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Tb co-doped films
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 235 (2020) 118260
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Dual coupled effects of low concentration gold nanorods on energy transfer and luminescence enhancement in Eu/Tb co-doped films Qingru Wang a,b,⁎, Jinhua Liu a, Kewei Huang a, Qingchao Chen a, Haochuan Dong a, Dong Zhang a, Qiang Shi a, Shuhong Li a,⁎⁎, Wenjun Wang a a b
School of Physical Science and Information Technology, Shandong Provincial Key Laboratory of Optical Communication Science and Technology, Liaocheng University, Liaocheng 252059, China Key Laboratory of Flexible Electronics & Institute of Advanced Materials, Nanjing Tech University, 30 South Puzhu Road, Nanjing 211816, China
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Article history: Received 28 October 2019 Received in revised form 13 March 2020 Accepted 14 March 2020 Available online xxxx Keywords: Eu Tb Plasmonic nanorods Energy transfer Enhanced luminescence
a b s t r a c t Eu/Tb co-doped films with low concentration gold nanorods have been prepared using the solution process. The luminescence spectra investigations indicate that the introduction of nanorods can effectively enhance the energy transfer from Tb to Eu under excitation of 292 nm, because of the plasmonic coupling with excited Tb complex. Under excitation of 360 nm, the emission at 612 nm is enhanced, the enhancement factor increases and then decreases as the molar ratio of Tb and Eu increases. The luminescence enhancement is attributed to the metal enhanced luminescence resulting from plasmonic coupling with excited Eu complex. The dual effects of LSPR on energy transfer and emission enhancement are both observed. More details on the luminescence of Eu/Tb co-doped films with nanorods are demonstrated, which gain a deeper understanding of the interactions luminescent-particle and luminescent-luminescent. © 2018 Published by Elsevier B.V.
1. Introduction The plasmonic metal nanoparticles with strong localized surface plasmon resonances (LSPRs) can serve as unique platforms of plasmon enhanced luminescence. They have extensively attracted great interest because of their applications in luminescence materials and devices [1–3]. Luminescence enhancement can be achieved by matching the LSPR wavelength with the excitation wavelength and by increasing the radiative decay rate of the emitters (Purcell effect). In addition, plasmonic nanoparticles are also capable of changing the energy transfer between two close molecules [4–6]. Despite the improved optical properties, the potential of metal enhanced luminescence (MEL) has not yet fully materialized due to the complexity of the photoluminescence process and the challenges of substrate fabrication [7]. Great efforts have been devoted to searching for further enhanced luminescence efficiency by using LSPR. Luminescent lanthanide compounds have been widely used in various fields, such as fluorescence materials [8–10], luminescence devices [11–13], fluorescence probes and labels in biological systems [14,15] due to their luminescence properties (wide spectral
⁎ Correspondence to: Q. Wang, School of Physical Science and Information Technology, Shandong Provinical Key Laboratory of Optical Communication Science and Technology, Liaocheng University, Liaocheng 252059, China. ⁎⁎ Corresponding author. E-mail address: [email protected] (Q. Wang).
https://doi.org/10.1016/j.saa.2020.118260 1386-1425/© 2018 Published by Elsevier B.V.
range, intense and sharp bands, and long lifetime). However, further development and applications of lanthanide compounds are greatly constrained by their underlying shortcoming such as significant non-radiative processes, low quantum efficiency, and white light generation displays. Recently, MEL has been explored for the enhancement of luminescence properties of lanthanide complex [15–19]. There is rarely report about the interaction and the energy transfer mechanism between two or more lanthanide compounds in presence of plasmonic nanoparticles. Of particular note is that the effect of surface plasmons on the energy transfer has been decoupled from the aforementioned enhancement in emission processes [20]. Many details about the mechanism of MEL in the system with two or more lanthanide compounds are still unclear at present. We have reported the influences of metal nanoparticles with high concentration on the emission of lanthanide complexes [18,19], however it is hard to determine the effect of nanoparticles on the energy transfer between lanthanide ions in this case. Moreover, the distribution of high concentration nanoparticles is usually not uniform which limit their applications in many devices. As a continual effort, in order to further gain the effects of plasmon nanoparticles on the systems with two lanthanide compounds and apply the samples in devices, we introduce a simple and scalable method to obtain the nearly uniform and large area gold nanorods, and then research the details about physical mechanism of effect of nanorods on the Eu\\Tb co-doped samples.
