PbS quantum dots (QDs) are widely studied because of the excellent properties including nonlinear optics [1], multiple exciton generation [2, 3], electroluminescence [4, 5] and photovoltaics [6–8] etc. Especially, PbS QDs exhibit a super-wide and multi-wavelength tunability from 825 nm to 2100 nm [9] attribute to the narrow bandgap energy and strong quantum size effect, which provides a promising application in wideband absorbers and multi-emission light sources. Moreover, such broad tunability makes it prospective in integrating multicolor QDs sources that covers the whole telecommunication window and gives promising applications in the field of chemical/biological sensing, near infrared LED and multicolor quantum dot lasers [10]. Most of PbS QDs were synthesized via colloidal [11, 12] and aqueous routes [13–16] in liquid solvents. These kinds of QDs are unstable and prone to aggregate, which make it difficult to be transformed into solid hosts and unsuitable for solid compact optoelectronics. A better solution and strategy is to immobilize QDs by solid hosts homogeneously. Therefore, a lot of attention has drawn to incorporate PbS QDs into solids. As representation, polymers and silica glasses [5, 17] were utilized to protect QDs from ambient environment damage. Nevertheless, it is hard to develop multi-functional composites in this system since QDs co-doping was difficult to achieve. Besides, macroporous matrices [10, 18] provide enough assembling space for QDs, but the emission instability caused by aggregation and oxidation is an existing problem. In addition, future industrial applications require stable compact composites for integrated optics and communication sources. How to obtain both controllable and stable luminescence of such composites remains a challenge. In this case, mesoporous glasses with nanoscale pores offer new possibilities to immobilize QDs. Recently, we reported a novel mesoporous AlPO₄ glass with controlled and accessible pores [19]. Compared with silicate glass, AlPO₄ provides a suitable reductive atmosphere in the mesoporous network [20] which is likely to have the advantage of antioxidation. On the other hand, the AlO₄ and PO₄ units in AlPO₄ exhibit more chemical activity than SiO₄ units to profit the adsorption [21, 22].

Here we designed different composited strategies to fabricate PbS QDs-AlPO₄ mesoporous glass (PbS-AlP) composites in pursuit of controllable luminescence for the first time. These composites showed tunable photoluminescence (PL) in near infrared range. Dramatically, a broad emission peak as wide as 242 nm and a bimodal luminescence placed at 970 and 1224 nm were achieved in AlPO₄ monolithic glasses, respectively. In addition, adjustable luminescence from 955 to 1061 nm were realized when pore diameter of mesoporous AlPO₄ glasses was modified.

1. Experimental methods

Sodium sulfide nonahydrate (Na₂S·9H₂O, 98%), A-monothioglycerol (TGL, 97%), 2,3-dimercapto-1-propanol (DTG, 98%), triethylamine (TEA, 99%) and aluminum L-lactate (98%) were purchased from Sigma-Aldrich. Phosphoric acid (H₃PO₄), ammonia, isopropanol and lead acetate (PbAc) were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were used as purchased. PbS QDs were synthesized via an aqueous route similar to Bakueva’s [13]. Typically, 15 mL of solution containing 0.25 mmol of PbAc, 1.5 mmol of TGL, and 0.5 mmol of DTG was adjusted to pH 11.2 by the addition of TEA with ice bath. A 0.1 M solution of Na₂S was added to the cooled mixture quickly under vigorous stirring (1200 rpm). The color of the solution changed from transparent to dark brown in a few minutes. The molar ratio of S/Pb varied from 0.2 to 0.8. In AlPO₄ synthesis, aluminum L-lactate and H₃PO₄ were used as precursors and mixed in distilled water with molar ratio of Al/P focus to 1:1. After gelling at room temperature and annealing at 600 °C, transparent and colorless AlPO₄ glasses were obtained [19]. PbS-AlP composites were prepared by dipping methods. In a typical procedure, 100 mg of AlPO₄ glasses were immersed in 5 mL PbS QDs dispersions for 15 min, followed by washing with absolute ethanol for several times to remove the organic solvent and ions on the surface of glass. The post-dipping glass were dried and subsequently stored in a desiccator. After dipping by PbS QDs, the color of AlPO₄ glass changed from transparent to uniformly red.

