Direct evidence on the energy transfer of nearinfrared emission in PbS quantum dot-doped glass Haipeng Wang,1,2 Guobo Wu,1,2 Jianrong Qiu,1 and Guoping Dong1,* 1
State Key Laboratory of Luminescent Materials and Device, School of Materials Science and Engineering, South China University of Technology, Guangzhou, China 2 Equal contribution to this work. * [email protected]
Abstract: PbS quantum dot (QD)-doped glass was prepared by the heat treatment of as-prepared glass, which was confirmed by transmission electron microscope (TEM), absorption and photoluminescence (PL) spectra. Choosing the glass heat treatment at 600°C for 24h as a representative sample, steady-state and transient-state PL, lifetime and time-resolve emission spectra (TRES) of PbS QD-doped glass were studied in detail. The results indicated that the lifetime spectra showed a similar variation tendency with the PL spectra. The steady-state, transient-state PL and TRES results first revealed the energy transfer process from smaller QDs with higher energy to bigger QDs with lower energy in the PbS QDdoped glass. ©2015 Optical Society of America OCIS codes: (160.2540) Fluorescent and luminescent materials; (160.2750) Glass and other amorphous materials; (300.6280) Spectroscopy, fluorescence and luminescence.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
V. I. Klimov, A. A. Mikhailovsky, S. Xu, A. Malko, J. A. Hollingsworth, C. A. Leatherdale, H. Eisler, and M. G. Bawendi, “Optical gain and stimulated emission in nanocrystal quantum dots,” Science 290(5490), 314–317 (2000). F. W. Wise, “Lead salt quantum dots: the limit of strong quantum confinement,” Acc. Chem. Res. 33(11), 773– 780 (2000). M. S. Gaponenko, A. A. Lutich, N. A. Tolstik, A. A. Onushchenko, A. M. Malyarevich, E. P. Petrov, and K. V. Yumashev, “Temperature-dependent photoluminescence of PbS quantum dots in glass: Evidence of exciton state splitting and carrier trapping,” Phys. Rev. B 82(12), 125320 (2010). A. P. Alivisatos, “Semiconductor clusters, nanocrystals, and quantum dots,” Science 271(5251), 933–937 (1996). A. I. Ekimov, A. L. Efros, and A. A. Onushchenko, “Quantum size effect in semiconductor microcrystals,” Solid State Commun. 88(11), 947–950 (1985). E. Hanamura, “Very large optical nonlinearity of semiconductor microcrystallites,” Phys. Rev. B Condens. Matter 37(3), 1273–1279 (1988). A. L. Efros and A. L. Efros, “Interband absorption of light in a semiconductor sphere,” Sov. Phys. Semicond. 16(7), 772–775 (1982). C. Liu, Y. K. Kwon, and J. Heo, “Temperature-dependent brightening and darkening of photoluminescence from PbS quantum dots in glasses,” Appl. Phys. Lett. 90(24), 241111 (2007). E. U. Rafailov, M. A. Cataluna, and W. Sibbett, “Mode-locked quantum-dot lasers,” Nat. Photonics 1(7), 395– 401 (2007). G. Dong, G. Wu, S. Fan, F. Zhang, Y. Zhang, B. Wu, Z. Ma, M. Peng, and J. Qiu, “Formation, near-infrared luminescence and multi-wavelength optical amplification of PbS quantum dot-doped silicate glasses,” J. NonCryst. Solids 383(1), 192–195 (2004). S. Hoogland, V. Sukhovatkin, I. Howard, S. Cauchi, L. Levina, and E. H. Sargent, “A solution-processed 1.53 mum quantum dot laser with temperature-invariant emission wavelength,” Opt. Express 14(8), 3273–3281 (2006). A. Rakovich, J. F. Donegan, V. Oleinikov, M. Molinari, A. Sukhanova, I. Nabiev, and Y. P. Rakovich, “Linear and nonlinear optical effects induced by energy transfer from semiconductor nanoparticles to photosynthetic biological systems,” J. Photochem. Photobiol. Photochem. Rev. 20, 17–32 (2014). F. Wang, W. B. Tan, Y. Zhang, X. Fan, and M. Wang, “Luminescent nanomaterials for biological labelling,” Nanotechnology 17(1), R1–R13 (2006). E. H. Sargent, “Infrared photovoltaics made by solution processing,” Nat. Photonics 3(6), 325–331 (2009).
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15. R. D. Schaller, V. M. Agranovich, and V. I. Klimov, “High-efficiency carrier multiplication through direct photogeneration of multi-excitons via virtual single-exciton states,” Nat. Phys. 1(3), 189–194 (2005). 16. P. Bhattacharya and Z. Mi, “Quantum-dot optoelectronic devices,” Proc. IEEE 95(9), 1723–1740 (2007). 17. 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). 18. R. Thielsch, T. Bohme, R. Reiche, D. Schlafer, H. D. Bauer, and H. Bottcher, “Quantum-size effects of PbS nanocrystallites in evaporated composite films,” Nanostruct. Mater. 10(2), 131 (1998). 19. J. E. Murphy, M. C. Beard, A. G. Norman, S. Ph. Ahrenkiel, J. C. Johnson, P. Yu, O. I. Mićić, R. J. Ellingson, and A. J. Nozik, “PbTe colloidal nanocrystals: Synthesis, characterization, and multiple exciton generation,” J. Am. Chem. Soc. 128(10), 3241–3247 (2006). 20. F. Pang, X. Sun, H. Guo, J. Yan, J. Wang, X. Zeng, Z. Chen, and T. Wang, “A PbS quantum dots fiber amplifier excited by evanescent wave,” Opt. Express 18(13), 14024–14030 (2010). 21. M. A. Hines and G. D. Scholes, “Colloida 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). 22. S. Fan, G. Wu, Y. Zhang, G. Chai, Z. Ma, J. Qiu, and G. Dong, “Novel visible emission and mechanism investigation from PbS nanoclusters-doped borosilicate glasses,” J. Am. Ceram. Soc. 97(1), 173–178 (2014). 23. M. S. Ghamsari, M. H. Majles Ara, S. Radiman, and X. H. Zhang, “Colloidal lead sulfide nanocrystals with strong green emission,” J. Lumin. 137, 241–244 (2013). 24. C. Liu, Y. K. Kwon, and J. Heo, “Novel nano-structured glasses containing semiconductor quantum dots: controlling the photoluminescence with phonons and photons,” J. Mater. Sci. Mater. Electron. 20(1), 282–285 (2009). 25. A. A. Sirotkin, L. D. Labio, A. I. Zagumennyi, Y. D. Zavartsev, S. A. Kutovoi, V. I. Vlasov, W. Luthy, T. Feurer, A. A. Onushchenko, and I. A. Shcherbakov, “Mode-locked diode-pumped vanadate lasers operated with PbS quantum dots,” Appl. Phys. B 94(3), 375–379 (2009). 