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2. Methods 2.1. Preparation of gold nanorods The gold nanorods were synthesized by using the method mentioned in Ref. [21, 22]. Firstly, the seed solution was prepared. 10 ml of 100 mM hexadecyltrimethyl ammonium bromide (CTAB) solution was mixed with 0.25 ml of 10 mM coloroarric acid (HAuCl4) and stirred vigorously, 0.6 ml of 10 mM ice-cold NaBH4 was added. Vigorous stirring of the seed solution was continued for 2 min. After the solution was stirred, it was kept at 25 °C. Then the growth solution was prepared. 50 ml 100 mM CTAB solution was added to 0.2 ml 10 mM AgNO3 solution. 2.5 ml 10 nM HAuCl4 was added to this solution. 0.3 ml 10 mM freshly prepared 100 mM ascorbic acid solution was added. The final step was the adding of 12 μl seed solution and the mixed solution was gently mixed. The solution was allowed to sit overnight to let the nanorods grow. The nanorods were purified via centrifugation then resuspended in deionized water. To get the well-distributed nanorods, the silicon wafers were sequentially pretreated by acetone, isopropanol, and deionized water in an ultrasonic bath for 15 min and further disposed for 15 min by plasma cleaner PDC-002. Then the nanorods solutions were drop-casted onto the cleaned silicon wafers. 2.2. Preparation of Tb(TMHD)3 and Eu(dbm)3phen co-doped PMMA films Tb(TMHD)3 and Eu(dbm)3phen were purchased from TCI and Alfa Aesar separately. Tb(TMHD)3, Eu(dbm)3phen and PMMA were dissolved in chloroform follow by spin-coating onto the pre-treated silica substrates. The weight ratio of Tb(TMHD)3, PMMA and chloroform was 0.01:1:200. The concentration of Eu(dbm)3phen was changed and the molar ratio of Tb(TMHD)3 and Eu(dbm)3phen was increased from 1:0 to 1:1, we choose six different molar ratios of 1:0.0, 1:0.2, 1:0.4, 1:0.6, 1:0.8 and 1:1.0 in our samples. Then the solution was stirred for about 2 h at ambient temperature. 50 μl of the prepared Eu/Tb codoped PMMA solution was spin-coated at 3000 rpm for 60 s on the nanorods covered wafer and then baked on hot plate for 30 min. For the ease of comparison, all procedures and parameters were optimized and kept constant throughout the experiment. 2.3. Characterization and PL measurement Scanning Electron Microscope (SEM) images of gold nanorods were taken on a ZISS sigma 300 electron microscope. The UV–vis–NIR absorption spectra were recorded on a HTACHI U-3310 spectrometer. The photoluminescence spectra and the luminescence decay curves of samples were measured by a Edinburgh FLS920 spectrophotometer system. The luminescence decays were measured by using the time-correlated single photon counting technique. 3. Results and discussion Before exploiting LSPR effects on the Tb\\Eu co-doped films, we first study the optical properties of Tb\\Eu co-doped films in absence of nanorods. Fig. 1 shows the absorption of Tb, Eu and Tb\\Eu co-doped films, separately. As shown in Fig. 1, Tb(TMHD)3 shows a absorption band between 250 nm–300 nm which comes from the absorption of ligand (TMHD), the peak locates at about 276 nm. Eu(dbm)3phen shows two absorption bands between 250–280 nm and 300 nm–380 nm, which separately come from the π–π* transitions of ligands (Phen) and (dbm), the peaks locate at 265 nm and 343 nm. The absorption at 343 nm is higher than that of 265 nm. The Eu\\Tb co-doped films shows the absorption both of Tb(TMHD)3 and Eu(dbm)3phen. Then, we study the excitation and emission of Eu, Tb and Eu/Tb codoped films. The excitation and emission of pure Eu and Tb are shown in Fig. S1 in Supporting Information. For Tb(TMHD)3, the excitation
Fig. 1. The UV–Vis absorption spectra of Tb(TMHD)3, Eu(dbm)3phen, and Tb(TMHD)3-Eu (dbm)3phen co-doped films.
peak at 292 nm corresponds to the π–π* transitions. For Eu(dbm) the excitation peak at 360 nm corresponds to the π–π* transitions of ligand dbm. The photoluminescence (PL) spectrum of pure Tb complex shows four typical emission peaks at 490 nm, 545 nm, 580 nm and 620 nm with 292 nm excitation. The PL spectrum of Eu shows emission peaks at 582 nm, 590 nm and 612 nm, respectively, under excitation of 360 nm. Fig. 2 shows the excitation and emission spectra of Tb\\Eu co-doped films. Fig. 2(a) shows the excitation spectra when the monitoring wavelength is 545 nm. The peak at 292 nm corresponds to the π–π* transition, which is consistent with the absorption spectra in Fig. 1. The excitation is decreased with the increasing of Eu. Fig. 2(b) shows the corresponding emission spectra under excitation of 292 nm. As shown in Fig. 2(b), the emissions of Tb and Eu complexes are both observed. With increasing of Eu complex, the emission of Tb is decreased and the emission of Eu complex is increased. The increased Eu emission and decreased Tb emission demonstrate the energy transfer from Tb to Eu [19,23]. The emission at 612 nm increases slightly and the spectral profiles changes sharply when the molar ratio is from 1:0.4 to 1:0.8. The 5D0-7F2 transition of Eu centered at 612 nm is overlapping with the 5D4-7F3 transition of Tb at 620 nm, both Eu and Tb contribute to the luminescence at this band which induces the slightly enhancement at 612 nm when the molar ratio is from 1:0.4 to 1:0.8. Fig. 2(c) shows the excitation spectra when the monitoring wavelength is 612 nm. The peak at 360 nm also comes from the π–π* transition of dbm. The excitation is increased with the increasing of Eu. Fig. 2 (d) shows the corresponding emission spectra of the Tb\\Eu co-doped films under excitation of 360 nm. As shown in Fig. 2(d), the PL intensity is increased with the increasing of Eu complex, the emission peak at 545 nm is very weak because there is no absorption of Tb complex at 360 nm. The decay dynamics of samples are also measured under the excitations of 292 nm and 360 nm respectively. The analysis of decay times was carried out using the FLS-920 fluorescence spectrometer. The values of decay times are presented in Table S1 in Supporting Information. The χ2 value indicates how well the fit is (1 indicates a best fit). Fig. 3 shows the decay times of 545 and 612 nm for Tb\\Eu co-doped samples. As shown in Fig. 3(a), in absence of Eu, the profile at 545 nm follows second order exponential nature. In presence of Eu, the profiles at 545 nm follow three order exponential nature. There is a fast-initial component observed in the decay curves, which is attributed to the non-radiative energy transfer from Tb to Eu with the increasing of Eu. The profiles at 612 nm under the excitation of 292 nm and 360 nm, as shown in Fig. 3(b) and (c), follow second order exponential nature because there are two different emitting sites in the mixed systems [19]. The complexes were doped in PMMA films, which result in that the Eu 3phen,
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Fig. 2. Excitation and emission spectra of Tb\ \Eu co-doped films. (a) The excitation when the monitoring wavelength is 545 nm, (b) the emission spectra under the excitation of 292 nm, (c) the excitation spectra when the monitoring wavelength is 612 nm, (d) the emission spectra under the excitation of 360 nm.