TEM images were obtained using a JEOL 2100F microscope. Samples were prepared by dipping a TEM grid in a diluted QDs and PbS-AlP suspension, which is prepared by grinding and dispersing PbS-AlP glass in absolute ethanol. XPS measurements were operated on K-Alpha (Thermo Fisher Scientific, USA) in ultrahigh vacuum (pressure<10⁻⁷ Pa) with spot size of 400 μm. PL spectra were recorded using an FLsp920 spectrometer pumped by a semiconductor laser diode at 808 nm. BET (Brunauer-Emmett-Teller) surface area measurements were obtained from a Micromeritics ASAP 2010 volumetric adsorption analyzer with N² as an adsorbate at 77 K. Glasses were grounded into small pieces to fit the BET tube. Prior to analysis, samples weighing from 0.1 to 0.3 g were outgassed for at least 6 h under vacuum. Mesopore size distributions were obtained by the BJH (Barrett-Joyner-Halenda) method, assuming a cylindrical pore model.

2. Results and discussion

Ascribed to the large surface area, sol-gel mesoporous AlPO₄ glass is found to be an excellent supported matrix for QDs adsorbing. The BET measurements of AlPO₄ glass characterized a mesoporous structure with large surface area and a nano scaled average pore width [19]. The mesporous framework allows the QDs to be separated. Figure 1 gives comparison of morphology and chemical state for PbS QDs and PbS-AlP composites. As shown in Fig. 1 (a), well dispersed PbS QDs of high crystallization with particle size as 3~5 nm were synthesized. After incorporation, distinguishable PbS QDs were found in AlPO₄ glass matrix as evidenced by Fig. 1(d), which demonstrated the successful embedding of QDs. Owing to the homogeneous disordered pore distribution of glass, PbS QDs were diffused and adsorbed in glass pores. The diameter of PbS in glass agreed well with the particle size of original PbS QDs.

 

Fig. 1 Comparison of PbS QDs and PbS-AlP. (a) TEM image of PbS QDs; (b) XPS of phosphorus 2p core levels and lead 4f core levels of PbS QDs (black), AlPO₄ glass (blue) and PbS-AlP (red), respectively; (c) TEM image of PbS-AlP, arrow in (c) points out the edge of glass; (d) Close-up of dashed box area in (c)

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To deeply understand the chemical components and environment of PbS-AlP composites, XPS were carried out on PbS QDs powder (precipitated from QDs solutions) and the center of fracture surface of AlPO₄ glass and PbS-AlP composites, respectively. As shown in Fig. 1(b), two characteristic peaks at binding energies of 137.1 and 142 eV ascribed to Pb 4f7/2 and Pb 4f5/2 core levels for PbS QDs also present at the spectra of PbS-AlP composites. Meanwhile, the AlPO₄ glass only has a single peak at 134.5 eV for P 2p. It therefore convinced the chemical stable solidification of PbS QDs in AlPO₄ network. In this case, AlPO₄ network has little influence on the chemical environment as well as surface state of PbS QDs and the optical properties were predominantly influenced by size effect as free QDs. As references, AlPO₄ glasses were only dipped by PbAc aqueous solution with same processing parameters as QDs [shown as line Pb²⁺-AlP in Fig. 1(b)]. the XPS results were proved to be same as pure AlPO₄ glass. It is believed that most of free Pb ions adsorbed by AlPO₄ glass can be easily rinsed away by common washing. The residual Pb ions attracted on glass pores surfaces made little contribution to XPS results. Overall, we can infer that PbS QDs were efficiently dispersed in mesoporous AlPO₄ glass in the case of PbS-AlP composites.

