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Luminescence of Nd3+ ions under excitation of CdSe quantum dots in a glass system: energy transfer E. O. Serqueira1,* and N. O. Dantas2 1

2

Departamento de Física, Instituto de Ciências Exatas e Naturais e Educação (ICENE), Universidade Federal do Triângulo Mineiro (UFTM), 38025-180, Uberaba MG, Brazil

Laboratório de Novos Materiais Isolantes e Semicondutores-LNMIS, Instituto de Física, Universidade Federal de Uberlândia, 38400-902, Uberlândia, Minas Gerais, Brazil *Corresponding author: [email protected]

Received September 2, 2013; revised November 18, 2013; accepted November 27, 2013; posted December 3, 2013 (Doc. ID 196286); published December 23, 2013 We report rare evidence of energy transfer from CdSe quantum dots (QDs) to Nd3 ions in a SNAB glass system using absorption electronic transitions of ion as a nonresonant excitation source. However, the luminescence band was observed at 880 nm (4 F3∕2 → 4 I9∕2 transition) under 409 nm excitation only when Nd3 ions were embedded in the SNAB glass system with CdSe QDs. This happened because the Nd3 ions absorbed the photons emitted by CdSe QDs with 409 nm excitation. Band overlap of the QD luminescence band and the ion optical absorption bands caused photon absorption and produced valleys in the QD luminescence spectrum. This overlap contributed to reduction in the lifetime 4 F3∕2 state. © 2013 Optical Society of America OCIS codes: (160.2750) Glass and other amorphous materials; (160.4670) Optical materials; (160.4760) Optical properties; (160.5690) Rare-earth-doped materials; (230.5590) Quantum-well, -wire and -dot devices; (160.4236) Nanomaterials. http://dx.doi.org/10.1364/OL.39.000131

Nd3 -doped materials show great efficiency and are therefore strong candidates for photonic devices such as fiber lasers, microchip lasers, and planar waveguides [1–5]. Most recent research has focused on the nearinfrared spectrum (800–1400 nm). In this range, Nd3 ions have radiative emissions at 880, 1060, and 1350 nm, which originate from electronic transitions between 4f N levels (4 F3∕2 → 4 I9∕2 , 4 F3∕2 → 4 I11∕2 and 4F 4 3 ions 3∕2 → I13∕2 ). Glasses are good hosts for Nd because they are mechanically and thermally stable, inexpensive, and possess thermal-conductivity and heat-capacity properties that are compatible with photonic devices. Novel glass compositions with improved spectroscopic and thermal properties are currently under investigation. Given the scientific importance, we studied energy transfer between CdS NCs and Nd3 ions [6] and CdSe NCs and Nd3 ions [7]. We focused on the luminescence (4 F3∕2 → 4 I9∕2 ) of these ions in an environment rich in CdSe quantum dots and using a nonresonant excitation source: the absorption transitions of Nd3 ions. According to quantum mechanics, an electronic transition between two states can be favored when the excitation photon energies of these states are exactly the same [8,9]. This is possible given that the transition probability of a level-two system can be found using the time-dependent perturbation theory [8,10]. The literature reports that resonant excitation favors electronic transition luminescence of rare Earth ions (RE3 ) [11]. However, studies report that RE3 ion luminescence can result of from nonresonant excitation via virtual Auger processes [12]. Others show results where the indirect excitation increased the luminescence of ZnO thin films doped with Eu3 ions at temperature low (85 K). This increase in Eu3 luminescence was attributed to energy transfer from the ZnO film to the ions [13]. The current study presents experimental evidence of luminescence under nonresonant excitation of Nd3 ions embedded in a SNAB glass system nanostructured with CdSe QDs. 0146-9592/14/010131-04$15.00/0

Samples were synthesized by the fusion method at 1300°C for 15 min. The SNAB glass system is well known [6,7,14–16] and the data composition is 40SiO2 , 30Na2 CO3 , 1Al2 O3 , and 29B2 O3 (mol. %). The mass of the dopant Nd2 O3 and CdSe corresponds to 2% in the weight of glass SNAB. Optical absorption (OA) spectra were obtained using a SHIMATZU 3600 spectrometer, and luminescence spectra were excited by a diode laser (λex:  409 nm) and an argon laser (Ar ) and collected using a CCD JAZ (350 to 1050 nm). Lifetime measurements were performed using the time-resolved luminescence (TRL) technique. TRL measurements were recorded with a digital phosphor oscilloscope (Tektronix DPO 2012 100 MHz − 1 GS∕s) and detected with a Si PIN photodiode (photoconductive mode, 200 to 1100 nm) and a filter. All measurements were taken at room temperature. Figure 1 shows the luminescence and OA spectra of the Nd3 ions embedded in the SNAB glass system. Figure 1(a) shows the luminescence spectra of the Nd3 ions with different excitation sources. The band centered at approximately 882 nm represents the electronic transition from the 4 F3∕2 state to the fundamental 4 I9∕2 state of the Nd3 ions. Clearly, the luminescence of this transition is strongly dependent on excitation source. Note that there is no luminescence at 882 nm with 409 nm excitation and, consequently, there were no photon emissions in the 4 F3∕2 → 4 I9∕2 transition of the Nd3 ions. This happens because 409 nm excitation does not coincide with an Nd3 absorption band. Thus there is no luminescence because quantum mechanics prohibit excitation photon absorption. Note that the energy of the excitation source is slightly above the 2 P1∕2 state of the Nd3 ions and that this state corresponds to the band centered at 432 nm (4 I9∕2 → 2 P1∕2 ) [Figs. 1(a) and 1(b)]. The absorption energy of this band is much lower than in other bands, indicating low-absorption probability. Also note that the 4 I9∕2 → 2 P1∕2 transition © 2014 Optical Society of America

