Open Access
Add to BibSonomy
Abstract
PbS quantum dots (QDs) with a mean radii of 1.27 nm to 7.32 nm were precipitated in alkaline-earth silicate glasses enriched in sulfur. A transmission electron microscope image and absorption spectra evidenced the narrow size dispersion of PbS QDs and discrete energy levels of PbS QDs. Upon above-band-gap excitation, symmetry of the photoluminescence from PbS QDs was strongly dependent on their size. Photoluminescence from small PbS QDs showed a long-wavelength tail and vice versa for large PbS QDs. The size dependence of full width at half maximum, PL energy and Stokes shift on the size of PbS QDs suggested that the electron/hole trap states of PbS QDs and the defects states on the interface between PbS QDs and glass matrix have great effect on the photoluminescence properties of PbS QDs.
© 2017 Optical Society of America
1. Introduction
Lead chalcogenide quantum dots (QDs) have attracted a lot of attention due to their potential applications in the near- and middle-infrared spectral range [1–11]. In the past decades, great progress has been achieved in synthesis of QDs through wet chemistry techniques [12]. Realization of the monodisperse QDs [12,13], modification of surface states of QDs [14–16], and understanding of the exciton fine structures [17,18] of QDs has boosted the potential applications in light-emitting diodes [19], photovoltaics [20], gain media [2,3], and biolables [1]. Precipitation of lead chalcogenide QDs has long been studied, and precipitation of lead chalcogenide QDs in glasses through melt-quenching and subsequent thermal treatment can guarantee the good thermal, chemical and mechanical stabilities, prevent the agglomeration of QDs and facilitate the fabrication of various optical devices [21].In addition, precipitation of the QDs inside the glass matrix can guarantee the chemical stability of QDs and photo-stability due to the inert nature of the glasses [22].However, for most of the applications, it is important to have QDs with narrow size dispersion, which is strongly dependent on the glass composition, heat-treatment conditions, and concentration ratio between lead and chalcogen, etc. In our previous work, it has been shown that oversaturation of lead chalcogenide in the glass was mainly determined by the chalcogen [23]. Oversaturation of chalcogen in the glass can alleviate the dependence of the QDs formation on the heat-treatment, and facilitate the formation of QDs with narrow size dispersion, which can be comparable to those synthesized through wet chemistry techniques [12,13].
In addition, photoluminescence of QDs formed in the glasses was strongly dependent on the size, surface conditions and size dispersion. Through wet chemistry techniques, monodisperse and surface passivated QDs can be easily obtained [16,24], which facilitated the understanding the peculiar properties of photoluminescence of the QDs [13,25,26].For glasses containing quantum dots, various photoluminescence phenomena have been reported, including multiple photoluminescence bands, reversible photo-darkening and photo-brightening, etc [27–31]. Several models and schematic energy diagrams have been proposed to explain the observed phenomena [27–30,32,33]. However, the broad size dispersion of quantum dots in the glasses and small size range of quantum dots investigated led to the discrepancy among different models. Therefore, it is necessary to investigate the size-dependent photoluminescence properties of PbS QDs with narrow size dispersion.
In this work, PbS QDs doped alkaline-earth silicate glasses were prepared through conventional melt-quenching and heat-treatment. PbS QDs with mean radii of 1.27 nm to 7.32 nm and narrow size dispersion were precipitated in the glasses. Photoluminescence spectra were recorded using above-band-gap excitation, and size-dependent photoluminescence properties were investigated.
2. Experiment
Glasses with nominal compositions of 50SiO2-25Na2O-5Al2O3-(10-x-y)ZnO-10MO-xZnS-yPbO (M = Ca, Sr, all in mol%) were prepared by conventional melt-quenching method. The nominal concentration ratios of ZnS and PbO were varied from 2.0:0.5 to 3.0:0.6 and 4.0:0.2.Glasses containing Ca and Sr were named as Cx or Sx series, where x represented the different concentration ratios of S and Pb (x = 1 for 2.0:0.5, x = 2 for 3.0:0.6, x = 3 for 4:0.2). Chemical powders with purity of >99.9% were weighted and thoroughly mixed. The mixed powders were melted in alumina crucibles at 1400 °C for 30 min under the ambient atmosphere. The melts were poured into a brass mold and pressed with another plate for quenching. The glasses thus obtained were annealed at 400 °C for 2 h to reduce the thermal stress.
Compositions of the as-prepared glasses were analyzed using the continuum source atomic absorption spectroscopy (CS-AAS, contrAA700, Germany). It was found that sulfur and lead showed some evaporation loss during the melting process, and the effective concentration ratios between S and Pb were shown in Table 1. Glass transition temperatures of these as-prepared glasses were found to be ~500 °C using the simultaneous thermal analyzer (STA449c/3/c/G, NETZSCH, Selb, Germany).Annealed glasses were cut into small species for heat-treatments at temperatures around or above the glass transition to precipitate PbS QDs. Size of PbS QDs were tuned by modulating the heat-treatment temperatures or durations. Additional heat-treatments at 520 °C (for S3 glasses) and 540 °C (for C3 glasses) for extended duration were carried out to investigate the effect of heat-treatment time on the growth and optical properties of PbS QDs in glasses. The heat-treated glasses were ground into powders, and the structure and distribution of the QDs in the glasses were characterized using a high-resolution transmission electron microscope (HR-TEM, JEM-2200FS, JEOL, Japan) with an image Cs-corrector and an Ω-filter. Absorption spectra of the as-prepared and heat-treated glasses were recorded using an UV/Vis/NIR spectrophotometer (UV3600, Shimadzu, Japan).The photoluminescence spectra of the heat-treated glasses were recorded using a combination of 800 nm laser, mechanical chopper, monochromator, detector, and lock-in amplifier. The 800 nm laser beam, which was modulated by the mechanical chopper, was focused into the specimens using a silica lens with focal length of 5 cm. Photoluminescence was collected at the direction perpendicular to the excitation beam and dispersed into a monochromator. The excitation irradiance was kept as ~70W/cm2. Either an InGaAs or InSb detector was used to detect the intensity of photoluminescence. Low temperature PL spectra were recorded with an additional cryostat (Optistat AC-V12, Oxford, Abingdon, UK).
