Near-infrared photoluminescence properties of PbS QDs embedded in glasses were investigated upon below-bandgap excitation. PbS QDs were precipitated in the glasses upon thermal treatment. Near-infrared anti-Stokes photoluminescence (ASPL) from PbS QDs was observed. Dependence of the ASPL on size and excitation power indicated that ASPL was phonon-assisted one-photon process. These near-infrared anti-Stokes photoluminescence of PbS QDs in glasses have potential applications for light conversion and laser cooling.
© 2017 Optical Society of America
Anti-Stokes photoluminescence is luminescence where the emitted photons are higher in energy than the excitation photons. ASPL, which violates of Stokes’ law in nature, was first observed in dye molecules , and then has been reported in various systems like bulk semiconductor and nanocrystals [2–4], polymers [5,6], and heterostructures [7,8] Due to the particular properties, ASPL has great potentials for applications in the multi-color displays , dynamical imaging microscopy , unconventional lasers , bio-imaging agents  and solid-state laser cooling devices .
Two-photon absorption process [3,4], Auger recombination [7,8], and one-photon absorption assisted by phonons [14–19] have been considered as the possible mechanisms leading to ASPL. In the two-photon absorption process, two photons are absorbed through a real or virtual intermediate state leading to higher energy recombination. Two photons absorption through a real intermediate state is common in ASPL for rare-earth ions while through a virtual intermediate state has been used to explain the ASPL in semiconductor nanocrystals [3,4]. The second process is an Auger-like recombination, where an electron-hole pair recombines nonradiatively and excites another electron or hole into higher energy states, which is common in heterostructures [7,8]. The third process involves one-photon phonon-assisted carrier excitation. The additional energy comes from the absorption of phonons, which has been reported to explain ASPL of QDs, like CdSe [14,15], InP , CdTe [14,16], PbS  and PbSe [18,19]. In general, intensity of ASPL (IASPL) based on the two-photon absorption shows a quadratic dependence on the excitation power (Iex) . For the ASPL induced by the Auger recombination, a cubic dependence on the excitation power should be observed , and for the phonon absorption assisted ASPL, it is linearly dependent on the excitation power [15,16,20].
Synthesis of quantum dots in glasses is particularly attractive since it is inexpensive and chemically durable with high mechanical strength . Furthermore, the versatility of glassy hosts allows fabrication of devices in the bulk, planar or fiber forms . However, there has been not much researches on the ASPL of QDs doped glasses.
In this work, a series of PbS QDs with different diameters were precipitated inside the silicate glasses. Upon excitation at the long wavelength side of the absorption bands, efficient anti-Stokes photoluminescence was observed. The anti-Stokes photoluminescence spectrum was significantly red shifted relative to the normal photoluminescence spectrum. Moreover, with the increase in the excitation intensity, center wavelength of the anti-Stokes photoluminescence band gradually shifted towards the short wavelength, approaching the photoluminescence with high energy excitation. Finally, mechanism of ASPL from PbS QDs in glasses was discussed based on the phonon-assisted one-photon process.
Glasses with nominal compositions of 50SiO2-25Na2O-5Al2O3-10BaO-5ZnO-2ZnS-0.8PbO (in mol%) were prepared by conventional melt-quenching method. Chemical powders with purity of >99.9% were weighted and thoroughly mixed before melting in alumina crucibles at 1400 °C for 30 min under the ambient atmosphere. The melts were quenched in a brass mold and pressed with another for quenching. The glasses thus obtained were annealed at 400 °C for 2 h to reduce the thermal stress. Annealed glasses were cut into small species for further heat treatments. PbS QDs were precipitated in the glasses by thermal treatment at temperatures near the glass transition. Average size of PbS QDs formed in the glasses was controlled by adjusting the heat-treatment temperature and duration.