After getting rid of the influence on surface chemical state, we tried to tune the emission of PbS-AlP by QDs particle size. In order to control and tailor the broadly tunable and bimodal luminescence of PbS-AlP composites, as shown in Fig. 2, two different approaches were designed to immobilize PbS QDs selectively for the purpose of co-doping in one AlPO₄ monolith. Prior dipping process, three PbS QDs dispersions (m, n and m/n) centered at different emission peak positions were deliberately prepared. Figure 3(a) gives a tunable luminescence placed at 1038 nm and 1360 nm for QDs m and n, respectively. Comparing with fresh QDs m, the emission peak position of ripened QDs n (After ripening at 50 °C for 24 hours) red shifted obviously. As well, the mixed solution of m and n (m/n, m:n = 1:1) showed a bimodal luminescence at both positions as QDs m and n. Afterwards, these dispersions were employed to dipping processes for obtaining composites with tunable luminescence.

 

Fig. 2 Dipping procedures of PbS-AlP composites (M + N) and (M/N) from (a) two-steps dipping method and (b) one-step dipping method. M and N refer to samples from the single dipping method

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Fig. 3 PL spectra for (a) PbS QDs and (b) PbS-AlP. M/N: nanocomposite prepared by one-step dipping method from m/n; M and N: nanocomposite prepared by single dipping method from m and n, respectively; (M + N): nanocomposite prepared by two-steps dipping method from m and n. Surface area and average pore diameter of the pure glass are 468 m²g⁻¹ and 5.6 nm

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As two-steps dipping method presented in Fig. 2(a), mesoporous AlPO₄ glass was first dipped into PbS QDs m, after 200 °C degassing for 6 hours at vacuum, it was then dipped in PbS QDs n. Finally, PbS-AlP composites (M + N) were achieved. In the same way, but reversed dipping order, composites (N + M) was prepared. For one-step dipping method [Fig. 2(b)], AlPO₄ glasses were directly dipped into the mixed QDs m/n to obtain composites M/N.

PL spectra of PbS-AlP samples were shown in Fig. 3(b). After single dipped by QDs m and n, the corresponding PbS-AlP composites M and N remain tunable luminescent but slightly blue-shift to positions around 1007 nm and 1264 nm in comparison to relevant QDs solutions. Meanwhile, PbS-AlP composite (M + N) presents broadly tunable emission around 1120 nm compared with samples M and N. In this kind of process driving by adsorption force, QDs m first fill most of the glass pores and a main size distribution was invariable in AlPO₄ glass. When next immersed in QDs n, a re-supplement process proceed, QD particles were absorbed by residual place. Thus, the spectrum was widened with broadening QDs size distribution. For comparison, if dipped in QDs n first and then m to give (N + M), this broad peak was still acquired [dash line in Fig. 3(b)]. To conclude, the re-supplement process plays a dominant role in widening the fluorescence of PbS-AlP composites, and the dipping order has a slight effect on the peak position. It seems that this peak position of composites tend to get close to the last QD solution from two-steps dipping method.

Remarkably, PbS-AlP (M/N) shows a bimodal luminescence with two emission peaks at 970 and 1224 nm, demonstrating two size distributions of embedded QDs. It is clear that the luminescence of QDs is determined by both particle size and size distribution [23]. The unique fluorescence of PbS-AlP composites from two-steps and one-step dipping methods can be attributed to broad and bimodal size distributions of QDs, respectively. If the mixed suspensions were comprised of QDs with two different sizes, the bimodal size distribution of QDs in this mixture retain and fill into the glass pores at the same time. As a result, this technique enables the bimodal emission of PbS-AlP composites. The emission peak positions (λ_em), full width of half maximum (FWHM) and calculated average sizes of QDs (R) of these samples are summarized in Table 1.