26. O. Qasaimeh, “Effect of doping on the optical characteristics of quantum-dot semiconductor optical amplifiers,” IEEE J. Lightw. Technol. 27(12), 1978–1984 (2009). 27. V. Sukhovatkin, S. Musikhin, I. Gorelikov, S. Cauchi, L. Bakueva, E. Kumacheva, and E. H. Sargent, “Roomtemperature amplified spontaneous emission at 1300 nm in solution-processed PbS quantum-dot films,” Opt. Lett. 30(2), 171–173 (2005). 28. N. F. Borrelli and D. W. Smith, “Quantum confinement of PbS microcrystals in glass,” J. Non-Cryst. Solids 180(1), 25–31 (1994). 29. K. Wundke, J. M. Auxier, A. Schülzgen, N. Peyghambarian, and N. F. Borrelli, “Room-temperature gain at 1.3 um in PbS-doped glasses,” Appl. Phys. Lett. 75(20), 3060 (1999). 30. J. M. Auxier, K. Wundke, A. Schülzgen, N. Peyghambarian, and N. F. Borrelli, “Luminescence and gain around 1.3 um in PbS quantum dots,” CLEO 2000 Technical Digest 385, 1–2 (2000). 31. K. Wundke, S. Pötting, J. Auxier, A. Schülzgen, N. Peyghambarian, and N. F. Borrelli, “PbS quantum-dot-doped glasses for ultrashort-pulse generation,” Appl. Phys. Lett. 76(1), 10–12 (2000). 32. G. P. Dong, B. T. Wu, F. T. Zhang, L. L. Zhang, M. Y. Peng, D. D. Chen, E. Wu, and J. R. Qiu, “Broadband near-infrared luminescence and tunable optical amplification around 1.55 μm and 1.33 μm of PbS quantum dots in glasses,” J. Alloys Compd. 509(38), 9335–9339 (2011). 33. 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). 34. 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). 35. C. Liu, J. Heo, X. H. Zhang, and J. L. Adam, “Photoluminescence of PbS quantum dots doped in glasses,” J. Non-Cryst. Solids 354(2-9), 618–623 (2008). 36. N. O. Dantas, F. Qu, A. F. G. Monte, R. S. Silva, and P. C. Morais, “Optical properties of IV–VI quantum dots doped in glass: Size-effects,” J. Non-Cryst. Solids 352(32–35), 3525–3529 (2006).
1. Introduction Semiconductor quantum dots (QDs) have been extensively investigated because of their unique optical and electronic properties compared with their bulk counterparts [1, 2]. These low dimension materials perform a physical nature between single molecules and bulk solid state, providing an opportunity to trace the evolution of electronic and optical properties of the matter from small atomic clusters to bulk solids [3]. The strong quantum confinement separating QDs into discrete electronic transitions depends on the size of QDs [4, 5]. Hanamura and Efros at al. have carried on the detailed studies on quantum confinement of QDs and their large nonlinearities [6, 7]. Because of the size effect, tunable spectral position of the absorption and emission bands by varying their size can be obtained from QDs [2, 8, 9]. This feature endows the various application of QDs, such as 1.3-1.55μm range of optical communications, biomedical fluorescent labeling, light emitting devices and solar cell, etc [8– 14]. And carrier multiplication effect should lead to substantial improvements in the
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Received 24 Mar 2015; revised 14 Jun 2015; accepted 15 Jun 2015; published 17 Jun 2015 29 Jun 2015 | Vol. 23, No. 13 | DOI:10.1364/OE.23.016723 | OPTICS EXPRESS 16724
performance of solar cells, low-threshold lasers and entangled photon sources, etc [15]. Among these narrow band gap IV–VI materials, PbS has attracted much attention recently, because of their large exciton Bohr radii (18nm) and nearly equivalent electron and hole effective masses, which makes them attractive as strong quantum confinement effect materials [16–20]. And their bandgap depend strongly on size, which can change from 0.41eV for bulk to 5.2eV for 1nm size at room temperature [21]. The abundant bandgap of PbS QDs make them promising for tunable visible and near-infrared (NIR) emissions [10, 22–24]. From the technological perspective, the preparation of PbS QDs in glasses have the advantages of chemical and mechanical stabilities, stabilizing QDs and being adapted to fiber-drawing and device manufacturing process, which can be further used into fiber lasers and amplifiers [20, 25–27]. Besides, the size distribution of QD in glass can be controlled by a dual-heat treatment [28, 29]. In this study, PbS QD-doped glasses with different size have been prepared by controlling the heat-treatment process. Time-resolve emission spectra (TRES) of PbS QD-doped glasses were studied for the first time. Combined with the steadystate, transient-state photoluminescence (PL) and TRES results, the energy transfer (ET) process from smaller QDs with higher energy to bigger QDs with lower energy was firstly revealed in PbS QD-doped glasses. 2. Experimental The nominal mass composition of glass sample was 56SiO2-6.33Al2O3-17.3B2O3-9.63Na2O7.1CaO-1.94PbO-0.85ZnS. 30g analytical grade raw materials were well mixed and melt at 1300°C for 1h under ambient atmosphere. Then the melt was poured onto a cold stainless steel to obtain transparent glass. The glass was cut into small pieces with a size of 10 × 10 mm and heat-treated at 560°C, 580°C, 600°C and 620°C for 24h. Then the obtained QDdoped glass samples were polished for optical measurements. The morphology and crystallization of PbS QDs in glasses were characterized by high-resolution transmission electron microscopy (HR-TEM, 2100F, JEOL, Japan). Absorption spectra were recorded using a Lambda 900 (Perkin Elmer, USA) spectrophotometer. And the PL spectra, lifetime decay profiles and TRES were measured by an Edinburgh FLS 920 instrument (Edinburgh Instruments, Royston, UK). 3. Results and discussion
Fig. 1. (a) Absorption spectra of glass heated at different temperatures for 24h. (b) PL spectra of glass heated at different temperatures for 24h excited at 468nm.