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Fig. 1. (a) Luminescence and (b) OA spectra of Nd3 ions embedded in the SNAB system. The electronic transition luminescence is strongly dependent on excitation source. Note that luminescence is not produced from the 409 nm.

is prohibited by the Judd–Ofelt selection rule [17–21] (electric dipole transition: ΔJ ≤ 6; ΔL ≤ 6; ΔS  0, magnetic dipole transition: ΔJ  0, 1; ΔL  0; ΔS  0). There is a wide band in the luminescence spectrum (409 nm excitation) associated with the host glass matrix [Fig. 1(a)]. This band overlaps the absorption bands of the Nd3 ions but does not contribute the energy transfer to the ions because the 882 nm luminescence of the 4 F3∕2 → 4 I9∕2 electronic transition does not occur. The results observed for the Nd3 ions embedded in the SNAB glass system demonstrate that the probability is very low that photons are absorbed from the 409 nm excitation source. Figure 2 shows the OA and luminescence spectra of CdSe QDs in the SNAB matrix [SNAB+2CdSe (wt. %)], heat-treated at 560°C for 0, 1, 2, 4, 10, 24, and 36 h. Figure 2(a) and 2(c) show that the OA and luminescence bands redshift with longer heat treatment. Therefore, heat treatment contributes to CdSe QD growth in the SNAB glass system. These quantum dots not only grow, but also develop quantum confinement properties because of heat treatment [22]. The average sizes of the QDs were calculated based on the effective-mass approximation theory and are shown in Fig. 2(b) as a function of heat-treatment time [16,22]. Well-defined QD growth kinetics can be seen as heat treatment lengthens. These results (Figs. 1 and 2) show that the SNAB glass system can be used for Nd3 doping and growth of CdSe QDs, which can be used to study energy transfer from QDs to RE3 ions under indirect excitation. 409 nm excitation from a diode laser can be used to study energy transfer from CdSe QDs to Nd3 ions. Samples with Nd3 ions and grown CdSe QDs were synthesized and heat-treated in the same way as the samples in Figs. 1 and 2. Figure 3 shows the OA and luminescence spectra of the CdSe QDs and Nd3 ions in the SNAB glass system fSNAB  2CdSe  Nd2 O3  wt: %g heat treated at 560°C for 0, 1, 2, 10, 24, and 36 h. Both the QD and Nd3 OA spectra indicate that CdSe QDs and Nd3 ions are embedded in the SNAB glass system. The weak intensity

Fig. 2. (a) OA spectra of CdSe QDs in the SNAB+2CdSe (wt. %) glass system heat-treated at 560°C for 0 at 36 hrs. (b) QD size relative to heat treatment. (c) Luminescence spectra of CdSe QDs in this glass system heat-treated at 560° C for 0 at 36 h.

and sharp absorption lines indicate single-site doping in the SNAB matrix but not the incorporation of Nd3 ions into the dot structure [6]. Here a wide band can be seen, which is associated with the great variation in the average sizes of the CdSe QDs [Fig. 3(a)]. Similar variation was not seen in the samples of Fig. 2, which indicates that RE3 ions influence the size dispersion of QDs grown in glass systems [16]. The OA bands of the Nd3 ions do not vary with heattreatment time and QD growth. This is strong evidence of the permanence of the ions in the glass phase [6,7,16]. Figure 3(b) shows the luminescence bands of the CdSe QDs and the Nd3 ions in the SNAB glass system under 409 nm excitation. The wide luminescence band associated with QDs has valleys [6,7] that coincide with the OA bands of the Nd3 ions. This clearly shows that the Nd3 ions are absorbing photons emitted from the CdSe QDs under 409 nm excitation. This band redshifts with QD growth, which was also observed in Fig. 2(c). The redshift increased the overlap of the wide luminescence band of the CdSe QDs with the absorption bands of the Nd3 ions. Note that greater overlap increased the valleys, indicating increased photon absorption by Nd3 ions embedded in the SNAB glass system. This

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Fig. 4. Lifetime of the 4 F3∕2 state of the Nd3 ions embedded in the SNAB glass system with CdSe QDs fSNAB  2CdSe  Nd2 O3  wt: %g heat-treated at 560°C for 0, 1, 2, 10, 24, and 36 h.