Table 1. Absorption peak energy, calculated average radii and size dispersion of PbS QDs formed in glasses
3. Results and discussion
Upon heat-treatment, color of the glasses gradually changed from yellow (as-prepared, abbreviated as AP) to brown, dark-brown and black as the heat-treatment temperature gradually increased. Changes in the color of the heat-treated glasses indicated the precipitation of PbS QDs in the glasses. In order to facilitate the characterization, C1 glass heat-treated at 540 °C for 30 h was examined by HR-TEM (Fig. 1). As shown in Fig. 1(a), nearly spherical nanocrystals were almost homogeneously precipitated inside the glass upon thermal treatment. The HR-TEM of one nanocrystal in Fig. 1(a) showed that the interplanar distance was 2.03 Å, comparable to the interplanar distance of (220) crystal plan of PbS crystal (JCPDS No.: 78-1054).Based on the TEM image, it was also found that size of the PbS QDs followed the Gaussian distribution with an average radius of 4.12 nm and a size dispersion of ~7.7%.
Fig. 1 (a) High-resolution transmission electron microscope image and (b) size distribution of nanocrystals formed in glass heat-treated at 540 °C for 30 h. Inset in (a) is an HR-TEM image of one nanocrystal formed in the glass.
To investigate the effects of heat-treatment temperature and duration on the growth and optical properties of PbS QDs in the glasses, absorption spectra of the as-prepared and heat-treated C1, C2, C3, S1, S2 and S3 glasses were recorded and illustrated in Fig. 2.For the as-prepared C1 glass, absorption showed a cut-off at ~378 nm. Upon thermal treatment at 500 °C for 10 h, the absorption cut-off edge red-shifted and a weak shoulder appeared at 677.3 nm (1.83 eV). With further increase in the heat-treatment temperature from 510 °C to 560 °C with a step of 10 °C, the absorption peaks appeared and the peak wavelength shifted from 772.4 nm (1.61 eV) to 970.9 nm (1.28 eV), 1154.4 nm (1.07 eV), 1513.7 nm (0.82 eV), 1774.2 nm (0.70 eV) and 1948.2 nm (0.64 eV), respectively. Shift in the absorption peaks with the increase in heat-treatment temperature indicated the continuous growth of PbS QDs in the glass. For C2, C3, S1, S2 and S3 series of glasses, absorption showed similar behaviors upon heat-treatment (Table 1).
Fig. 2 Absorption spectra of (a) C1, (b) C2 and (c) C3 series, and (d) S1, (e) S2 and (f) S3 series glasses heat-treated at various conditions. In all spectra, (1) represents the as-prepared glasses, and (2)-(7) represent the heat-treatment temperatures of 500 °C, 510 °C, 520 °C, 530 °C, 540 °C, 550 °C, and 560 °C, respectively.
Several theoretical models have been proposed to predict the size-dependent effective band gap energy of lead chalcogenide QDs [34–37]. For PbS QDs formed in the glasses, average size of PbS QDs estimated using the hyperbolic band model matched well with size found in the TEM images [21,34,38,39].For the C3 glass heat-treated at 540 °C for 30 h, the average radius was calculated to be 4.33 nm, comparable to the size obtained from the TEM image (4.12 nm), which in turn confirmed that the average radii of PbS QDs (Table 1) calculated using the hyperbolic band model were very close to the actual size of PbS QDs precipitated in the glasses. As a result, the calculated radii (Table 1) of PbS QDs were used as the size of PbS QDs in this work. For all these glass specimens containing PbS QDs, it was found that the average radii of the PbS QDs increased monotonically with the increase in thermal treatment temperature. This phenomenon was consistent with those previously reported [21,38–41].
Another feature of these absorption spectra of PbS QDs was the appearance of multiple peaks as the increase in the heat-treatment temperature. It was found that the absorption induced by PbS QDs only appeared as broad shoulders when the heat-treatment temperature was low. At higher heat-treatment temperature, these absorption peaks became much sharper and several additional peaks at higher energies were observed. Further increase in the heat-treatment temperature led to the broadening of the absorption peaks again (Fig. 2). The sharpness of the absorption peaks and the appearance of the multiple absorption peaks were closely related to the size dispersion of the PbS QDs [42, 43].