and Tb ions located in more than one non-equivalent site, so the luminescence of Tb and Eu are not one exponential [19]. Next, we focus on the LSPR effects of gold nanorods on the luminescence properties of Tb\\Eu co-doped films. Here, gold nanorods were selected because they have two plasmon resonance bands in the visible region, which near to the emissions of Tb complex and Eu complex. In addition, gold nanorods typically have strong LSPR strength and
scattering property. The resonant absorption spectrum of nanorods we used is shown in Fig. 4(a), inset shows the size and morphology of nanorods. The size of nanorods is about ~50 nm. The gold nanorods exhibit two plasmon resonance bands at 519 nm and 663 nm corresponding to the transverse surface plasmon resonance (T-LSPR) and longitudinal surface plasmon resonance (L-LSPR) [24]. Fig. 4 (b) demonstrates the distribution of nanorods on substrate. As shown
Fig. 3. PL decay profiles of Tb\ \Eu co-doped films. (a) The decay profiles at 545 nm under the excitation of 292 nm, (b) the decay profiles at 612 nm under the excitation of 292 nm and (c) the decay profiles at 612 nm under the excitation of 360 nm.
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Fig. 4. SEM images and the plasmon absorption bands of nanorods. (a) The absorption of nanorods, inset shows the size and shape of nanorods. (b) The distribution of nanorods on the substrate.
in Fig. 4(b), the distribution is low density and almost uniform over a large area. The effects of nanorods on the emissions of Tb\\Eu co-doped systems are observed from detail characterizations. Here, we elaborate the effects on the luminescence of sample with the molar ratio 1:0.6. The results of the other four molar ratios also demonstrate the similar change. Fig. 5 shows the corresponding spectra when the molar ratio of Tb and Eu is 1:0.6. Fig. 5(a) shows the excitation spectra when the monitoring wavelength is 545 nm, the excitation peak is located at 292 nm. The excitation intensity is decreased in presence of nanorods. Fig. 5(b) shows the corresponding PL spectra under the excitation of 292 nm. As compared to the case without nanorods, the emission at 545 nm is further decreased and the emission at 612 nm is further increased in presence of nanorods. The intensity of emission at 490 nm is also slightly reduced. It suggests that the gold nanorods enhance the energy transfer from Tb to Eu. Under the excitation of 292 nm, the Tb complex is excited. The excited Tb would interact with T-LSPR of gold nanorods because the T-LSPR of nanorods is close to the emission of Tb at 545 nm, which result in the alteration of their relaxation process [1,3,25,26]. As we know, when the plasmon peak is located in between the intrinsic emission peak of Tb complex and the absorption peak of Eu complex according to the previous theoretical investigation, the energy transfer from Tb to Eu would be enhanced [20,27]. The energy transfer efficiency (ETE) between Tb and Eu can be calculated by following formula [28]: Is η ¼ 1− I so where Iso is the emission intensity of Tb in the absence of Eu, Is is the emission intensity of Tb in the presence of Eu. In presence of nanorods, the energy transfer rate will be modulated and the energy transfer efficiency η will thereafter be altered. The emission intensity ratio of Tb to Eu can be modulated by the LSPR through changing the value of ETE [20]. To examine the effect of the plasmon on the value of ETE from Tb to Eu by analyzing the emission spectra, the calculated ETE is shown in Fig. 5(c). In absence of nanorods, the ETE increases from 42% to 92% with increasing of Eu complex. The ETE depends on the distance of two molecules. The distance between Tb ions and Eu ions decreases with increasing of Eu complex, which results in the increase of the ETE. In presence of nanorods, the ETE is enhanced and it increased from 57% to 96% with the increasing of Eu. And the ETE enhancement at low molar ratio is greater than that at high molar ratio. In parallel, Fig. 5(d) shows the excitation spectra when the monitoring wavelength is 612 nm, the peak is also 360 nm. The excitation intensity is increased in presence of nanorods. Under excitation of 360 nm, the Eu complex is excited. The L-LSPR of nanorods is closed to the
emissions of Eu complex. Therefore, the excited Eu complex could interact with gold nanorods, which alter the relaxation process of Eu. Fig. 5 (e) shows the PL spectra when the molar ratio of Tb and Eu is 1:0.6. As shown in Fig. 5(e), the emission at 612 nm is enhanced, the enhancement factor is about 2.0. For other four molar ratios, the emission at 612 nm also be enhanced. The corresponding enhancement factor at different molar ratios is shown in Fig. 5(f). As shown, the enhancement factor increases and then decreases again with the increasing of Eu complex. The results show that the luminescence enhancement factor strongly relates to the composition of the lanthanide complexes. The overall enhancement factor of 612 nm under excitation of 360 nm is greater than that at 292 nm. The excitation efficiency of Eu complex under excitation of 360 nm is greater than that at 292 nm. More excited Eu complex could interact with LSPR of Au nanorods, which induces the greater enhancement factor under excitation of 360 nm. To better understand the effects of nanorods on the Tb\\Eu codoped films, the PL decays of complexes in presence of nanorods were also studied. Fig. S2 in Supporting Information shows the PL decays of samples in presence of nanorods. The decay times of samples with nanorods is shown in Table S2 in Supporting Information. Compared with the pure complexes, Fig. 6 show