Figure 1(a) shows the absorption spectra of the as-prepared and heat-treated glass samples. An obvious red shift of the absorption edge can be observed with the increase of the heattreated temperature. It can be ascribed to the formation and growth of PbS QDs in glass matrix with the increase of the heat-treated temperature. It is noticed that there is no obvious absorption band of the sample heat-treated at 560°C. When the heat-treated temperature reaches 580°C, a slightly absorption band appears and the absorption band of PbS QDs gradually shifts to the longer wavelength with the increase of heat-treated temperature. The absorption band is ascribed to the generation of electron and hole pairs induced by the
#236890 © 2015 OSA
Received 24 Mar 2015; revised 14 Jun 2015; accepted 15 Jun 2015; published 17 Jun 2015 29 Jun 2015 | Vol. 23, No. 13 | DOI:10.1364/OE.23.016723 | OPTICS EXPRESS 16725
excitation photon. The QD-doped glass heat-treated at 600°C for 24h shows stronger absorption intensity than that of the sample heat-treated at 580°C and 620°C, but with the smallest full width at half maximum (FWHM) of these samples. It can be deduced that more PbS QDs with narrow size distribution are generated in the QD-doped glass heat-treated at 600°C, while the size of PbS QDs in the glass heat-treated at 560°C and 620°C seems slightly unconformity. Comparing our results with the works of Borrelli et al. [28–31], which prepared PbS QD-doped glasses with narrow size distribution by a dual-heat treatment, it can be deduced that the narrow size distribution of PbS QDs can also be obtained by the composition regulation of glass matrix [32]. With the advantage of the number density of PbS QDs with similar size, the glass heat-treated at 600°C shows the strongest absorption intensity and the smallest FWHM. According to previous works, the average size of PbS QDs in glass can be estimated by the following equation [33,34] 2 2 E g π 2 2 (1) = + E g g ∗ m R Where R is the average radius of PbS QDs, ħ is Planck's constant, m* is the reduced mass, Eg(R) is the effective energy of PbS QDs, and Eg is the bandgap energy of bulk PbS semiconductor. For bulk PbS semiconductor, Eg = 0.41 eV and m* = 0.085me, where me is the static electron mass. Since PbS is a direct bandgap semiconductor, the effective energy of PbS QDs can be evaluated by the fitting of the absorption spectra [33, 35]. The effective energy of PbS QDs in glasses heated at 580°C, 600°C and 620°C is fitted to be 1.63eV, 1.13eV and 0.91eV, respectively. Based on Eq. (1), the average radius of PbS QDs in glasses heated at 580°C, 600°C and 620°C is estimated to be 1.72nm, 2.58nm and 3.35nm, respectively.
( E ( R ))
2
Fig. 2. (a) TEM image of glass heated at 600°C for 24 h. The inset shows the HR-TEM image of a PbS QD. (b) The size distribution of PbS QDs in glasses corresponding to TEM image.
Figure 2(a) shows the TEM and HR-TEM images of the glass heat-treated at 600°C for 24h. It is noticed that the PbS QDs are distributed uniformly in the glass matrix, which is in agreement with the result indicated from the absorption spectra as shown in Fig. 1(a). The morphology of QDs is quasi-spherical, while the size distribution of QDs is mainly between 1.8 and 4.6nm (Fig. 2(b)), with an average size of ~2.7nm, which is consistent with the fitting result of the absorption spectra shown in Fig. 1(a). The relatively large size distribution of PbS QDs is probably due to the high doping concentration of PbS in glass preparation, which results in the inhomogeneous migration of S source and growth of PbS nuclei during the formation of PbS QD. The inset of Fig. 2 shows the HR-TEM image of a single PbS QD. A group of lattice fringe with a space of ~0.21nm is clearly observed in the image, which can be assigned to the (220) plane of cubic PbS crystals (JCPDF: 65-0132). Figure 1(b) illustrates the PL spectra of QD-doped glass under an excitation of 468 nm xenon lamp. Although PbS QDs are confirmed to begin forming in the glass heated at 560°C, the PbS QDs are too small to be considered as PbS nuclei, and the PL band is too weak to be detected as shown in Fig. 1(b). Due to the growth of PbS QDs in the glass matrix, broadband
#236890 © 2015 OSA
Received 24 Mar 2015; revised 14 Jun 2015; accepted 15 Jun 2015; published 17 Jun 2015 29 Jun 2015 | Vol. 23, No. 13 | DOI:10.1364/OE.23.016723 | OPTICS EXPRESS 16726
PL spectra occur when the heat-treated temperature reaches 580°C. As shown in Fig. 1(b), an obvious red shift of PL band can be observed with the increase of heat-treated temperature. The center of PL band corresponding to 580°C, 600°C, 620°C is 1010nm, 1270nm and 1420nm, respectively. The red shift of PL band confirms the PbS QDs gradually grow with the increase of heat-treated temperature. It can be deduced that the PL wavelength of PbS QD-doped glass can be widely tuned by controlling the heat treatment condition. The FWHM of the QD-doped glass heat-treated at 580°C, 600°C and 620°C is 200nm, 170nm and 230nm, respectively. In addition, the QD-doped glass heat-treated at 600°C illustrates the strongest PL intensity, which is corresponding to the results of the absorption spectra.
Fig. 3. (a) The decay profiles of the QD-doped glass heat-treated at 580°C, 600°C and 620°C detected at 1010nm, 1270nm and 1420nm emission, respectively. (b) Measured lifetime of PbS QD-doped glass heat-treated at 600°C for 24h as function of wavelength at intervals of 25nm.
Fig. 4. (a) The time-resolve PL spectra for the QD-doped glass heat-treated at 600°C for 24h. (b) The transient-state and steady-state PL spectra for the QD-doped glass heat-treated at 600°C.