Glass transition temperatures of these as-prepared glasses were found to be ~480 °C using the simultaneous thermal analyzer (STA449c/3/c/G, NETZSCH, Selb, Germany). X-ray diffraction (λ = 1.54056Å, D8 Advance, Bruker, Karlsruhe, Germany) patterns were recorded to illustrate the structural changes after thermal treatment, and the patterns were recorded at a scanning rate of 2 °/min with a step size of 0.02°. Absorption spectra of the as-prepared and heat-treated samples were recorded using a UV/Vis/NIR spectrophotometer (UV3600, Shimadzu, Kyoto, Japan). Lasers with wavelength of 800 nm, 1319 nm, and 1532 nm were used as the excitation sources. The excitation laser beams were modulated by a mechanical chopper at a frequency of 50 Hz and focused into the specimens using a silica lens with a focal length of 5 cm. Photoluminescence (PL) was collected at the direction perpendicular to the excitation beam and dispersed into a monochromator. A combination of InGaAs (or InSb) detector and lock-in amplifier was used to detect the intensity of photoluminescence.
3. Results and discussion
Figure 1 shows the X-ray diffraction patterns of as-prepared (AP) and heat-treated glasses at temperatures ranging from 530 °C and 540 °C for different durations. For the AP glass, only one broad halo was observed, indicating the non-crystalline nature of the as-prepared glass. Once the glasses were heat-treated, obvious change from yellow to dark brown and black was observed when the heat-treatment temperature and time increased from 530 °C for 8 and 10 h to 540 °C for 6 and 8 h, indicating the formation of nanocrystals in the glasses. However, due to the small size and low volume fraction of the nanocrystals, no diffraction peaks can be observed from glasses heat-treated at 530 °C for 8 h. When the heat-treated for longer time and at higher temperatures, several peaks appeared in the diffraction patterns, corresponding to the face-centered cubic PbS crystal (JCPDS No.: 65-157). These results showed that PbS nanocrystals were formed in the glasses upon thermal treatment. Average diameters of PbS nanocrystals were calculated to be 5.4 nm and 7.2 nm for glasses heat-treated at 530 °C for 10 h and 540 °C for 8 h, respectively, using the (220) diffraction peak and the Scherrer equation:
Figure 2 shows the absorption and photoluminescence (PL) spectra of the heat-treated glasses. Using multiple Gaussian function simulation, it was found that the peak wavelength of the lowest absorption bands gradually shifted from 1002 nm to 1188 nm, 1323 nm and 1391 nm when the heat-treatment temperatures increased from 530 °C for 8 and 10 h to 540 °C for 6 and 8 h, respectively. Red-shift in the absorption bands with increase in the heat-treatment temperature or duration indicated the growth of the PbS QDs. Average diameters of PbS QDs were found to be 3.9 ± 0.25 nm, 4.7 ± 0.25 nm, 5.3 ± 0.27 nm, and 5.8 ± 0.34 nm for glasses heat-treated at 530 °C for 8 and 10 h, 540 °C for 6 and 8 h, respectively, using the hyperbolic band model relation :23,24]. Such a big Stokes shift indicated that photoluminescence of PbS QDs were from the surface trap states .
Figure 3 shows the absorption and photoluminescence spectra of glass heat-treated at 530 °C for 10 h. Lasers with wavelength of 800 nm, 1319 nm and 1532 nm were used as the excitation. When the glass was excited at 800 nm, photoluminescence of PbS QDs appeared at 1314 nm with a Stokes shift of 101 meV. When the glass was excited at 1319 nm, where the glass had an absorption coefficient of 3.14 cm−1, one PL band with a center wavelength at 1338 nm was observed and part of the photoluminescence was located at the shorter wavelength compared to the excitation. When the glass was excited at 1532 nm, where the glass had only weak background absorption coefficient of 0.13 cm−1, the photoluminescence band was mainly composed of ASPL with a center wavelength of ~1390 nm and a blue-shift of ~82 meV compared to the excitation energy. It has to be pointed out that the ASPL even extended to ~1100 nm (1.112 eV), showing a maximum anti-Stokes shift (ΔEmax) of 318 meV. In addition, full width at half maximum (FWHM) of these photoluminescence bands increased from 107 nm to 152 nm and 177 nm when the excitation wavelength increased from 800 nm to 1319 nm and 1532 nm, respectively. It has to be pointed out that all the photoluminescence spectra observed were non-symmetric in nature.