In order to get further information on the size dispersion of PbS QDs formed in the glasses, the absorption spectra of heat-treated glasses were simulated using multiple Gaussian functions (Fig. 3).Since the cut-off absorption edge of glass matrix was far from the absorption induced by PbS QDs, and the scattering loss induced by the small-sized PbS QDs was negligible in this spectral range, all the absorption observed were ascribed to the electronic transitions of PbS QDs formed in the glasses. Taken C3 glass heat-treated at 540 °C for 10 h as an example, the absorption spectrum can be nicely reproduced using six Gaussian functions (including the peak marked by an asterisk), and these Gaussian functions matched very well with the peaks or pumps observed in the absorption spectrum as well as the negative dips in the second derivative (dark grey line in Fig. 3) of the absorption spectrum [42,43]. This confirmed that the multiple Gaussian simulations yield reasonable excitonic absorption peaks. The size dispersion () of PbS QDs formed in the glasses were then calculated using the full width at half maximum of the lowest excitonic absorption peaks (i.e. peak 1 in Fig. 3), and the calculated size dispersion was listed in Table 1. It can be found from Table 1 that size dispersion of PbS QDs decreased firstly and increased again as the heat-treatment temperature increased from 500 °C to 570 °C. Among them, 90% of PbS QDs was found to have size dispersion smaller than 10%, and 34% of PbS QDS was found to have size dispersion smaller than 6%. These results showed PbS QDs formed in the glasses through heat-treatment had a small dispersion, even comparable to those synthesized through chemical routes [42,43]. In addition, Table 1 also evidenced that heat-treatment temperature and duration had different effects on size dispersion of PbS QDs in the glasses. High heat-treatment temperature can easily lead to the increase in size dispersion due to the Ostwald ripening effect [44]. However, increase in the duration of heat-treatment can lead to the efficient narrowing of size dispersion. For C3 glasses, when heat-treated at 540 °C, the size dispersion decreased from 5.49% to 4.99%, 4.99% and 4.94% when the duration increased from 10 h to 20 h, 30 h and 40 h, respectively. For S3 glasses, when heat-treated at 520 °C, the size dispersion decreased from 6.27% to 5.44%, 5.22% and 5.23% when the duration increased from 10 h to 20 h, 30 h and 40 h, respectively. This phenomenon was very similar to the “self-focusing” effect of quantum dots synthesized through chemical routes [45], except the increase in size with prolongation of heat-treatment duration. This indicated that prolongation of heat-treatment can lead to the precipitation of quantum dots with small size dispersion.
Fig. 3 Multiple Gaussian functions simulation of the absorption spectrum of C3 glass heat-treated at 540 °C for 10 h. The open circles represent the recorded absorption data, and the solid lines are the Gaussian functions. The dark grey line at the top is the second derivative of the absorption spectrum.
Due to the small size dispersion, most of the absorption spectra can be simulated using five or six Gaussian functions (Fig. 3). The electronic transition energy of each peak (marked in Fig. 3) was summarized in Fig. 4(a). These peak energies were plotted versus the inverse square of radius. Since the effective band gap energy of quantum dot was dependent on its radius [34–37], the following equation was used to simulate the peak energies [43]:
where x represents the peaks shown in Fig. 3, Egis the bulk band gap energy of PbS crystal (0.41 eV at room temperature), Ax represents the quantum confinement energy (proportional to ) and Bx represents the Coulomb attraction (proportional to ). The spatial correlation term () was removed from Eq. (1) due to the large dielectric constant of PbS [33]. The fitting yielded the following relations for the electronic transitions:Fig. 4 (a) Transition energies of peaks obtained from multiple Gaussian simulations versus r−2, (b) exciton confinement energies as a function of the first exciton confinement energy.
It was found that all the electronic transitions of PbS QDs in the glasses can be reasonably simulated using Eq. (1), even including the peak marked with an asterisk in Fig. 3. Instead of the random distribution of the values in Ref. 43, the values of and obtained in this work almost increased monotonically as the electronic transitions moved to high energy side. In the effective mass approximation [46], the “normalized confinement energy”, i.e. the confinement energy of an electronic transition divided by the confinement energy of the first electronic transition [43,46], which should remain constant for comparable degree of confinement, was plotted as a function of confinement energy of the first electronic transition in Fig. 4(b).
It was found that the normalized confinement energies were nearly constant at the following values:
These values were found to be comparable to, but slightly larger than those reported in Ref. 43.The main reason for the larger values may most probably come from the size dependence of the confinement energy, which decreased as the band gap energy decreased. The peak marked by an asterisk was observed in all of the glasses containing PbS QDs with size dispersion smaller than 6%, scaled similarly with the radii of PbS QDs as other electronic transitions (Fig. 4). However, origin of this peak was still in debate [42,43].
The above results confirmed that PbS QDs with narrow size dispersion were precipitated in the glasses upon heat-treatment, facilitating the study of the photoluminescence properties. Figure 5 shows the photoluminescence spectra of the glasses containing PbS QDs. Similar to those observed in the absorption spectra (Fig. 2), peak wavelength of the photoluminescence bands shifted towards long wavelength side as the heat-treatment temperature increased, indicating that these photoluminescence was closely related to the size of PbS QDs, consistent with those reported previously [21,38–41]. By adjusting the heat-treatment temperature, photoluminescence can be tuned in the range of 800 nm to 2400 nm. Besides, symmetry of these photoluminescence spectra was also strongly dependent on the size of PbS QDs. It was found that for PbS QDs with large band gap energies, the photoluminescence bands showed tail extending towards long wavelength side, and for PbS QDs with small band gap energy energies, the photoluminescence bands showed tail at the short wavelength side. For PbS QDs with intermediate band gap energies, the photoluminescence band looked almost symmetric (Fig. 5). Changes in the symmetry of the photoluminescence bands indicated that additional factors, in addition to the quantum confinement effect, had important effects on the photoluminescence properties of PbS QDs. Similar phenomena have also been reported [25,47].
Fig. 5 Photoluminescence spectra of (a) C1, (b) C2 and (c) C3 series, and (d) S1, (e) S2 and (f) S3 series glasses heat-treated at various conditions. The number on the top of each curve represents the heat-treatment temperature. All the photoluminescence spectra were normalized for clear comparison.