Figure 3(a) illustrates the PL lifetime decay profiles of the QD-doped glasses heat-treated at different temperatures detected at 1010nm, 1270nm and 1420nm, respectively. The corresponding fluorescence decay profiles are fitted by the single-exponential model. It is noticed that the QD-doped glass heat-treated at 600°C for 24h has the longest lifetime of 7.54μs, while the lifetime of the QD-doped glass heat-treated at 620°C and 580°C is 6.38μs and 4.11μs. Compared Fig. 3(a) with Fig. 1(b), it can be noticed that the QD-doped glass has longer lifetime with intense PL. Hence, it can be deduced that the lifetime of PbS QD-doped glass relatives to their PL intensity. In order to prove this deduction, a series of lifetimes of the QD-doped glass heat-treated at 600°C detected from 1100nm to 1400nm with an interval of 25nm were carried out in Fig. 3(b). As shown in Fig. 3(b), the variation tendency of lifetimes detected from 1100nm to 1400nm shows a similar profile with that of PL spectra. This consistency demonstrates the PL intensity strongly impacts the lifetime in the PbS QD#236890 © 2015 OSA
Received 24 Mar 2015; revised 14 Jun 2015; accepted 15 Jun 2015; published 17 Jun 2015 29 Jun 2015 | Vol. 23, No. 13 | DOI:10.1364/OE.23.016723 | OPTICS EXPRESS 16727
doped glasses in a way. Wundke et al. have pointed out the luminescence originated from two sources: free excitons and shallow trap states [29–31]. Generally speaking, the latter with a binding energy of about 30 meV have a longer lifetime. And our result is probably ascribed to a combined effect of ET along with multi-channel relaxation of free exciton states and trapped-states. In order to further investigate the ET process between PbS QDs with different sizes, TRES as well as transient-state and steady-state PL spectra of the QD-doped glass heattreated at 600°C were measured. Figure 4(a) depicts the TRES of QD-doped glass heattreated at 600°C in the time scale of 0–25μs. A series of emission spectra at different decay time can be observed clearly from the spectra. The PL spectra appear a peak at 1220nm at beginning, and finally the peak is redshift to 1270nm at 25μs. This continuous redshift about 50nm with decay time is clearly evident as shown in Fig. 4(a). Figure 4(b) depicts the transient-state and steady-state PL spectra of the QD-doped glass heat-treated at 600°C. The PL spectra appear a peak at 1220nm in transient-state and 1270nm in steady-state, respectively. From the previous works [29–31], the different excitation power can lead to the shift of PL, but in this measurement, the fluctuation of excitation power is weak. Therefore, the excitation power effect on the shift of PL is neglected in the following discussion. These results indicate the arrow of ET in the PbS QD-doped glass is from PbS QDs with higher energy to the PbS QDs with lower energy. Since the wavelength of PL peak relates to the size of PbS QDs as shown in Fig. 1(b), it can be deduced that the emission generated from smaller PbS QDs will create electron-hole pairs with smaller bandgap of larger PbS QDs to emit longer wavelength PL. The schematic ET diagram is shown in Fig. 5. Electron energy levels in QDs are not continuous but quantized, and energy gap will increase with the decrease of the size of QDs due to the quantum confined effect. Electron–hole pairs originate from smaller PbS QDs under the excitation of Xeon lamp. The PL with high energy emission through the recombination of these electron–hole pairs may be absorbed by larger PbS QDs to create electron-hole pairs with smaller bandgap. Dantas et al. [36] have reported that the energy intensity generated in the QD-doped glass strongly influenced the ET process. The lower the detection energy, the larger the diffusion length. This behavior results from the ET between different subsets of QDs. It indicates that photons emitted by smaller QDs can only be absorbed by even larger QDs, which means the incident light can only create the electron– hole pairs in larger QDs. Considering the dispersion of QDs with different sizes in the glass matrix, with the ET from smaller PbS QDs, a PL spectrum generated by the majority larger PbS QDs will be observed as shown in Fig. 5. This also explains the unsymmetrical sidebands at measured lifetime spectra in Fig. 3(b). In previous works [29–31], Auxier et al. have proposed that the higher quantum efficiency (QE) trapped-state PL was redshift relative to the free exciton PL, which probably also contribute to the PL redshift in present works. According to the above ET mechanism, when the QDs are on resonance ET, there will be a loss of energy. When the QDs distribution is uneven, the probability of ET will increase, meanwhile more photons will loss and PL intensity weaken. And according to the following equation, 1 (2) k F + ki Where τ is the total lifetime, kF is the radiative decay rate constant, and ki is the nonradiative decay rate constant. The emitting decay rates will increase with the ET leading to the decrease of PL lifetime. Hence, as PL intensity increases, the excitation energy loss reduces by nonradiative process, and the PL lifetime becomes longer, which leads to the similarity of the spectra in Fig. 1(b) and Fig. 3(b).
τ=
#236890 © 2015 OSA
Received 24 Mar 2015; revised 14 Jun 2015; accepted 15 Jun 2015; published 17 Jun 2015 29 Jun 2015 | Vol. 23, No. 13 | DOI:10.1364/OE.23.016723 | OPTICS EXPRESS 16728
Fig. 5. Schematic energy transfer process between PbS QDs with different size.
4. Conclusion In conclusion, PbS QD-doped glass was prepared by the heat-treatment of as-prepared glass. The absorption and PL spectra demonstrated that the optical properties of PbS QD-doped glass can be widely tuned by carefully controlling the size and distribution of PbS QDs in glass. A series of lifetime decay profiles detected at whole PL band were detected in detail, which indicated that the variation tendency of lifetime spectra shows a similar profile with that of PL spectra. More importantly, the steady-state, transient-state PL spectra and TRES results directly revealed the ET process from smaller QDs with higher energy to bigger QDs with lower energy in PbS QD-doped glasses for the first time. Acknowledgments This work was supported by the National Natural Science Foundation of China (61475047, 51102096), the Guangdong Natural Science Foundation for Distinguished Young Scholars (2014A030306045), and the Pearl River S&T Nova Program of Guangzhou (2014J2200083). The authors are grateful to Mr. Weixi Ma for a part of sample preparation.