Fig. 3. (a) OA and (b) luminescence of CdSe QDs and Nd3 ions in the SNAB fSNAB  2CdSe  Nd2 O3  wt: %g glass system heat-treated at 560°C for 0, 1, 2, 10, 24, and 36 h.

process clearly demonstrates energy transfer from CdSe QDs to Nd3 ions. The luminescence band centered at approximately 882 nm (4 F3∕2 → 4 I9∕2 ) from the Nd3 ions under 409 nm excitation provides additional clear evidence of energy transfer. As also observed in Fig. 1, the Nd3 ions do not produce a luminescence band under this excitation source. Figure 4 shows the lifetime of the 4 F3∕2 state of the Nd3 ions embedded in the SNAB glass system with CdSe QDs (SNAB  2CdSe  Nd2 O3  wt: %) and heattreated at 560°C for 0, 1, 2, 4, 10, 24, and 36 h. Here, lifetime decreases sharply as the size of CdSe QDs increases. This decrease is due to the overlapping of the CdSe QD luminescence bands with the OA bands of the Nd3 ions. The OA bands are fixed, and the luminescence of the QDs can shift with changes in the average size of the QDs, which can, in turn, be controlled by the heat-treatment time. This provides strong evidence of energy transfer from CdSe QDs to Nd3 ions in the SNAB glass matrix using a nonresonant excitation source. The lifetime of RE3 ions embedded in a host material using a direct excitation source is generally longer than under indirect excitation [13]. The current study shows that lifetime decreases when luminescence bands overlap OA bands. This indicates that the radiative decay of electronic transition luminescence of Nd3 ions is stimulated [4] by photon emissions from CdSe QDs, which reduces the lifetime of the 4 F3∕2 state of the Nd3 ions (Fig. 4).

Our results show that Nd3 ions remain in the glass system during heat treatment. We also found rare evidence of energy transfer from CdSe QDs to Nd3 ions by observing the 4 F3∕2 → 4 I9∕2 luminescence band under nonresonant excitation. Finally, we have demonstrated that the lifetime of the 4 F3∕2 state of the neodymium ions decreased as the CdSe QD luminescence bands became resonant with the 4 F5∕2  2 H9∕2 and 4 F3∕2 states. This favors electron pumping in the absorption transition as well as in the decay stimulated by the 4 F3∕2 state and allows the luminescence transition to appear, producing the band at approximately 882 nm and decreasing lifetime. We believe that these results will inspire further investigation into similar systems with potential device or laser applications. The authors acknowledge financial support from the Brazilian agencies CAPES, CNPq, and FAPEMIG. References 1. F. Chen, Y. Tan, D. Jaque, L. Wang, X.-L. Wang, and K.-M. Wang, Appl. Phys. Lett. 92, 021110 (2008). 2. P. Molina, D. Sarkar, M. O. Ramirez, J. G. Sole, L. E. Bausa, B. J. Garcia, and J. E. M. Santiuste, Appl. Phys. Lett. 90, 141901 (2007). 3. A. Jha, B. Richards, G. Jose, T. Teddy-Fernandez, P. Joshi, X. Jiang, and J. Lousteau, Prog. Mat. Sci. 57, 1426 (2012). 4. D. Biggemann and L. R. Tessler, Opt. Mater. 27, 773 (2005). 5. H. N. Zhang, X. H. Chen, Q. P. Wang, X. Y. Zhang, J. Chang, L. Gao, H. B. Shen, Z. H. Cong, Z. J. Liu, X. T. Tao, and P. Li, Opt. Lett. 38, 3075 (2013). 6. N. O. Dantas, E. O. Serqueira, A. P. Carmo, M. J. V. Bell, V. Anjos, and G. E. Marques, Opt. Lett. 35, 1329 (2010). 7. N. O. Dantas, E. O. Serqueira, V. Anjos, and M. J. V. Bell, Appl. Phys. Lett. 101, 121903 (2012). 8. A. Yariv, Quantum Electronics (Wiley, 1975). 9. D. J. Griffiths, Introduction To Quantum Mechanics, 2nd ed. (Pearson Education, 2005). 10. N. Dudovich, D. Oron, and Y. Silberberg, Phys. Rev. Lett. 88, 123004 (2002). 11. T. Kushida, E. Takushi, and Y. Oka, J. Lumin. 12–13, 723 (1976).

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12. I. N. Yassievich, Opt. Mater. 33, 1079 (2011). 13. Y. Terai, K. Yamaoka, K. Yoshida, T. Tsuji, and Y. Fujiwara, Physica E 42, 2834 (2010). 14. E. O. Serqueira, N. O. Dantas, and M. J. V. Bell, Chem. Phys. Lett. 508, 125 (2011). 15. E. O. Serqueira, N. O. Dantas, G. H. Silva, V. Anjos, M. J. V. Bell, and M. A. Pereira-da-Silva, Chem. Phys. Lett. 504, 67 (2011). 16. E. O. Serqueira, N. O. Dantas, V. Anjos, M. A. Pereirada-Silva, and M. J. V. Bell, J. Lumin. 131, 1401 (2011).

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Luminescence of Nd3+ ions under excitation of CdSe quantum dots in a glass system: energy transfer.

We report rare evidence of energy transfer from CdSe quantum dots (QDs) to Nd3+ ions in a SNAB glass system using absorption electronic transitions of...
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