In order to the get more information on the photoluminescence of PbS QDs, photoluminescence spectra of these PbS QDs were simulated using multiple Gaussian functions. It was found that these photoluminescence spectra can be nicely fitted using dual Gaussian functions. Photoluminescence from PbS QDs formed in C2 glasses heat-treated at 520 °C, 530 °C, and 550 °C were simulated as examples, as shown in Fig. 6(a). For PbS QDs with large band gap energy of 1.57eV (C2 glass heat-treated at 520 °C), peaks of the Gaussian functions were found at 1.23eV (FWHM: 185.5 meV) and 1.20eV (FWHM: 283 meV). As the band gap energy of PbS QDs decreased to 1.13 eV (C2 glass heat-treated at 530 °C), peaks of Gaussian functions were found at 0.993 eV but different FWHM values of 94.6 meV and 178 meV. With further decrease in the band gap energy of PbS QDs down to 0.7 eV (C2 glass heat-treated at 560 °C), Gaussian functions peaked at 0.70 eV with a FWHM value of 65.5 meV and 0.72 eV with a FWHM value of 99.7 meV can yield a nice fitting to the photoluminescence spectrum. All the simulation results were summarized in Fig. 6.These size-dependent photoluminescence line-shapes were very similar to a recent report by Caram et al. [25]
Fig. 6 (a) Simulation of photoluminescence of PbS QDs formed in C2 glasses heat-treated at 520 °C, 530 °C, and 560 °C. The blue lines are Gaussian functions for the P1 and P2 peaks, and red lines are summation of blue lines. Open circles represents the experimental data. (b) Full width at half maximum values of the lowest excitonic absorption peak (solid squares), P1 peak (solid circles) and P2 peak (open circles). (c) Energy of P1 (solid circles) and P2 (open circles) peaks. The solid line represents the photoluminescence energy with zero-Stokes shift. (d) Stokes shift of P1 (solid circles) and P2 (open circles) peaks.
From these Gaussian fittings, several features were observed. First, in most cases, these two Gaussian functions had big difference in the FWHM values, as shown in Fig. 6(b). In the following part, the narrow Gaussian peak was named as P1 and the broad Gaussian peak was named as P2. For all the PbS QDs studied in this work, the FWHM values obtained from the simulation of the photoluminescence spectra were summarized in Fig. 6(b), together with the FWHM values of the lowest excitonic absorption peak. It was found that FWHM values of the absorption peaks, P1 and P2 peaks all decreased as the band gap energies of the PbS QDs decreased. As the bandgap energy decreased (or increase in size), the FWHM values of the lowest absorption peaks also decreased, consistent with the recent report by Campos et al. [13]. Exciton-phonon coupling, exciton fine structure, and spectral diffusion have been considered to exert large effect on the FWHM of the lowest absorption peaks [48–50].When the band gap energies of PbS QDs were ~1.5 eV or above, FWHM values of P2 was comparable to that of absorption peak. At lower band gap energy of PbS QDs, the FWHM values of P2 became larger than that of the absorption, and the difference became larger as the decrease in the band gap energy of PbS QDs. Difference in the FWHM values of P2 and absorption peak indicated that the photoluminescence peak P2 was probably from some defects states instead of the intrinsic band edge photoluminescence from PbS QDs [51]. In terms of photoluminescence peak P1, the FWHM values was smaller than that of the absorption peak when the band gap energies of PbS QDs was larger than 0.9eV, and comparable with that of the absorption peak when the band gap energy of the PbS QDs was smaller than 0.9 eV.
Second, energies of the photoluminescence P1 and P2 changed their relative location as the band gap energies of PbS QDs changed. For small PbS QDs with large band gap energies (>1.5 eV), energy of P1 was larger than that of P2. For PbS QDs with band gap energies of 1.2-1.5 eV, energies of P1 and P2 were comparable to each other. With further decrease in the band gap energies of PbS QDs, energy of P1 became smaller than that of P2, shown in Fig. 6(c). This size-dependent shift of P1 and P2 peaks was consistent with the recent report [25]. For P1 peak, its energy decreased monotonically as the band gap energy of PbS QDs decreased, and its energy never exceeded the band gap energy of the PbS QDs. While for P2 peak, it also decreased monotonically as the band gap energies of PbS QDs decreased, but it exceeded the band gap energy of PbS QDs with band gap energies smaller than 0.9 eV. These changes indicated that P1 and P2 peaks had different origins and P1 peak was closely related to the confinement effect [25,47].
Third, when the band gap energy of PbS QDs was larger than 0.9 eV, energies of the photoluminescence peak P1 and P2 showed large deviation from the zero-Stokes line (the black solid line) in Fig. 6(c), leading to the presence of large Stokes shift of the P1 and P2 peaks in Fig. 6(d). When the bandgap energies of PbS QDs decreased from 1.82 eV to 0.97 eV, the Stokes shift decreased from 532.5 meV to 61.3 meV. With further decrease in the band gap energies of PbS QDs, Stokes shifts of P1 peak gradually decreased to zero, as shown in Fig. 6(d). While, P2 peak showed anti-Stokes shift as the band gap energies of PbS QDs decreased. It has been shown that both intervalley splitting and exchange splitting can contribute to the Stokes shift [17]. Stokes shift induced by these splitting effects was strongly dependent on the size of QDs, and it decreased from more than 100 meV to zero as the size of QDs increased [17,52]. However, the huge anti-Stokes shift observed from large QDs indicated that P2 peak was probably associated defect states which were naturally present in the glasses, such as non-bridging oxygen, structural modifiers and other disordered structures located at the interface between PbS QDs and glass matrices.
