. The diffraction spots shown in Fig. 8(c) exclusively evidenced that nanocrystal formed in the glasses had a face centered cubic structure as that of PbS crystal. For the glass obtained through two-step heat-treatment, the size of the nanocrystals was found to be ~20 nm, which is apparently larger than that (~10 nm) calculated from the absorption peak (Fig. 9(a)). The high resolution TEM image showed that the core was surrounded by another crystalline phase (Fig. 9(b)). The fast Fourier transformation image (inset in Fig. 9(b)) showed these diffraction spots were from two kinds of crystalline phases. Considering the XRD patterns of glasses heat-treated at various conditions, we believed PbS QDs acted as nucleating agents for Na2SrSi2O6 nanocrystal and PbS/Na2SrSi2O6 core/shell nanocrystals probably formed in the glass. However, due to the mismatch in the crystal structure of PbS QDs and Na2SrSi2O6 nanocrystal, enhancement in the photoluminescence efficiency of PbS QDs was not observed. But, this result provides us the opportunity to form the shell on QDs in the glass in order to passivate the surface defects of QDs.
Fig. 8 (a) TEM image, (b) high resolution TEM image of a single nanocrystal and (c) the fast Fourier transformation image from Fig. 8(b). The SP glass was heat-treated at 540 °C for 30 h.
Fig. 9 (a) TEM image, and (b) high resolution TEM image of a single nanocrystal. Inset in Fig. 9(b) is the fast Fourier transformation image from the core/shell structured nanocrystal. The SP glass was heat-treated at 540 °C for 30 h followed by a second step heat-treatment at 560 °C for 10 h.
5. Conclusion
In summary, PbS QDs with average diameter of 7.2-10.0 nm and Na2SrSi2O6 nanocrystals with average diameter of 30-45 nm were precipitated in the glasses through thermal treatment. The growth behavior of PbS QDs in glasses with respect to heat-treatment temperatures and durations was monitored with absorption and photoluminescence spectra. The formation of Na2SrSi2O6 nanocrystals was identified with X-ray diffraction patterns. The results suggested that PbS QDs acted as nucleating agents for Na2SrSi2O6 nanocrystals and promote their formation in glasses. High resolution transmission electron microscopy analysis evidenced that PbS/Na2SrSi2O6 core/shell nanocrystals were formed in the glass.
Acknowledgments
This work was supported by the Natural Science Foundation of Hubei Province (Grant No.: 2012FFA024, 2013CFA008), National Natural Science Foundation of China (Grant No.: 51202170), Program for New Century Excellent Talents in University (Grant No.: NCET-13-0943), and Chutian Scholar Program of Hubei Province.
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