#236890 © 2015 OSA
Received 24 Mar 2015; revised 14 Jun 2015; accepted 15 Jun 2015; published 17 Jun 2015 29 Jun 2015 | Vol. 23, No. 13 | DOI:10.1364/OE.23.016723 | OPTICS EXPRESS 16729
State Key Laboratory of Luminescent Materials and Device, School of Materials Science and Engineering, South China University of Technology, Guangzhou, China 2 Equal contribution to this work. * [email protected]
Abstract: PbS quantum dot (QD)-doped glass was prepared by the heat treatment of as-prepared glass, which was confirmed by transmission electron microscope (TEM), absorption and photoluminescence (PL) spectra. Choosing the glass heat treatment at 600°C for 24h as a representative sample, steady-state and transient-state PL, lifetime and time-resolve emission spectra (TRES) of PbS QD-doped glass were studied in detail. The results indicated that the lifetime spectra showed a similar variation tendency with the PL spectra. The steady-state, transient-state PL and TRES results first revealed the energy transfer process from smaller QDs with higher energy to bigger QDs with lower energy in the PbS QDdoped glass. ©2015 Optical Society of America OCIS codes: (160.2540) Fluorescent and luminescent materials; (160.2750) Glass and other amorphous materials; (300.6280) Spectroscopy, fluorescence and luminescence.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
V. I. Klimov, A. A. Mikhailovsky, S. Xu, A. Malko, J. A. Hollingsworth, C. A. Leatherdale, H. Eisler, and M. G. Bawendi, “Optical gain and stimulated emission in nanocrystal quantum dots,” Science 290(5490), 314–317 (2000). F. W. Wise, “Lead salt quantum dots: the limit of strong quantum confinement,” Acc. Chem. Res. 33(11), 773– 780 (2000). M. S. Gaponenko, A. A. Lutich, N. A. Tolstik, A. A. Onushchenko, A. M. Malyarevich, E. P. Petrov, and K. V. Yumashev, “Temperature-dependent photoluminescence of PbS quantum dots in glass: Evidence of exciton state splitting and carrier trapping,” Phys. Rev. B 82(12), 125320 (2010). A. P. Alivisatos, “Semiconductor clusters, nanocrystals, and quantum dots,” Science 271(5251), 933–937 (1996). A. I. Ekimov, A. L. Efros, and A. A. Onushchenko, “Quantum size effect in semiconductor microcrystals,” Solid State Commun. 88(11), 947–950 (1985). E. Hanamura, “Very large optical nonlinearity of semiconductor microcrystallites,” Phys. Rev. B Condens. Matter 37(3), 1273–1279 (1988). A. L. Efros and A. L. Efros, “Interband absorption of light in a semiconductor sphere,” Sov. Phys. Semicond. 16(7), 772–775 (1982). C. Liu, Y. K. Kwon, and J. Heo, “Temperature-dependent brightening and darkening of photoluminescence from PbS quantum dots in glasses,” Appl. Phys. Lett. 90(24), 241111 (2007). E. U. Rafailov, M. A. Cataluna, and W. Sibbett, “Mode-locked quantum-dot lasers,” Nat. Photonics 1(7), 395– 401 (2007). G. Dong, G. Wu, S. Fan, F. Zhang, Y. Zhang, B. Wu, Z. Ma, M. Peng, and J. Qiu, “Formation, near-infrared luminescence and multi-wavelength optical amplification of PbS quantum dot-doped silicate glasses,” J. NonCryst. Solids 383(1), 192–195 (2004). S. Hoogland, V. Sukhovatkin, I. Howard, S. Cauchi, L. Levina, and E. H. Sargent, “A solution-processed 1.53 mum quantum dot laser with temperature-invariant emission wavelength,” Opt. Express 14(8), 3273–3281 (2006). A. Rakovich, J. F. Donegan, V. Oleinikov, M. Molinari, A. Sukhanova, I. Nabiev, and Y. P. Rakovich, “Linear and nonlinear optical effects induced by energy transfer from semiconductor nanoparticles to photosynthetic biological systems,” J. Photochem. Photobiol. Photochem. Rev. 20, 17–32 (2014). F. Wang, W. B. Tan, Y. Zhang, X. Fan, and M. Wang, “Luminescent nanomaterials for biological labelling,” Nanotechnology 17(1), R1–R13 (2006). E. H. Sargent, “Infrared photovoltaics made by solution processing,” Nat. Photonics 3(6), 325–331 (2009).
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15. R. D. Schaller, V. M. Agranovich, and V. I. Klimov, “High-efficiency carrier multiplication through direct photogeneration of multi-excitons via virtual single-exciton states,” Nat. Phys. 1(3), 189–194 (2005). 16. P. Bhattacharya and Z. Mi, “Quantum-dot optoelectronic devices,” Proc. IEEE 95(9), 1723–1740 (2007). 17. 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). 18. R. Thielsch, T. Bohme, R. Reiche, D. Schlafer, H. D. Bauer, and H. Bottcher, “Quantum-size effects of PbS nanocrystallites in evaporated composite films,” Nanostruct. Mater. 10(2), 131 (1998). 19. J. E. Murphy, M. C. Beard, A. G. Norman, S. Ph. Ahrenkiel, J. C. Johnson, P. Yu, O. I. Mićić, R. J. Ellingson, and A. J. Nozik, “PbTe colloidal nanocrystals: Synthesis, characterization, and multiple exciton generation,” J. Am. Chem. Soc. 128(10), 3241–3247 (2006). 20. F. Pang, X. Sun, H. Guo, J. Yan, J. Wang, X. Zeng, Z. Chen, and T. Wang, “A PbS quantum dots fiber amplifier excited by evanescent wave,” Opt. Express 18(13), 14024–14030 (2010). 21. M. A. Hines and G. D. Scholes, “Colloida 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). 22. S. Fan, G. Wu, Y. Zhang, G. Chai, Z. Ma, J. Qiu, and G. Dong, “Novel visible emission and mechanism investigation from PbS nanoclusters-doped borosilicate glasses,” J. Am. Ceram. Soc. 97(1), 173–178 (2014). 23. M. S. Ghamsari, M. H. Majles Ara, S. Radiman, and X. H. Zhang, “Colloidal lead sulfide nanocrystals with strong green emission,” J. Lumin. 137, 241–244 (2013). 24. C. Liu, Y. K. Kwon, and J. Heo, “Novel nano-structured glasses containing semiconductor quantum dots: controlling the photoluminescence with phonons and photons,” J. Mater. Sci. Mater. Electron. 20(1), 282–285 (2009). 25. A. A. Sirotkin, L. D. Labio, A. I. Zagumennyi, Y. D. Zavartsev, S. A. Kutovoi, V. I. Vlasov, W. Luthy, T. Feurer, A. A. Onushchenko, and I. A. Shcherbakov, “Mode-locked diode-pumped vanadate lasers operated with PbS quantum dots,” Appl. Phys. B 94(3), 375–379 (2009). 