Changes in the peak energies, FWHM and Stokes shift of P1 and P2 peaks observed in this work (Fig. 6) indicated that P1 and P2 peaks had different origins, which were schematically illustrated in Fig. 7. For PbS QDs embedded in glasses, there existed electron trap states and hole trap states (red dashed lines in Fig. 7) due to the presence of the unpassivated surface states [27,53,54]. When the PbS QDs was photo-excited, the photo-generated electron will be trapped by the electron trap states located at below the state of PbS QDs. Radiative transition of the trapped electrons to the state of PbS QDs gave the photoluminescence corresponding to P1 peak with a Stokes shift of . It has been proposed that between the electron trap states and state of PbS QDs was strongly dependent on the size of QDs [39,47,52,], and decreased as the size of QDs increased. When the size of QDs was large enough, the electron trap states can locate at or above the lowest excitonic state in the conduction band of QDs [55]. In such case, Stokes shift of P1 peak would decrease as the size of PbS QDs increase, and zero-Stokes shift of P1 peak can be expected for large PbS QDs, consistent with Stokes shift observed in Fig. 6(d).When zero-Stokes shift was observed, FWHM of the P1 peak was comparable to that of the absorption peak shown in Fig. 6(b), and therefore, P1 peak photoluminescence was mostly composed of the intrinsic emission ( transition) of PbS QDs due to the rapid electronic cooling of trapped electrons to the state of PbS QDs. This schematic energy diagram was consistent with that proposed by Caram et al. [25]
Fig. 7 Schematic energy diagram for small, medium and large PbS QDs. ETS, HTS, DS represent the electron trap states, hole trap states and defect states, respectively. Dashed (P1) and dotted (P2) arrows represent the radiative transitions from ETS and DS to 1Sh state of PbS QDs. The solid arrow is the radiative transition from 1Se→1Sh.
Compared with P1 peak, changes in the FWHM, Stokes shift, and peak energy of P2 peak with band gap energies of PbS QDs showed that P2 peak was not from the electron trap states on the surface of PbS QDs, but from the defect states located on the interface between PbS QDs and glass matrix (DS shown in Fig. 7), and their energy position and distribution of these defect states was closely related to the size of PbS QDs. For small PbS QDs, the defect states showed a wide distribution (thin lines in Fig. 7) with an energy distribution of and formed a defect states band (DS) with a center slightly below the electron trap states. With the increase in the size of PbS QDs, DS band moved up relative to the 1Se state of PbS QDs and overlapped with electron trap states. As a consequence, the P1 and P2 peaks overlapped with each other. With further increased in the size of PbS QDs, the DS band can even located above the 1Se state of PbS QDs, leading to the anti-Stokes photoluminescence (P2 peak with negative Stokes shift).
Effects of trap states and defect states on the photoluminescence properties were further investigated using the low temperature photoluminescence spectra (Fig. 8). For PbS QDs with the smallest size (formed in glass heat-treated at 520 °C), the trap states located at higher energy than defect states (Fig. 7), and the photo-generated electrons will be trapped by the trap states first. As the temperature decreased, photoluminescence intensity increased and the maxima of the PL bands gradually shifted towards high energy side. This observation was consistent with that reported by Caram et al. [25], indicating that trap states were more emissive than the defect states. For larger PbS QDs in glasses heat-treated at 520 °C, the trap states and defect states overlapped with each other (Fig. 7) and equilibrium between these two states can be easily achieved. As a result, both trap states and defect states contributed to the photoluminescence, and center of the PL bands did not change with temperature. This observation was also consistent with that reported by Caram et al. [25]. However, for the largest PbS QDs formed in glass heat-treated at 560 °C, center of the PL bands shifted initially towards low energy side as the temperature increased and shifted towards high energy side with further increase in the temperature. At low temperature, the photoluminescence was composed of the intrinsic emission from PbS QDs, emission from trap states and defects states. As the temperature increased, effective band gap energy of PbS QDs increased [35], and emission from trap states became significant, leading to the red-shift of the photoluminescence band. With further increase in the temperature, the trap states increased in energy as the effective band gap energy of PbS QDs increased, and the overall photoluminescence band showed blue-shift with further increase in temperature. It was also observed that photoluminescence efficiency or quantum yield of PbS QDs was strongly dependent on temperature, and it decreased as the temperature increased as evidenced from the decrease in the PL intensity with the increase in temperature. Similar phenomena were observed from chemically synthesized PbS QDs [25]. The temperature dependent photoluminescence of PbS QDs further confirmed that photoluminescence of PbS QDs was mainly determined by the size-dependent trap states and defect states.
Fig. 8 Low temperature (50-250 K) photoluminescence spectra of PbS QDs formed in glasses heat-treated at 520 °C, 530°C and 560 °C, respectively. The dashed lines indicate the peak energies recorded at 250 K.
4. Conclusion
In this work, PbS QDs were precipitated in glasses through heat-treatment. Both absorption and photoluminescence of PbS QDs were tuned in a wide spectral range by adjusting the heat-treatment temperature and duration. Transmission electron microscope image and multiple Gaussian simulation of the absorption peaks showed that PbS QDs precipitated in the glasses have a narrow size distribution. Line-shapes of photoluminescence from PbS QDs showed a strong dependence on the size. Small PbS QDs showed a photoluminescence tail at the long wavelength side, and vice versa for large PbS QDs quantum dots. Changes in the photoluminescence of PbS QDs indicated that there existed surface trap states on the surface of quantum dots and defect states at the interface between quantum dots and glass matrix had important effects on the photoluminescence properties (such as peak energy, full width at half maximum and Stokes shift) of PbS QDs. These effects suggested that surface trap states and defect states were strongly dependent on the size of quantum dots.
Funding
Program for New Century Excellent Talents in University (Grant No.: NCET-13-0943); Chutian Scholar Program of Hubei Province.