Similar results are also observed in other samples in Fig. 4. Figure 4(a) shows the ASPL of 5.3 nm-sized PbS QDs upon 1532 nm laser excitation, together with the absorption spectrum and the normal PL (i.e. photoluminescence recorded upon 800 nm laser excitation) spectrum. Compared to the normal photoluminescence, the ASPL showed an obvious red-shift, indicating that the ASPL may originate a different set of defect states. With the increase in the excitation power, center wavelength of the ASPL band gradually shifted towards short wavelength, approaching the normal PL. Similar phenomenon was observed for 5.8 nm-sized PbS QDs shown in Fig. 4(b). When the excitation power increased from 0.2 W to 1.0 W, the ASPL band shifted from ~1498 nm to ~1474 nm, nearly overlapping with the normal PL band. These observations indicated that the surface trap states (which was response for the normal PL) and the defect states (which was responsible for the ASPL) were strongly dependent on the size of PbS QDs.
ASPL photoluminescence properties of 3.9 nm (excited at 1319 nm), 5.3 nm (excited at 1532 nm) and 5.8 nm (excited at 1532 nm) were shown in Fig. 5. For 3.9 nm-sized PbS QDs, photoluminescence intensity monotonically increased and peak wavelength of the ASPL band gradually blue-shifted from 1254 nm to 1240 nm as the excitation intensity increased from 50 mW to 1000 mW shown in Fig. 5(a). For 5.3 nm-sized PbS QDs, intensity of the ASPL firstly increased to the maximum as the excitation intensity increased from 100 mW to 1000 mW, and then decreased with further increase in the excitation intensity shown in Fig. 5(b). At the meantime, peak wavelength of the ASPL band gradually blue-shifted from 1462 nm to 1442 nm as the excitation intensity increased. For 5.8 nm-sized PbS QDs shown in Fig. 5(c), intensity of the ASPL also increased to the maximum as the excitation intensity increased from 10 mW to 300 mW and decreased with further increase in excitation intensity. Peak wavelength of the ASPL band gradually blue-shifted from 1498 nm to 1472 nm. The largest Stokes shifts () between the maximum ASPL energy and the excitation energy were found to be 240 meV, 395 meV and 430 meV for 3.9 nm, 5.3 nm and 5.8 nm-sized PbS QDs, respectively, much larger than most of the reported values [16,17,20].
Integral intensity of the ASPL band was strongly dependent on the excitation power shown in Fig. 5(d). For 3.9 nm-sized PbS QDs, the integral intensity of the ASPL band showed a linear dependence on the excitation power with a slope of 0.9974. For 5.3 nm-sized PbS QDs, the integral intensity of the ASPL band initially increased linearly with a slope of 0.712 as the excitation power increased. With further increase in the excitation power, the integral intensity of the ASPL deviated from the linear and started to decrease as the increase in the excitation power. For 5.8 nm-sized PbS QDs, the integral ASPL intensity also increased linearly with a slope of 0.754 as the excitation power increased, and further increase in the excitation power led to the decrease in the integral ASPL intensity. The linear and sublinear dependence of the integral intensity of ASPL on the excitation power showed that one-photon process, instead of the two-photon excitation and Auger recombination, was responsible for the ASPL [16,20]. For 5.3 nm and 5.8 nm-sized PbS QDs, the integral intensity of ASPL band decreased with excitation power at high pump power. This phenomenon was due to the photo-induced photo-darkening, consistent with our previous work .