the PL decays with and without nanorods when the molar ratio of Tb and Eu is 1:0.6. As shown in Fig. 6, the decay time is decreased for the main two emissions of 544 nm and 612 nm in presence of nanorods. Under excitation of 292 nm, the decay time of 545 nm is decreased from 1157 μs to 1089 μs, the decay time of 612 nm is decreased from 787 μs to 738 μs. Under excitation of 360 nm, the decay time at 612 nm is decreased from 611 μs to 578 μs. As we known, the luminescence intensity I and the lifetime τ of organic materials in a free space can be defined as I ∝ I0ελclrrτ, τ 1 [19], where rr is the radiative decay rate and rnr is the r r þ r nr nonradiative decay rate, I0 is the intensity of the excitation wavelength, ελ is the dielectric constant at the emission wavelength, c is the velocity of light, l is the thickness of sample. For Tb\\Eu co-doped films, c and l are constant. The effects of plasmonic nanoparticles on the luminescent are determined by the relative extent of three aspects, excitation enhancement which is induced by the increased light absorption, emission enhancement which is induced by increased radiative decay and quenching which is induced by increased non-radiative decay [29]. In this paper, the LSPR bands of nanorods mainly influence the emission process rather than excitation process. The shortened decay in presence of nanorods indicates that either the radiative decay rate rr or the nonradiative decay rate rnr was increased. Under the excitation of 292 nm, we observed luminescence quenching at 545 nm and enhancement at 612 nm. Hence, the decreased decay at 545 nm can be explained ¼
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Fig. 5. The optical properties of Tb\ \Eu co-doped films in presence of nanorods. (a) The excitation spectra when the monitoring wavelength is 545 nm (b) The corresponding emission spectra when the molar ratio of Tb and Eu is 1:0.6, and (c) The ETE at different molar ratio of Tb and Eu; (d) The excitation spectra when the monitoring wavelength is 612 nm, (e) The corresponding emission spectra when the molar ratio of Tb and Eu is 1:0.6, and (f) the enhancement factor at different molar ratio of Tb and Eu.
as an increase in the nonradiative decay rate and the decreased decay at 612 nm is induced by an increase in the radiative decay rate, which demonstrates the energy transfer from Tb to Eu is enhanced. The mechanism is shown in Fig. 7. Under the excitation of 292 nm, the Tb complex is excited and the energy transfer from Tb to Eu occurs. The T-LSPR close to the emission band of Tb complex, so the nanorods mainly interact with the excited Tb complex. The plasmonic coupling with excited Tb complex could be strong in such a way that the nonradiative decay rates would become faster. Therefore, the energy transfer efficiency is
enhanced in presence of nanorods. And the increased nonradiative decay rate induces the decreased emission and the decreased decay time at 545 nm. Meanwhile, for the emission at 612 nm, the increased energy transfer further enhances the radiative decay rate and luminescence intensity. Under excitation of 360 nm, only the Eu complex is excited, the nanorods interact with more excited Eu due to the good match between L-LSPR band of nanorods with the emission band of Eu complex, which induces an increase in the radiative decay rate at 612 nm thus the luminescence is enhanced and the decay time is decreased.
Fig. 6. The PL decay profiles of Tb\ \Eu co-doped films with nanorods when the molar ratio of Tb and Eu is 1:0.6. (a) The decay profiles at 545 nm under excitation of 292 nm, (b) The decay profiles at 612 nm under excitation of 292 nm and (c) The decay profiles at 612 nm under excitation of 360 nm.
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Shandong Province (Grant No. 2019KJJ019); Introduction and Cultivation Plan of Youth Innovation Talents for Universities of Shandong Province; and The Special Construction Project Fund of Shandong Province Taishan Scholars. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.saa.2020.118260. References
Fig. 7. Schematic of the energy transfer and emission processes of samples in presence of gold nanorods with low concentration.
4. Conclusions In this study, we investigated the effects of plasmon nanoparticles on Tb\\Eu co-doped films. By analyzing the steady state and time resolved luminescence, we discuss the interaction among Tb, Eu and gold nanorods. Under excitation of 292 nm, the emission at 545 nm is decreased and the emission at 612 nm is increased. The luminescence and decay times investigations reveal that the energy transfer efficiency from Tb to Eu is enhanced in presence of nanorods. Under excitation of 360 nm, the emission at 612 nm is enhanced because of the direct interaction between nanorods and excited Eu, the enhancement is related to the molar ratio of Tb and Eu. The dual effects of LSPR on energy transfer and emission enhancement are both observed. More details on the two or more lanthanide doped samples with nanorods are further demonstrated, which would give us a deeper understanding on interactions luminescent-particle and luminescent-luminescent. CRediT authorship contribution statement Qingru Wang: Conceptualization, Methodology, Investigation, Writing - original draft, Writing - review & editing. Jinhua Liu: Validation, Formal analysis, Visualization, Writing - review & editing. Kewei Huang: Validation, Formal analysis, Visualization, Data curation. Qingchao Chen: Validation, Formal analysis, Visualization, Data curation. Haochuan Dong: Validation, Formal analysis, Visualization, Data curation. Dong Zhang: Writing - review & editing. Qiang Shi: Writing - review & editing. Shuhong Li: Resources, Writing - review & editing, Supervision, Data curation. Wenjun Wang: Resources, Writing - review & editing, Supervision, Project administration, Funding acquisition. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement Shandong Provincial Natural Science Foundation (ZR2019MF068, ZR2018MA039); National Natural Science Foundation Program of China (NSFC) (61775089 and 11504390); Shandong Province Higher Educational Science and Technology Program (J17KA175); Alliance Fund of Shandong Provincial Key Laboratory (SDKL20016038); Science and Technology Plan of Youth Innovation Team for Universities of