Table 1. Details of PL spectra in Fig. 3

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As stated above, the PL spectra of PbS-AlP composites are deliberately tuned by changing QDs sources and processing methods. It should be noted that the structural properties of mesoporous AlPO₄ glass, especially the average pore diameter, can be adjusted by controlling processing parameters (pH, heating processing, etc.) on the basis of our previous work [19]. Furthermore, in order to tailor the tunable luminescence at different wavelength, a series of mesoporous AlPO₄ glasses with diverse surface areas and average pore diameters were synthesized to incorporate the same freshly PbS QDs. As a result, PL spectra of PbS-AlP composites present a marked redshift from 955 nm to 1061 nm with increasing pore diameters (D) as depicted in Fig. 4.It is worth noting that the pore diameters of these glasses are controlled in a narrow size distribution, which enables the selective adsorption of PbS QDs and gives rise to different QDs size distributions in glass. It therefore signifies the PbS QDs with larger diameter are selectively solidified in AlPO₄ glasses with larger pore size.

 

Fig. 4 PL spectra with adjustable emission from 955 to1061 nm for PbS-AlP composites from mesoporous AlPO₄ glasses with corresponding average pore diameters from 2.4 to 6.6 nm (shown as solid triangle in the graph below). Inset: time evolution of emission peak compared to other work

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We further investigated the stability of our PbS-AlP systems. Extend research showed that the emission stability is much better than QDs in regular macroporous matrix. The inset of Fig. 4 gives the fact that an excellent small blue shift less than 40 nm of PL spectra for PbS-AlP composites after 6 months storage. We plotted experimental data on the emission peak position as a function of storage time. Our data were indicated by the black squares as shown in inset of Fig. 4. The red and blue triangle lines were taken from Litvin and Ihly et al. [18, 24]. The PL spectra were measured immediately after dipping and after a period of time. All samples were stored at room temperature in an unconfined box kept in dark during that time and measured after 10, 60, 180 days storage. In a same condition, fluorescence of PbS QDs reported by Litvin et al. showed blue shift as much as 300 nm after 90 days storage in a macroporous matrix. On the other hand, Ihly et al. reported a blue shift as much as 169 nm after 30 days storage of solid state PbS QDs without any host. By comparison, QDs embedded in mesoporous AlPO₄ glass showed much better stability. We attribute this improvement to the active chemical atmosphere existed in AlPO₄ mesoporous glass. The Al ions are reported to provide a suitable atmosphere for the abnormal reduction of ions in the air [20]. The AlPO₄ glass is of alternative AlO₄ and PO₄ tetrahedral units, and the high amount of aluminum is responsible for stable emission of QDs prevent from oxidation. Additionally, compared this all-inorganic QDs-AlPO₄ glass with QDs-polymer systems, the geometrical or electronic structure of organic polymer changes more easily under light irradiation and thermal agitation, which gives rise to fluorescence quenching [10]. The fluorescent stability and thermal stability of this all-inorganic nanocrystal-in-AlPO₄ glass composites are much more improved, which indicates that the mesoporous AlPO₄ glass is a potential novel excellent host for QDs in application of tunable optical devices.

3. Conclusion

We report here the broadly tunable and controllable bimodal luminescent PbS-AlP composites by incorporating PbS QDs into mesoporous AlPO₄ glasses for the first time. Two-steps and one-step dipping method have been employed to obtain PbS-AlP composites with wideband (242 nm) and bimodal photoluminescence, respectively. Further research shows that the PL spectra of PbS-AlP composites were tuned from 955 to 1061 nm as a consequence of selective adsorption by pores of glasses. Our results imply that the mesoporous AlPO₄ glass is suitable in embedding PbS QDs and tailoring the luminescence. This finding greatly extends the domain of QDs composites research and significantly offers an excellent approach to tailor and design the stable luminescence of QDs. Remarkably, The blue shift of initial emitting wavelength for PbS is declined in AlPO₄ glass host and exhibited strong emission stability. In particular, the PbS-AlP composites with controllable emission provide a new candidate for applications among telecommunications and infrared optics.