26. O. Qasaimeh, “Effect of doping on the optical characteristics of quantum-dot semiconductor optical amplifiers,” IEEE J. Lightw. Technol. 27(12), 1978–1984 (2009). 27. V. Sukhovatkin, S. Musikhin, I. Gorelikov, S. Cauchi, L. Bakueva, E. Kumacheva, and E. H. Sargent, “Roomtemperature amplified spontaneous emission at 1300 nm in solution-processed PbS quantum-dot films,” Opt. Lett. 30(2), 171–173 (2005). 28. N. F. Borrelli and D. W. Smith, “Quantum confinement of PbS microcrystals in glass,” J. Non-Cryst. Solids 180(1), 25–31 (1994). 29. K. Wundke, J. M. Auxier, A. Schülzgen, N. Peyghambarian, and N. F. Borrelli, “Room-temperature gain at 1.3 um in PbS-doped glasses,” Appl. Phys. Lett. 75(20), 3060 (1999). 30. J. M. Auxier, K. Wundke, A. Schülzgen, N. Peyghambarian, and N. F. Borrelli, “Luminescence and gain around 1.3 um in PbS quantum dots,” CLEO 2000 Technical Digest 385, 1–2 (2000). 31. K. Wundke, S. Pötting, J. Auxier, A. Schülzgen, N. Peyghambarian, and N. F. Borrelli, “PbS quantum-dot-doped glasses for ultrashort-pulse generation,” Appl. Phys. Lett. 76(1), 10–12 (2000). 32. G. P. Dong, B. T. Wu, F. T. Zhang, L. L. Zhang, M. Y. Peng, D. D. Chen, E. Wu, and J. R. Qiu, “Broadband near-infrared luminescence and tunable optical amplification around 1.55 μm and 1.33 μm of PbS quantum dots in glasses,” J. Alloys Compd. 509(38), 9335–9339 (2011). 33. 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). 34. 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). 35. C. Liu, J. Heo, X. H. Zhang, and J. L. Adam, “Photoluminescence of PbS quantum dots doped in glasses,” J. Non-Cryst. Solids 354(2-9), 618–623 (2008). 36. N. O. Dantas, F. Qu, A. F. G. Monte, R. S. Silva, and P. C. Morais, “Optical properties of IV–VI quantum dots doped in glass: Size-effects,” J. Non-Cryst. Solids 352(32–35), 3525–3529 (2006).
1. Introduction Semiconductor quantum dots (QDs) have been extensively investigated because of their unique optical and electronic properties compared with their bulk counterparts [1, 2]. These low dimension materials perform a physical nature between single molecules and bulk solid state, providing an opportunity to trace the evolution of electronic and optical properties of the matter from small atomic clusters to bulk solids [3]. The strong quantum confinement separating QDs into discrete electronic transitions depends on the size of QDs [4, 5]. Hanamura and Efros at al. have carried on the detailed studies on quantum confinement of QDs and their large nonlinearities [6, 7]. Because of the size effect, tunable spectral position of the absorption and emission bands by varying their size can be obtained from QDs [2, 8, 9]. This feature endows the various application of QDs, such as 1.3-1.55μm range of optical communications, biomedical fluorescent labeling, light emitting devices and solar cell, etc [8– 14]. And carrier multiplication effect should lead to substantial improvements in the
#236890 © 2015 OSA
Received 24 Mar 2015; revised 14 Jun 2015; accepted 15 Jun 2015; published 17 Jun 2015 29 Jun 2015 | Vol. 23, No. 13 | DOI:10.1364/OE.23.016723 | OPTICS EXPRESS 16724
performance of solar cells, low-threshold lasers and entangled photon sources, etc [15]. Among these narrow band gap IV–VI materials, PbS has attracted much attention recently, because of their large exciton Bohr radii (18nm) and nearly equivalent electron and hole effective masses, which makes them attractive as strong quantum confinement effect materials [16–20]. And their bandgap depend strongly on size, which can change from 0.41eV for bulk to 5.2eV for 1nm size at room temperature [21]. The abundant bandgap of PbS QDs make them promising for tunable visible and near-infrared (NIR) emissions [10, 22–24]. From the technological perspective, the preparation of PbS QDs in glasses have the advantages of chemical and mechanical stabilities, stabilizing QDs and being adapted to fiber-drawing and device manufacturing process, which can be further used into fiber lasers and amplifiers [20, 25–27]. Besides, the size distribution of QD in glass can be controlled by a dual-heat treatment [28, 29]. In this study, PbS QD-doped glasses with different size have been prepared by controlling the heat-treatment process. Time-resolve emission spectra (TRES) of PbS QD-doped glasses were studied for the first time. Combined with the steadystate, transient-state photoluminescence (PL) and TRES results, the energy transfer (ET) process from smaller QDs with higher energy to bigger QDs with lower energy was firstly revealed in PbS QD-doped glasses. 2. Experimental The nominal mass composition of glass sample was 56SiO2-6.33Al2O3-17.3B2O3-9.63Na2O7.1CaO-1.94PbO-0.85ZnS. 30g analytical grade raw materials were well mixed and melt at 1300°C for 1h under ambient atmosphere. Then the melt was poured onto a cold stainless steel to obtain transparent glass. The glass was cut into small pieces with a size of 10 × 10 mm and heat-treated at 560°C, 580°C, 600°C and 620°C for 24h. Then the obtained QDdoped glass samples were polished for optical measurements. The morphology and crystallization of PbS QDs in glasses were characterized by high-resolution transmission electron microscopy (HR-TEM, 2100F, JEOL, Japan). Absorption spectra were recorded using a Lambda 900 (Perkin Elmer, USA) spectrophotometer. And the PL spectra, lifetime decay profiles and TRES were measured by an Edinburgh FLS 920 instrument (Edinburgh Instruments, Royston, UK). 3. Results and discussion
Fig. 1. (a) Absorption spectra of glass heated at different temperatures for 24h. (b) PL spectra of glass heated at different temperatures for 24h excited at 468nm.