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Dual coupled effects of low concentration gold nanorods on energy transfer and luminescence enhancement in Eu/Tb co-doped films Qingru Wang a,b,⁎, Jinhua Liu a, Kewei Huang a, Qingchao Chen a, Haochuan Dong a, Dong Zhang a, Qiang Shi a, Shuhong Li a,⁎⁎, Wenjun Wang a a b
School of Physical Science and Information Technology, Shandong Provincial Key Laboratory of Optical Communication Science and Technology, Liaocheng University, Liaocheng 252059, China Key Laboratory of Flexible Electronics & Institute of Advanced Materials, Nanjing Tech University, 30 South Puzhu Road, Nanjing 211816, China
a r t i c l e
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Article history: Received 28 October 2019 Received in revised form 13 March 2020 Accepted 14 March 2020 Available online xxxx Keywords: Eu Tb Plasmonic nanorods Energy transfer Enhanced luminescence
a b s t r a c t Eu/Tb co-doped films with low concentration gold nanorods have been prepared using the solution process. The luminescence spectra investigations indicate that the introduction of nanorods can effectively enhance the energy transfer from Tb to Eu under excitation of 292 nm, because of the plasmonic coupling with excited Tb complex. Under excitation of 360 nm, the emission at 612 nm is enhanced, the enhancement factor increases and then decreases as the molar ratio of Tb and Eu increases. The luminescence enhancement is attributed to the metal enhanced luminescence resulting from plasmonic coupling with excited Eu complex. The dual effects of LSPR on energy transfer and emission enhancement are both observed. More details on the luminescence of Eu/Tb co-doped films with nanorods are demonstrated, which gain a deeper understanding of the interactions luminescent-particle and luminescent-luminescent. © 2018 Published by Elsevier B.V.
1. Introduction The plasmonic metal nanoparticles with strong localized surface plasmon resonances (LSPRs) can serve as unique platforms of plasmon enhanced luminescence. They have extensively attracted great interest because of their applications in luminescence materials and devices [1–3]. Luminescence enhancement can be achieved by matching the LSPR wavelength with the excitation wavelength and by increasing the radiative decay rate of the emitters (Purcell effect). In addition, plasmonic nanoparticles are also capable of changing the energy transfer between two close molecules [4–6]. Despite the improved optical properties, the potential of metal enhanced luminescence (MEL) has not yet fully materialized due to the complexity of the photoluminescence process and the challenges of substrate fabrication [7]. Great efforts have been devoted to searching for further enhanced luminescence efficiency by using LSPR. Luminescent lanthanide compounds have been widely used in various fields, such as fluorescence materials [8–10], luminescence devices [11–13], fluorescence probes and labels in biological systems [14,15] due to their luminescence properties (wide spectral
⁎ Correspondence to: Q. Wang, School of Physical Science and Information Technology, Shandong Provinical Key Laboratory of Optical Communication Science and Technology, Liaocheng University, Liaocheng 252059, China. ⁎⁎ Corresponding author. E-mail address: [email protected] (Q. Wang).
https://doi.org/10.1016/j.saa.2020.118260 1386-1425/© 2018 Published by Elsevier B.V.
range, intense and sharp bands, and long lifetime). However, further development and applications of lanthanide compounds are greatly constrained by their underlying shortcoming such as significant non-radiative processes, low quantum efficiency, and white light generation displays. Recently, MEL has been explored for the enhancement of luminescence properties of lanthanide complex [15–19]. There is rarely report about the interaction and the energy transfer mechanism between two or more lanthanide compounds in presence of plasmonic nanoparticles. Of particular note is that the effect of surface plasmons on the energy transfer has been decoupled from the aforementioned enhancement in emission processes [20]. Many details about the mechanism of MEL in the system with two or more lanthanide compounds are still unclear at present. We have reported the influences of metal nanoparticles with high concentration on the emission of lanthanide complexes [18,19], however it is hard to determine the effect of nanoparticles on the energy transfer between lanthanide ions in this case. Moreover, the distribution of high concentration nanoparticles is usually not uniform which limit their applications in many devices. As a continual effort, in order to further gain the effects of plasmon nanoparticles on the systems with two lanthanide compounds and apply the samples in devices, we introduce a simple and scalable method to obtain the nearly uniform and large area gold nanorods, and then research the details about physical mechanism of effect of nanorods on the Eu\\Tb co-doped samples.