References and links
1. M. Bruchez Jr, M. Moronne, P. Gin, S. Weiss, and A. P. Alivisatos, “Semiconductor nanocrystals as fluorescent biological labels,” Science 281(5385), 2013–2016 (1998). [CrossRef] [PubMed]
2. V. I. Klimov, A. A. Mikhailovsky, S. Xu, A. Malko, J. A. Hollingsworth, C. A. Leatherdale, H. J. Eisler, and M. G. Bawendi, “Optical gain and stimulated emission in nanocrystal quantum dots,” Science 290, 314–317 (2000). [CrossRef] [PubMed]
3. R. D. Schaller, M. A. Petruska, and V. I. J. Klimov, “Tunable Near-Infrared Optical Gain and Amplified Spontaneous Emission Using PbSe Nanocrystals,” J. Phys. Chem. B 107(50), 13765–13768 (2003). [CrossRef]
4. L. Bakueva, S. Musikhin, M. A. Hines, T. W. F. Chang, M. Tzolov, G. D. Scholes, and E. H. Sargent, “Size-tunable infrared (1000–1600 nm) electroluminescence from PbS quantum-dot nanocrystals in a semiconducting polymer,” Appl. Phys. Lett. 82(17), 2895–2897 (2003). [CrossRef]
5. A. M. Malyarevich, V. G. Savitsji, P. V. Prokoshin, N. N. Posnov, and K. V. Yumashev, “Glass doped with PbS quantum dots as a saturable absorber for 1-µm neodymium lasers,” J. Opt. Soc. Am. B 19(1), 28–32 (2002). [CrossRef]
6. A. M. Malyarevich, I. A. Denisov, V. G. Savitsky, K. V. Yumashev, and A. A. Lipovskii, “Glass doped with PbS quantum dots for passive Q switching of a 1.54- microm laser,” Appl. Opt. 39(24), 4345–4347 (2000). [CrossRef] [PubMed]
7. M. S. Gaponenko, V. E. Kisel, N. V. Kuleshov, A. M. Malyarevich, K. V. Yumashev, and A. A. Onushchenko, “Passive mode locking of diode-pumped Tm:KYW laser with PbS quantum-dot-doped glass,” Laser Phys. Lett. 7(4), 286–289 (2010). [CrossRef]
8. I. A. Denisov, N. A. Skoptsov, M. S. Gaponenko, A. M. Malyarevich, K. V. Yumashev, and A. A. Lipovskii, “Passive mode locking of 2.09 microm Cr,Tm,Ho:Y3Sc2Al3O12 laser using PbS quantum-dot-doped glass,” Opt. Lett. 34(21), 3403–3405 (2009). [CrossRef] [PubMed]
9. P. T. Guerreiro, S. Ten, N. F. Borrelli, J. Butty, G. E. Jabbour, and N. Peyghambarian, “PbS quantum-dot doped glasses as saturable absorbers for mode locking of a Cr: forsterite laser,” Appl. Phys. Lett. 71(12), 1595–1597 (1997). [CrossRef]
10. A. A. Lagatsky, C. G. Leburn, C. T. A. Brown, W. Sibbett, A. M. Malyarevich, V. G. Savitski, K. V. Yumashev, E. L. Raaben, and A. A. Zhilin, “Passive mode locking of a Cr4+:YAG laser by PbS quantum-dot-doped glass saturable absorber,” Opt. Commun. 241(4–6), 449–454 (2004). [CrossRef]
11. F. Yue, J. W. Tomm, D. Kruschke, P. Glas, K. A. Bzheumikhov, and Z. Ch. Margushev, “PbS:glass as broad-bandwidth near-infrared light source material,” Opt. Express 21(2), 2287–2296 (2013). [CrossRef] [PubMed]
12. M. A. Hines and G. D. Scholes, “Colloidal PbS nanocrystals with size-tunable near-infrared emission: Observation of post-synthesis self-narrowing of the particle size distribution,” Adv. Mater. 15(21), 1844–1849 (2003). [CrossRef]
13. L. Cademartiri, J. Bertolotti, R. Sapienza, D. S. Wiersma, G. von Freymann, and G. A. Ozin, “Multigram scale, solventless, and diffusion-controlled route to highly monodisperse PbS nanocrystals,” J. Phys. Chem. B 110(2), 671–673 (2006). [CrossRef] [PubMed]
14. M. P. Campos, M. P. Hendricks, A. N. Beecher, W. Walravens, R. A. Swain, G. T. Cleveland, Z. Hens, M. Y. Sfeir, and J. S. Owen, “A library of selenourea precursors to PbSe nanocrystals with size distributions near the homogeneous limit,” J. Am. Chem. Soc. 139(6), 2296–2305 (2017). [CrossRef] [PubMed]
15. J. McBride, J. Treadway, L. C. Feldman, S. J. Pennycook, and S. J. Rosenthal, “Structural basis for near unity quantum yield core/shell nanostructures,” Nano Lett. 6(7), 1496–1501 (2006). [CrossRef] [PubMed]
16. D. Zherebetskyy, M. Scheele, Y. Zhang, N. Bronstein, C. Thompson, D. Britt, M. Salmeron, P. Alivisatos, and L.-W. Wang, “Hydroxylation of the surface of PbS nanocrystals passivated with oleic acid,” Science 344(6190), 1380–1384 (2014). [CrossRef] [PubMed]
17. I. Moreels, Y. Justo, B. De Geyter, K. Haustraete, J. C. Martins, and Z. Hens, “Size-tunable, bright, and stable PbS quantum dots: a surface chemistry study,” ACS Nano 5(3), 2004–2012 (2011). [CrossRef] [PubMed]