Figure 1(a) shows the absorption spectra of the as-prepared and heat-treated glass samples. An obvious red shift of the absorption edge can be observed with the increase of the heattreated temperature. It can be ascribed to the formation and growth of PbS QDs in glass matrix with the increase of the heat-treated temperature. It is noticed that there is no obvious absorption band of the sample heat-treated at 560°C. When the heat-treated temperature reaches 580°C, a slightly absorption band appears and the absorption band of PbS QDs gradually shifts to the longer wavelength with the increase of heat-treated temperature. The absorption band is ascribed to the generation of electron and hole pairs induced by the
#236890 © 2015 OSA
Received 24 Mar 2015; revised 14 Jun 2015; accepted 15 Jun 2015; published 17 Jun 2015 29 Jun 2015 | Vol. 23, No. 13 | DOI:10.1364/OE.23.016723 | OPTICS EXPRESS 16725
excitation photon. The QD-doped glass heat-treated at 600°C for 24h shows stronger absorption intensity than that of the sample heat-treated at 580°C and 620°C, but with the smallest full width at half maximum (FWHM) of these samples. It can be deduced that more PbS QDs with narrow size distribution are generated in the QD-doped glass heat-treated at 600°C, while the size of PbS QDs in the glass heat-treated at 560°C and 620°C seems slightly unconformity. Comparing our results with the works of Borrelli et al. [28–31], which prepared PbS QD-doped glasses with narrow size distribution by a dual-heat treatment, it can be deduced that the narrow size distribution of PbS QDs can also be obtained by the composition regulation of glass matrix [32]. With the advantage of the number density of PbS QDs with similar size, the glass heat-treated at 600°C shows the strongest absorption intensity and the smallest FWHM. According to previous works, the average size of PbS QDs in glass can be estimated by the following equation [33,34] 2 2 E g π 2 2 (1) = + E g g ∗ m R Where R is the average radius of PbS QDs, ħ is Planck's constant, m* is the reduced mass, Eg(R) is the effective energy of PbS QDs, and Eg is the bandgap energy of bulk PbS semiconductor. For bulk PbS semiconductor, Eg = 0.41 eV and m* = 0.085me, where me is the static electron mass. Since PbS is a direct bandgap semiconductor, the effective energy of PbS QDs can be evaluated by the fitting of the absorption spectra [33, 35]. The effective energy of PbS QDs in glasses heated at 580°C, 600°C and 620°C is fitted to be 1.63eV, 1.13eV and 0.91eV, respectively. Based on Eq. (1), the average radius of PbS QDs in glasses heated at 580°C, 600°C and 620°C is estimated to be 1.72nm, 2.58nm and 3.35nm, respectively.
( E ( R ))
2
Fig. 2. (a) TEM image of glass heated at 600°C for 24 h. The inset shows the HR-TEM image of a PbS QD. (b) The size distribution of PbS QDs in glasses corresponding to TEM image.
Figure 2(a) shows the TEM and HR-TEM images of the glass heat-treated at 600°C for 24h. It is noticed that the PbS QDs are distributed uniformly in the glass matrix, which is in agreement with the result indicated from the absorption spectra as shown in Fig. 1(a). The morphology of QDs is quasi-spherical, while the size distribution of QDs is mainly between 1.8 and 4.6nm (Fig. 2(b)), with an average size of ~2.7nm, which is consistent with the fitting result of the absorption spectra shown in Fig. 1(a). The relatively large size distribution of PbS QDs is probably due to the high doping concentration of PbS in glass preparation, which results in the inhomogeneous migration of S source and growth of PbS nuclei during the formation of PbS QD. The inset of Fig. 2 shows the HR-TEM image of a single PbS QD. A group of lattice fringe with a space of ~0.21nm is clearly observed in the image, which can be assigned to the (220) plane of cubic PbS crystals (JCPDF: 65-0132). Figure 1(b) illustrates the PL spectra of QD-doped glass under an excitation of 468 nm xenon lamp. Although PbS QDs are confirmed to begin forming in the glass heated at 560°C, the PbS QDs are too small to be considered as PbS nuclei, and the PL band is too weak to be detected as shown in Fig. 1(b). Due to the growth of PbS QDs in the glass matrix, broadband
#236890 © 2015 OSA
Received 24 Mar 2015; revised 14 Jun 2015; accepted 15 Jun 2015; published 17 Jun 2015 29 Jun 2015 | Vol. 23, No. 13 | DOI:10.1364/OE.23.016723 | OPTICS EXPRESS 16726
PL spectra occur when the heat-treated temperature reaches 580°C. As shown in Fig. 1(b), an obvious red shift of PL band can be observed with the increase of heat-treated temperature. The center of PL band corresponding to 580°C, 600°C, 620°C is 1010nm, 1270nm and 1420nm, respectively. The red shift of PL band confirms the PbS QDs gradually grow with the increase of heat-treated temperature. It can be deduced that the PL wavelength of PbS QD-doped glass can be widely tuned by controlling the heat treatment condition. The FWHM of the QD-doped glass heat-treated at 580°C, 600°C and 620°C is 200nm, 170nm and 230nm, respectively. In addition, the QD-doped glass heat-treated at 600°C illustrates the strongest PL intensity, which is corresponding to the results of the absorption spectra.
Fig. 3. (a) The decay profiles of the QD-doped glass heat-treated at 580°C, 600°C and 620°C detected at 1010nm, 1270nm and 1420nm emission, respectively. (b) Measured lifetime of PbS QD-doped glass heat-treated at 600°C for 24h as function of wavelength at intervals of 25nm.
Fig. 4. (a) The time-resolve PL spectra for the QD-doped glass heat-treated at 600°C for 24h. (b) The transient-state and steady-state PL spectra for the QD-doped glass heat-treated at 600°C.