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2. Methods 2.1. Preparation of gold nanorods The gold nanorods were synthesized by using the method mentioned in Ref. [21, 22]. Firstly, the seed solution was prepared. 10 ml of 100 mM hexadecyltrimethyl ammonium bromide (CTAB) solution was mixed with 0.25 ml of 10 mM coloroarric acid (HAuCl4) and stirred vigorously, 0.6 ml of 10 mM ice-cold NaBH4 was added. Vigorous stirring of the seed solution was continued for 2 min. After the solution was stirred, it was kept at 25 °C. Then the growth solution was prepared. 50 ml 100 mM CTAB solution was added to 0.2 ml 10 mM AgNO3 solution. 2.5 ml 10 nM HAuCl4 was added to this solution. 0.3 ml 10 mM freshly prepared 100 mM ascorbic acid solution was added. The final step was the adding of 12 μl seed solution and the mixed solution was gently mixed. The solution was allowed to sit overnight to let the nanorods grow. The nanorods were purified via centrifugation then resuspended in deionized water. To get the well-distributed nanorods, the silicon wafers were sequentially pretreated by acetone, isopropanol, and deionized water in an ultrasonic bath for 15 min and further disposed for 15 min by plasma cleaner PDC-002. Then the nanorods solutions were drop-casted onto the cleaned silicon wafers. 2.2. Preparation of Tb(TMHD)3 and Eu(dbm)3phen co-doped PMMA films Tb(TMHD)3 and Eu(dbm)3phen were purchased from TCI and Alfa Aesar separately. Tb(TMHD)3, Eu(dbm)3phen and PMMA were dissolved in chloroform follow by spin-coating onto the pre-treated silica substrates. The weight ratio of Tb(TMHD)3, PMMA and chloroform was 0.01:1:200. The concentration of Eu(dbm)3phen was changed and the molar ratio of Tb(TMHD)3 and Eu(dbm)3phen was increased from 1:0 to 1:1, we choose six different molar ratios of 1:0.0, 1:0.2, 1:0.4, 1:0.6, 1:0.8 and 1:1.0 in our samples. Then the solution was stirred for about 2 h at ambient temperature. 50 μl of the prepared Eu/Tb codoped PMMA solution was spin-coated at 3000 rpm for 60 s on the nanorods covered wafer and then baked on hot plate for 30 min. For the ease of comparison, all procedures and parameters were optimized and kept constant throughout the experiment. 2.3. Characterization and PL measurement Scanning Electron Microscope (SEM) images of gold nanorods were taken on a ZISS sigma 300 electron microscope. The UV–vis–NIR absorption spectra were recorded on a HTACHI U-3310 spectrometer. The photoluminescence spectra and the luminescence decay curves of samples were measured by a Edinburgh FLS920 spectrophotometer system. The luminescence decays were measured by using the time-correlated single photon counting technique. 3. Results and discussion Before exploiting LSPR effects on the Tb\\Eu co-doped films, we first study the optical properties of Tb\\Eu co-doped films in absence of nanorods. Fig. 1 shows the absorption of Tb, Eu and Tb\\Eu co-doped films, separately. As shown in Fig. 1, Tb(TMHD)3 shows a absorption band between 250 nm–300 nm which comes from the absorption of ligand (TMHD), the peak locates at about 276 nm. Eu(dbm)3phen shows two absorption bands between 250–280 nm and 300 nm–380 nm, which separately come from the π–π* transitions of ligands (Phen) and (dbm), the peaks locate at 265 nm and 343 nm. The absorption at 343 nm is higher than that of 265 nm. The Eu\\Tb co-doped films shows the absorption both of Tb(TMHD)3 and Eu(dbm)3phen. Then, we study the excitation and emission of Eu, Tb and Eu/Tb codoped films. The excitation and emission of pure Eu and Tb are shown in Fig. S1 in Supporting Information. For Tb(TMHD)3, the excitation
Fig. 1. The UV–Vis absorption spectra of Tb(TMHD)3, Eu(dbm)3phen, and Tb(TMHD)3-Eu (dbm)3phen co-doped films.
peak at 292 nm corresponds to the π–π* transitions. For Eu(dbm) the excitation peak at 360 nm corresponds to the π–π* transitions of ligand dbm. The photoluminescence (PL) spectrum of pure Tb complex shows four typical emission peaks at 490 nm, 545 nm, 580 nm and 620 nm with 292 nm excitation. The PL spectrum of Eu shows emission peaks at 582 nm, 590 nm and 612 nm, respectively, under excitation of 360 nm. Fig. 2 shows the excitation and emission spectra of Tb\\Eu co-doped films. Fig. 2(a) shows the excitation spectra when the monitoring wavelength is 545 nm. The peak at 292 nm corresponds to the π–π* transition, which is consistent with the absorption spectra in Fig. 1. The excitation is decreased with the increasing of Eu. Fig. 2(b) shows the corresponding emission spectra under excitation of 292 nm. As shown in Fig. 2(b), the emissions of Tb and Eu complexes are both observed. With increasing of Eu complex, the emission of Tb is decreased and the emission of Eu complex is increased. The increased Eu emission and decreased Tb emission demonstrate the energy transfer from Tb to Eu [19,23]. The emission at 612 nm increases slightly and the spectral profiles changes sharply when the molar ratio is from 1:0.4 to 1:0.8. The 5D0-7F2 transition of Eu centered at 612 nm is overlapping with the 5D4-7F3 transition of Tb at 620 nm, both Eu and Tb contribute to the luminescence at this band which induces the slightly enhancement at 612 nm when the molar ratio is from 1:0.4 to 1:0.8. Fig. 2(c) shows the excitation spectra when the monitoring wavelength is 612 nm. The peak at 360 nm also comes from the π–π* transition of dbm. The excitation is increased with the increasing of Eu. Fig. 2 (d) shows the corresponding emission spectra of the Tb\\Eu co-doped films under excitation of 360 nm. As shown in Fig. 2(d), the PL intensity is increased with the increasing of Eu complex, the emission peak at 545 nm is very weak because there is no absorption of Tb complex at 360 nm. The decay dynamics of samples are also measured under the excitations of 292 nm and 360 nm respectively. The analysis of decay times was carried out using the FLS-920 fluorescence spectrometer. The values of decay times are presented in Table S1 in Supporting Information. The χ2 value indicates how well the fit is (1 indicates a best fit). Fig. 3 shows the decay times of 545 and 612 nm for Tb\\Eu co-doped samples. As shown in Fig. 3(a), in absence of Eu, the profile at 545 nm follows second order exponential nature. In presence of Eu, the profiles at 545 nm follow three order exponential nature. There is a fast-initial component observed in the decay curves, which is attributed to the non-radiative energy transfer from Tb to Eu with the increasing of Eu. The profiles at 612 nm under the excitation of 292 nm and 360 nm, as shown in Fig. 3(b) and (c), follow second order exponential nature because there are two different emitting sites in the mixed systems [19]. The complexes were doped in PMMA films, which result in that the Eu 3phen,
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Fig. 2. Excitation and emission spectra of Tb\ \Eu co-doped films. (a) The excitation when the monitoring wavelength is 545 nm, (b) the emission spectra under the excitation of 292 nm, (c) the excitation spectra when the monitoring wavelength is 612 nm, (d) the emission spectra under the excitation of 360 nm.