18. J. M. An, A. Franceschetti, and A. Zunger, “The excitonic exchange splitting and radiative lifetime in PbSe quantum dots,” Nano Lett. 7(7), 2129–2135 (2007). [CrossRef]
19. J. M. An, A. Franceschetti, S. V. Dudiy, and A. Zunger, “The peculiar electronic structure of PbSe quantum dots,” Nano Lett. 6(12), 2728–2735 (2006). [CrossRef] [PubMed]
20. G. J. Supran, K. W. Song, G. W. Hwang, R. E. Correa, J. Scherer, E. A. Dauler, Y. Shirasaki, M. G. Bawendi, and V. Bulović, “High-performance shortwave-infrared light-emitting devices using core-shell (PbS-CdS) colloidal quantum dots,” Adv. Mater. 27(8), 1437–1442 (2015). [CrossRef] [PubMed]
21. J. Tang, K. W. Kemp, S. Hoogland, K. S. Jeong, H. Liu, L. Levina, M. Furukawa, X. Wang, R. Debnath, D. Cha, K. W. Chou, A. Fischer, A. Amassian, J. B. Asbury, and E. H. Sargent, “Colloidal-quantum-dot photovoltaics using atomic-ligand passivation,” Nat. Mater. 10(10), 765–771 (2011). [CrossRef] [PubMed]
22. C. Liu and J. Heo, “Lead Chalcogenide Quantum Dot-Doped Glasses for Photonic Devices,” Int. J. Appl. Glass Sci. 4(3), 163–173 (2013). [CrossRef]
23. N. Han, C. Liu, J. Zhang, X. Zhao, J. Heo, and Y. Jiang, “Infrared photoluminescence from lead sulfide quantum dots in glasses enriched in sulfur,” J. Non-Cryst. Solids 391(3), 39–42 (2014). [CrossRef]
24. I. Moreels, K. Lambert, D. Smeets, D. De Muynck, T. Nollet, J. C. Martins, F. Vanhaecke, A. Vantomme, C. Delerue, G. Allan, and Z. Hens, “Size-dependent optical properties of colloidal PbS quantum dots,” ACS Nano 3(10), 3023–3030 (2009). [CrossRef] [PubMed]
25. J. R. Caram, S. N. Bertram, H. Utzat, W. R. Hess, J. A. Carr, T. S. Bischof, A. P. Beyler, M. W. B. Wilson, and M. G. Bawendi, “PbS Nanocrystal Emission Is Governed by Multiple Emissive States,” Nano Lett. 16(10), 6070–6077 (2016). [CrossRef] [PubMed]
26. A. P. Litvin, A. A. Babaev, P. S. Parfenov, E. V. Ushakova, M. A. Baranov, O. V. Andreeva, K. Berwick, A. V. Fedorov, and A. V. Baranov, “Photoluminescence of Lead Sulfide Quantum Dots of Different Sizes in a Nanoporous Silicate Glass Matrix,” J. Phys. Chem. 121(15), 8645–8652 (2017).
27. D. Kim, T. Kuwabara, and M. Nakayama, “Photoluminescence properties related to localized states in colloidal PbS quantum dots,” J. Lumin. 119–120(7), 214–218 (2006). [CrossRef]
28. T. Miyoshi, K. Nitta, H. Ohkuni, F. Ikeda, and N. Matsuo, “Laser-Induced Reversion of Photodarkening in CdS-Doped Glass,” Jpn. J. Appl. Phys. 36(11), 6726–6727 (1997). [CrossRef]
29. T. Miyoshi, A. Hirano, T. Suenaga, J. Nagata, T. Nagai, and N. Matsuo, “Photodarkening and Photobrightening in Glasses Doped with CdS and CdSxSe1-x Nanocrystals,” Jpn. J. Appl. Phys. 39(11), 6290–6292 (2000). [CrossRef]
30. L. A. Padilha, A. A. R. Neves, C. L. Cesar, L. C. Barbosa, and C. H. Brito Cruz, “Recombination processes in CdTe quantum-dot-doped glasses,” Appl. Phys. Lett. 85(15), 3256–3258 (2004). [CrossRef]
31. C. Liu, Y. K. Kwon, and J. Heo, “Optical modulation of near-infrared photoluminescence from lead sulfide quantum dots in glasses,” Appl. Phys. Lett. 94(2), 021103 (2009). [CrossRef]
32. N. O. Dantas, G. L. Fernandes, and A. C. A. Silva, “Controlling the growth of ultrasmall CdTe quantum dots and the diffusion of cadmium vacancies: Thermal annealing,” J. Alloys Compd. 637, 466–470 (2015). [CrossRef]
33. E. S. FreitasNeto, A. C. A. Silva, S. W. da Silva, P. C. Morais, J. A. Gómez, O. Baffa, and N. O. Dantas, “Raman spectroscopy of very small Cd1−xCoxS quantum dots grown by a novel protocol: direct observation of acoustic-optical phonon coupling,” J. Raman Spectrosc. 44(7), 1022–1032 (2013). [CrossRef]
34. Y. Wang, A. Suna, W. Mahler, and R. Kasowski, “PbS in polymers: From molecules to bulk solids,” J. Chem. Phys. 87(12), 7315–7322 (1987). [CrossRef]
35. I. Kang and F. W. Wise, “Electronic structure and optical properties of PbS and PbSe quantum dots,” J. Opt. Soc. Am. B 14(7), 1632–1646 (1997). [CrossRef]
36. Y. Nosaka, “Finite depth spherical well model for excited states of ultrasmall semiconductor particles. An application,” J. Phys. Chem. 95(13), 3591–3597 (1991). [CrossRef]
37. R. S. Kane, R. E. Cohen, and R. Silbey, “Theoretical Study of the Electronic Structure of PbS Nanoclusters,” J. Phys. Chem. 100(19), 7928–7932 (1996). [CrossRef]