Figure 3(a) illustrates the PL lifetime decay profiles of the QD-doped glasses heat-treated at different temperatures detected at 1010nm, 1270nm and 1420nm, respectively. The corresponding fluorescence decay profiles are fitted by the single-exponential model. It is noticed that the QD-doped glass heat-treated at 600°C for 24h has the longest lifetime of 7.54μs, while the lifetime of the QD-doped glass heat-treated at 620°C and 580°C is 6.38μs and 4.11μs. Compared Fig. 3(a) with Fig. 1(b), it can be noticed that the QD-doped glass has longer lifetime with intense PL. Hence, it can be deduced that the lifetime of PbS QD-doped glass relatives to their PL intensity. In order to prove this deduction, a series of lifetimes of the QD-doped glass heat-treated at 600°C detected from 1100nm to 1400nm with an interval of 25nm were carried out in Fig. 3(b). As shown in Fig. 3(b), the variation tendency of lifetimes detected from 1100nm to 1400nm shows a similar profile with that of PL spectra. This consistency demonstrates the PL intensity strongly impacts the lifetime in the PbS QD#236890 © 2015 OSA
Received 24 Mar 2015; revised 14 Jun 2015; accepted 15 Jun 2015; published 17 Jun 2015 29 Jun 2015 | Vol. 23, No. 13 | DOI:10.1364/OE.23.016723 | OPTICS EXPRESS 16727
doped glasses in a way. Wundke et al. have pointed out the luminescence originated from two sources: free excitons and shallow trap states [29–31]. Generally speaking, the latter with a binding energy of about 30 meV have a longer lifetime. And our result is probably ascribed to a combined effect of ET along with multi-channel relaxation of free exciton states and trapped-states. In order to further investigate the ET process between PbS QDs with different sizes, TRES as well as transient-state and steady-state PL spectra of the QD-doped glass heattreated at 600°C were measured. Figure 4(a) depicts the TRES of QD-doped glass heattreated at 600°C in the time scale of 0–25μs. A series of emission spectra at different decay time can be observed clearly from the spectra. The PL spectra appear a peak at 1220nm at beginning, and finally the peak is redshift to 1270nm at 25μs. This continuous redshift about 50nm with decay time is clearly evident as shown in Fig. 4(a). Figure 4(b) depicts the transient-state and steady-state PL spectra of the QD-doped glass heat-treated at 600°C. The PL spectra appear a peak at 1220nm in transient-state and 1270nm in steady-state, respectively. From the previous works [29–31], the different excitation power can lead to the shift of PL, but in this measurement, the fluctuation of excitation power is weak. Therefore, the excitation power effect on the shift of PL is neglected in the following discussion. These results indicate the arrow of ET in the PbS QD-doped glass is from PbS QDs with higher energy to the PbS QDs with lower energy. Since the wavelength of PL peak relates to the size of PbS QDs as shown in Fig. 1(b), it can be deduced that the emission generated from smaller PbS QDs will create electron-hole pairs with smaller bandgap of larger PbS QDs to emit longer wavelength PL. The schematic ET diagram is shown in Fig. 5. Electron energy levels in QDs are not continuous but quantized, and energy gap will increase with the decrease of the size of QDs due to the quantum confined effect. Electron–hole pairs originate from smaller PbS QDs under the excitation of Xeon lamp. The PL with high energy emission through the recombination of these electron–hole pairs may be absorbed by larger PbS QDs to create electron-hole pairs with smaller bandgap. Dantas et al. [36] have reported that the energy intensity generated in the QD-doped glass strongly influenced the ET process. The lower the detection energy, the larger the diffusion length. This behavior results from the ET between different subsets of QDs. It indicates that photons emitted by smaller QDs can only be absorbed by even larger QDs, which means the incident light can only create the electron– hole pairs in larger QDs. Considering the dispersion of QDs with different sizes in the glass matrix, with the ET from smaller PbS QDs, a PL spectrum generated by the majority larger PbS QDs will be observed as shown in Fig. 5. This also explains the unsymmetrical sidebands at measured lifetime spectra in Fig. 3(b). In previous works [29–31], Auxier et al. have proposed that the higher quantum efficiency (QE) trapped-state PL was redshift relative to the free exciton PL, which probably also contribute to the PL redshift in present works. According to the above ET mechanism, when the QDs are on resonance ET, there will be a loss of energy. When the QDs distribution is uneven, the probability of ET will increase, meanwhile more photons will loss and PL intensity weaken. And according to the following equation, 1 (2) k F + ki Where τ is the total lifetime, kF is the radiative decay rate constant, and ki is the nonradiative decay rate constant. The emitting decay rates will increase with the ET leading to the decrease of PL lifetime. Hence, as PL intensity increases, the excitation energy loss reduces by nonradiative process, and the PL lifetime becomes longer, which leads to the similarity of the spectra in Fig. 1(b) and Fig. 3(b).
τ=
#236890 © 2015 OSA
Received 24 Mar 2015; revised 14 Jun 2015; accepted 15 Jun 2015; published 17 Jun 2015 29 Jun 2015 | Vol. 23, No. 13 | DOI:10.1364/OE.23.016723 | OPTICS EXPRESS 16728
Fig. 5. Schematic energy transfer process between PbS QDs with different size.
4. Conclusion In conclusion, PbS QD-doped glass was prepared by the heat-treatment of as-prepared glass. The absorption and PL spectra demonstrated that the optical properties of PbS QD-doped glass can be widely tuned by carefully controlling the size and distribution of PbS QDs in glass. A series of lifetime decay profiles detected at whole PL band were detected in detail, which indicated that the variation tendency of lifetime spectra shows a similar profile with that of PL spectra. More importantly, the steady-state, transient-state PL spectra and TRES results directly revealed the ET process from smaller QDs with higher energy to bigger QDs with lower energy in PbS QD-doped glasses for the first time. Acknowledgments This work was supported by the National Natural Science Foundation of China (61475047, 51102096), the Guangdong Natural Science Foundation for Distinguished Young Scholars (2014A030306045), and the Pearl River S&T Nova Program of Guangzhou (2014J2200083). The authors are grateful to Mr. Weixi Ma for a part of sample preparation.
#236890 © 2015 OSA
Received 24 Mar 2015; revised 14 Jun 2015; accepted 15 Jun 2015; published 17 Jun 2015 29 Jun 2015 | Vol. 23, No. 13 | DOI:10.1364/OE.23.016723 | OPTICS EXPRESS 16729