and Tb ions located in more than one non-equivalent site, so the luminescence of Tb and Eu are not one exponential [19]. Next, we focus on the LSPR effects of gold nanorods on the luminescence properties of Tb\\Eu co-doped films. Here, gold nanorods were selected because they have two plasmon resonance bands in the visible region, which near to the emissions of Tb complex and Eu complex. In addition, gold nanorods typically have strong LSPR strength and
scattering property. The resonant absorption spectrum of nanorods we used is shown in Fig. 4(a), inset shows the size and morphology of nanorods. The size of nanorods is about ~50 nm. The gold nanorods exhibit two plasmon resonance bands at 519 nm and 663 nm corresponding to the transverse surface plasmon resonance (T-LSPR) and longitudinal surface plasmon resonance (L-LSPR) [24]. Fig. 4 (b) demonstrates the distribution of nanorods on substrate. As shown
Fig. 3. PL decay profiles of Tb\ \Eu co-doped films. (a) The decay profiles at 545 nm under the excitation of 292 nm, (b) the decay profiles at 612 nm under the excitation of 292 nm and (c) the decay profiles at 612 nm under the excitation of 360 nm.
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Fig. 4. SEM images and the plasmon absorption bands of nanorods. (a) The absorption of nanorods, inset shows the size and shape of nanorods. (b) The distribution of nanorods on the substrate.
in Fig. 4(b), the distribution is low density and almost uniform over a large area. The effects of nanorods on the emissions of Tb\\Eu co-doped systems are observed from detail characterizations. Here, we elaborate the effects on the luminescence of sample with the molar ratio 1:0.6. The results of the other four molar ratios also demonstrate the similar change. Fig. 5 shows the corresponding spectra when the molar ratio of Tb and Eu is 1:0.6. Fig. 5(a) shows the excitation spectra when the monitoring wavelength is 545 nm, the excitation peak is located at 292 nm. The excitation intensity is decreased in presence of nanorods. Fig. 5(b) shows the corresponding PL spectra under the excitation of 292 nm. As compared to the case without nanorods, the emission at 545 nm is further decreased and the emission at 612 nm is further increased in presence of nanorods. The intensity of emission at 490 nm is also slightly reduced. It suggests that the gold nanorods enhance the energy transfer from Tb to Eu. Under the excitation of 292 nm, the Tb complex is excited. The excited Tb would interact with T-LSPR of gold nanorods because the T-LSPR of nanorods is close to the emission of Tb at 545 nm, which result in the alteration of their relaxation process [1,3,25,26]. As we know, when the plasmon peak is located in between the intrinsic emission peak of Tb complex and the absorption peak of Eu complex according to the previous theoretical investigation, the energy transfer from Tb to Eu would be enhanced [20,27]. The energy transfer efficiency (ETE) between Tb and Eu can be calculated by following formula [28]: Is η ¼ 1− I so where Iso is the emission intensity of Tb in the absence of Eu, Is is the emission intensity of Tb in the presence of Eu. In presence of nanorods, the energy transfer rate will be modulated and the energy transfer efficiency η will thereafter be altered. The emission intensity ratio of Tb to Eu can be modulated by the LSPR through changing the value of ETE [20]. To examine the effect of the plasmon on the value of ETE from Tb to Eu by analyzing the emission spectra, the calculated ETE is shown in Fig. 5(c). In absence of nanorods, the ETE increases from 42% to 92% with increasing of Eu complex. The ETE depends on the distance of two molecules. The distance between Tb ions and Eu ions decreases with increasing of Eu complex, which results in the increase of the ETE. In presence of nanorods, the ETE is enhanced and it increased from 57% to 96% with the increasing of Eu. And the ETE enhancement at low molar ratio is greater than that at high molar ratio. In parallel, Fig. 5(d) shows the excitation spectra when the monitoring wavelength is 612 nm, the peak is also 360 nm. The excitation intensity is increased in presence of nanorods. Under excitation of 360 nm, the Eu complex is excited. The L-LSPR of nanorods is closed to the
emissions of Eu complex. Therefore, the excited Eu complex could interact with gold nanorods, which alter the relaxation process of Eu. Fig. 5 (e) shows the PL spectra when the molar ratio of Tb and Eu is 1:0.6. As shown in Fig. 5(e), the emission at 612 nm is enhanced, the enhancement factor is about 2.0. For other four molar ratios, the emission at 612 nm also be enhanced. The corresponding enhancement factor at different molar ratios is shown in Fig. 5(f). As shown, the enhancement factor increases and then decreases again with the increasing of Eu complex. The results show that the luminescence enhancement factor strongly relates to the composition of the lanthanide complexes. The overall enhancement factor of 612 nm under excitation of 360 nm is greater than that at 292 nm. The excitation efficiency of Eu complex under excitation of 360 nm is greater than that at 292 nm. More excited Eu complex could interact with LSPR of Au nanorods, which induces the greater enhancement factor under excitation of 360 nm. To better understand the effects of nanorods on the Tb\\Eu codoped films, the PL decays of complexes in presence of nanorods were also studied. Fig. S2 in Supporting Information shows the PL decays of samples in presence of nanorods. The decay times of samples with nanorods is shown in Table S2 in Supporting Information. Compared with the pure complexes, Fig. 6 show the PL decays with and without nanorods when the molar ratio of Tb and Eu is 1:0.6. As shown in Fig. 6, the decay time is decreased for the main two emissions of 544 nm and 612 nm in presence of nanorods. Under excitation of 292 nm, the decay time of 545 nm is decreased from 1157 μs to 1089 μs, the decay time of 612 nm is decreased from 787 μs to 738 