38. C. Liu and J. Heo, “Band Gap and Diameter Modulation of Quantum Dots in Glasses,” Int. J. Appl. Glass Sci. 6(4), 329–338 (2015). [CrossRef]
39. N. Han, C. Liu, Z. Zhao, J. Zhang, J. Han, and X. Zhao, “Quantum Dots in Glasses: Size-Dependent Stokes Shift by Lead Chalcogenide,” Int. J. Appl. Glass Sci. 6(4), 339–344 (2015). [CrossRef]
40. C. Liu, J. Heo, X. Zhang, and J. L. Adam, “Photoluminescence of PbS quantum dots embedded in glasses,” J. Non-Cryst. Solids 354(2-9), 618–623 (2008). [CrossRef]
41. S. M. Shim, C. Liu, Y. K. Kwon, and J. Heo, “Lead Sulfide Quantum Dots Formation in Glasses Controlled by Erbium Ions,” J. Am. Ceram. Soc. 93(10), 3092–3094 (2010). [CrossRef]
42. R. Koole, G. Allan, C. Delerue, A. Meijerink, D. Vanmaekelbergh, and A. J. Houtepen, “Optical Investigation of Quantum Confinement in PbSe Nanocrystals at Different Points in the Brillouin Zone,” Small 4(1), 127–133 (2008). [CrossRef] [PubMed]
43. L. Cademartiri, E. Montanari, G. Calestani, A. Migliori, A. Guagliardi, and G. A. Ozin, “Size-Dependent Extinction Coefficients of PbS Quantum Dots,” J. Am. Chem. Soc. 128(31), 10337–10346 (2006). [CrossRef] [PubMed]
44. S. V. Gaponenko, “Optical properties of semiconductor nanocrystals, (Chapter 3, in the series of) Cambridge Studies in Modern Optics,” Cambridge University Press, United Kingdom, 55–61 (1998).
45. N. G. Bastús, J. Comenge, and V. Puntes, “Kinetically controlled seeded growth synthesis of citrate-stabilized gold nanoparticles of up to 200 nm: size focusing versus Ostwald ripening,” Langmuir 27(17), 11098–11105 (2011). [CrossRef] [PubMed]
46. B. L. Wehrenberg, C. Wang, and P. Guyot-Sionnest, “Interband and Intraband Optical Studies of PbSe Colloidal Quantum Dots,” J. Phys. Chem. B 106(41), 10634–10640 (2002). [CrossRef]
47. E. V. Ushakova, A. P. Litvin, P. S. Parfenov, A. V. Fedorov, M. Artemyev, A. V. Prudnikau, I. D. Rukhlenko, and A. V. Baranov, “Anomalous size-dependent decay of low-energy luminescence from PbS quantum dots in colloidal solution,” ACS Nano 6(10), 8913–8921 (2012). [CrossRef] [PubMed]
48. J. Cui, A. P. Beyler, L. F. Marshall, O. Chen, D. K. Harris, D. D. Wanger, X. Brokmann, and M. G. Bawendi, “Direct probe of spectral inhomogeneity reveals synthetic tunability of single-nanocrystal spectral linewidths,” Nat. Chem. 5(7), 602–606 (2013). [CrossRef] [PubMed]
49. J. Cui, A. P. Beyler, I. Coropceanu, L. Cleary, T. R. Avila, Y. Chen, J. M. Cordero, S. L. Heathcote, D. K. Harris, O. Chen, J. Cao, and M. G. Bawendi, “Evolution of the Single-Nanocrystal Photoluminescence Linewidth with Size and Shell: Implications for Exciton-Phonon Coupling and the Optimization of Spectral Linewidths,” Nano Lett. 16(1), 289–296 (2016). [CrossRef] [PubMed]
50. O. Chen, J. Zhao, V. P. Chauhan, J. Cui, C. Wong, D. K. Harris, H. Wei, H.-S. Han, D. Fukumura, R. K. Jain, and M. G. Bawendi, “Compact high-quality CdSe-CdS core-shell nanocrystals with narrow emission linewidths and suppressed blinking,” Nat. Mater. 12(5), 445–451 (2013). [CrossRef] [PubMed]
51. J. E. Lewis, S. Wu, and X. J. Jiang, “Unconventional gap state of trapped exciton in lead sulfide quantum dots,” Nanotechnology 21(45), 455402 (2010). [CrossRef] [PubMed]
52. E. Lifshitz, M. Brumer, A. Kigel, A. Sashchiuk, M. Bashouti, M. Sirota, E. Galun, Z. Burshtein, A. Q. Le Quang, I. Ledoux-Rak, and J. Zyss, “Air-Stable PbSe/PbS and PbSe/PbSexS1-x Core-Shell Nanocrystal Quantum Dots and Their Applications,” J. Phys. Chem. B 110(50), 25356–25365 (2006). [CrossRef] [PubMed]
53. A. Lobo, T. Möller, M. Nagel, H. Borchert, S. G. Hickey, and H. Weller, “Photoelectron Spectroscopic Investigations of Chemical Bonding in Organically Stabilized PbS Nanocrystals,” J. Phys. Chem. B 109(37), 17422–17428 (2005). [CrossRef] [PubMed]
54. N. B. Pendyala and K. S. R. Koteswara Rao, “Identification of surface states in PbS quantum dots by temperature dependent photoluminescence,” J. Lumin. 128(11), 1826–1830 (2008). [CrossRef]
55. E. Poles, D. C. Selmarten, O. I. Mićić, and A. J. Nozik, “Anti-Stokes photoluminescence in colloidal semiconductor quantum dots,” Appl. Phys. Lett. 75(7), 971–973 (1999). [CrossRef]