The excitation power dependent blue-shift and non-symmetry of the ASPL bands indicated that the ASPL was composed of two bands. Using dual Gaussian function simulation, one high energy band (P1) with narrow band width and one low energy band (P2) with broad band width were obtained shown in Figs. 5(e) and 5(f). It was found that the high energy bands (P1) were very close to the corresponding normal PL bands of 3.9 nm, and 5.3 nm and 5.8 nm-sized PbS QDs shown in Fig. 5(e) and Fig. 2, indicating that these P1 bands were related to the surface trap states [15,16,20]. As shown in Fig. 5(e), for 3.9 nm-sized PbS QDs, energy difference between P1 and P2 bands maintained nearly constant at 60 meV as the excitation power increased. For 5.3 nm-sized PbS QDs, energy of P1 band remained almost constant, and the energy difference between P1 and P2 bands decreased from 19.4 meV to 13.3 meV as the excitation power increased. While, for 5.8 nm-sized PbS QDs, energy of P1 band showed some variation around 0.84 eV. The energy difference between P1 and P2 bands decreased from 17.1 meV to zero as the excitation power increased to 1400 mW, and P1 and P2 bands overlapped with each other at higher excitation power. In addition, full width at half maximum (FWHM) of P1 and P2 bands showed distinct features in Fig. 5(f). For 3.9 nm-sized PbS QDs, FWHM values of P1 and P2 bands were found to be ~125 meV and ~175 meV, respectively, and FWHM values of these two bands did not change obviously with excitation power. For 5.3 nm-sized PbS QDs, P1 and P2 bands had FWHM values of ~100 meV and ~175 meV, respectively. For 5.8 nm-sized PbS QDs, FWHM value of P1 band increased from ~110 meV to ~165 meV, and that of P2 band increased from ~180 meV to ~330 meV with the increase in the excitation power. These features further confirmed that P1 and P2 bands had different origins, and probably, P2 bands were related to the defects states which located at lower energy compared to the surface trap states.
Based on the above results, the anti-Stokes photoluminescence of PbS QDs in glasses can be explained by the following model (Fig. 6). Due to the presence of dangling bonds on the surface of PbS QDs, these dangling bonds formed the surface trap states (STS) located at below the state of PbS QDs [15,16,24]. Besides, the defects located at the interface between the surface of PbS QDs and the glass matrix formed the defect states (DS) located at below the of PbS QDs. Upon above-bandgap excitation (such as 800 nm excitation used in this work), the electrons were excited from the state into the high energy conduction levels, and these excited electrons relaxed to the states by dissipating excess energy into heat. The electrons at state can be trapped by the STS, and relaxed to the state radiatively, yielding to normal PL (red arrow in Fig. 6). When the electron trapped at STS on the top of the state was excited into the DS upon below-bandgap excitation (such as 1532 nm excitation, dark grey arrow), part of the electrons radiatively relaxed to the state with the appearance of ASPL (P2 band), and part of the electrons non-radiatively relaxed to the bottom STS or (dashed line in Fig. 6) with the dissipation of thermal energy and rise in the local temperature. With the increase in the excitation power, by absorbing the phonon energy, electrons trapped in the DS can be further excited to high energy states, even the STS, leading to the blue-shift in the ASPL, appearance of P1 band together with the P2 band and non-symmetric shape of the ASPL band. As the size of PbS QDs increased, energy gap between the STS and DS () decreased . As a result, for large PbS QDs, energy separation between P1 and P2 decreased as the excitation power increased.
In conclusion, various size PbS QDs were precipitated in host glass by controlling temperature and time of thermal treatment. Near-infrared anti-Stokes photoluminescence (ASPL) from PbS QDs was observed upon below-bandgap excitation. Anti-Stokes shift as large as 430 meV was observed. The linear or sublinear dependence of the integral intensity of the ASPL showed the ASPL was a one-photon process. Due to the presence of defect states at the interface between PbS QDs and glass matrix, the ASPL showed strong dependence on the excitation power. The ASPL band shifted towards short wavelength side, along with the broadening the photoluminescence as the excitation power increased. The large anti-Stokes shift obtained upon below-bandgap excitation indicated that the ASPL has great potential for spectral conversion and solid state laser cooling.
Program for New Century Excellent Talents in University (No. NCET-13-0943), Chutian Scholar Program of Hubei Province, National Nature Science Foundation of China (No. 51602235).
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