μs. Under excitation of 360 nm, the decay time at 612 nm is decreased from 611 μs to 578 μs. As we known, the luminescence intensity I and the lifetime τ of organic materials in a free space can be defined as I ∝ I0ελclrrτ, τ 1 [19], where rr is the radiative decay rate and rnr is the r r þ r nr nonradiative decay rate, I0 is the intensity of the excitation wavelength, ελ is the dielectric constant at the emission wavelength, c is the velocity of light, l is the thickness of sample. For Tb\\Eu co-doped films, c and l are constant. The effects of plasmonic nanoparticles on the luminescent are determined by the relative extent of three aspects, excitation enhancement which is induced by the increased light absorption, emission enhancement which is induced by increased radiative decay and quenching which is induced by increased non-radiative decay [29]. In this paper, the LSPR bands of nanorods mainly influence the emission process rather than excitation process. The shortened decay in presence of nanorods indicates that either the radiative decay rate rr or the nonradiative decay rate rnr was increased. Under the excitation of 292 nm, we observed luminescence quenching at 545 nm and enhancement at 612 nm. Hence, the decreased decay at 545 nm can be explained ¼
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Fig. 5. The optical properties of Tb\ \Eu co-doped films in presence of nanorods. (a) The excitation spectra when the monitoring wavelength is 545 nm (b) The corresponding emission spectra when the molar ratio of Tb and Eu is 1:0.6, and (c) The ETE at different molar ratio of Tb and Eu; (d) The excitation spectra when the monitoring wavelength is 612 nm, (e) The corresponding emission spectra when the molar ratio of Tb and Eu is 1:0.6, and (f) the enhancement factor at different molar ratio of Tb and Eu.
as an increase in the nonradiative decay rate and the decreased decay at 612 nm is induced by an increase in the radiative decay rate, which demonstrates the energy transfer from Tb to Eu is enhanced. The mechanism is shown in Fig. 7. Under the excitation of 292 nm, the Tb complex is excited and the energy transfer from Tb to Eu occurs. The T-LSPR close to the emission band of Tb complex, so the nanorods mainly interact with the excited Tb complex. The plasmonic coupling with excited Tb complex could be strong in such a way that the nonradiative decay rates would become faster. Therefore, the energy transfer efficiency is
enhanced in presence of nanorods. And the increased nonradiative decay rate induces the decreased emission and the decreased decay time at 545 nm. Meanwhile, for the emission at 612 nm, the increased energy transfer further enhances the radiative decay rate and luminescence intensity. Under excitation of 360 nm, only the Eu complex is excited, the nanorods interact with more excited Eu due to the good match between L-LSPR band of nanorods with the emission band of Eu complex, which induces an increase in the radiative decay rate at 612 nm thus the luminescence is enhanced and the decay time is decreased.
Fig. 6. The PL decay profiles of Tb\ \Eu co-doped films with nanorods when the molar ratio of Tb and Eu is 1:0.6. (a) The decay profiles at 545 nm under excitation of 292 nm, (b) The decay profiles at 612 nm under excitation of 292 nm and (c) The decay profiles at 612 nm under excitation of 360 nm.
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Shandong Province (Grant No. 2019KJJ019); Introduction and Cultivation Plan of Youth Innovation Talents for Universities of Shandong Province; and The Special Construction Project Fund of Shandong Province Taishan Scholars. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.saa.2020.118260. References
Fig. 7. Schematic of the energy transfer and emission processes of samples in presence of gold nanorods with low concentration.
4. Conclusions In this study, we investigated the effects of plasmon nanoparticles on Tb\\Eu co-doped films. By analyzing the steady state and time resolved luminescence, we discuss the interaction among Tb, Eu and gold nanorods. Under excitation of 292 nm, the emission at 545 nm is decreased and the emission at 612 nm is increased. The luminescence and decay times investigations reveal that the energy transfer efficiency from Tb to Eu is enhanced in presence of nanorods. Under excitation of 360 nm, the emission at 612 nm is enhanced because of the direct interaction between nanorods and excited Eu, the enhancement is related to the molar ratio of Tb and Eu. The dual effects of LSPR on energy transfer and emission enhancement are both observed. More details on the two or more lanthanide doped samples with nanorods are further demonstrated, which would give us a deeper understanding on interactions luminescent-particle and luminescent-luminescent. CRediT authorship contribution statement Qingru Wang: Conceptualization, Methodology, Investigation, Writing - original draft, Writing - review & editing. Jinhua Liu: Validation, Formal analysis, Visualization, Writing - review & editing. Kewei Huang: Validation, Formal analysis, Visualization, Data curation. Qingchao Chen: Validation, Formal analysis, Visualization, Data curation. Haochuan Dong: Validation, Formal analysis, Visualization, Data curation. Dong Zhang: Writing - review & editing. Qiang Shi: Writing - review & editing. Shuhong Li: Resources, Writing - review & editing, Supervision, Data curation. Wenjun Wang: Resources, Writing - review & editing, Supervision, Project administration, Funding acquisition. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement Shandong Provincial Natural Science Foundation (ZR2019MF068, ZR2018MA039); National Natural Science Foundation Program of China (NSFC) (61775089 and 11504390); Shandong Province Higher Educational Science and Technology Program (J17KA175); Alliance Fund of Shandong Provincial Key Laboratory (SDKL20016038); Science and Technology Plan of Youth Innovation Team for Universities of
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