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In situ isothermal monitoring of the enhancement and quenching of Sm3+ photoluminescence in Ag co-doped glass
Solid State Communications 152 (2012) 1786–1790
Contents lists available at SciVerse ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
In situ isothermal monitoring of the enhancement and quenching of Sm3 þ photoluminescence in Ag co-doped glass Jose´ A. Jime´nez a,n, Mariana Sendova b a b
Department of Chemistry, University of North Florida, Jacksonville, FL 32224, USA Optical Spectroscopy & Nano-Materials Lab, New College of Florida, Sarasota, FL 34243, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 May 2012 Received in revised form 15 May 2012 Accepted 14 June 2012 by R. Phillips Available online 19 June 2012
A novel in situ concurrent photoluminescence and absorption microspectroscopy technique is proposed for the real-time monitoring of the optical properties of rare-earth and noble metal co-doped dielectrics during isothermal processing. The technique has been successfully applied to an Ag and Sm co-doped phosphate glass, where both enhancement and quenching regimes of Sm3 þ luminescence have been observed well separated in time, practical for an ‘optical tuning’ of the solid-state luminescent material. Further, relating simultaneously the luminescence with the time evolution in optical absorption allowed for discerning the effects of Ag non-plasmonic clusters as enhancers and nanoparticles as quenchers. Opportunities come forward for studying a variety of systems with relevance to a wide range of applications. & 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Glasses A. Nanostructures D. Luminescence D. Optical properties
Following the works of Malta et al. [1], Hayakawa et al. [2], and ¨ Strohhofer and Polman [3], numerous investigations have been carried out on the effect of noble metals on the photoluminescence (PL) of rare-earth (RE) ions in dielectric hosts [4–10]. The aim of obtaining an enhanced light emission from the REs has been reported as feasible by having the metal in different physico-chemical states such as ions [3,6,11], sub-nanometric clusters [5,7,9,12], and plasmonic nanoparticles (NPs) [1,2,4,8,10]. Nevertheless, the influence of the plasmonic NPs has been a subject of discussion [3,5–7,11] due to the fact that the they are effective as PL quenchers [3,6,11,13]. The situation becomes rather complicated when several oxidation and aggregation states of the metal are co-existent in a matrix, such as the metal NP precursors (e.g. single ions, dimers, etc.) and the NPs themselves, all of which may influence RE luminescence in different ways [5,7,11]. As a consequence, an energy transfer [14] mechanism proceeding via atomic or molecule-like metallic species could be mistakenly attributed to so-called local-field enhancements from plasmonic metal particles. It is then essential to distinguish between the possible contributors to the increase in RE luminescence in order to be able to arrive at proper conclusions about the mechanisms operating, and as a result be effective in tailoring material properties for applications [7].
n
Corresponding author. Tel.: þ1 904 620 1963; fax: þ1 904 620 3535. E-mail address: [email protected] (J.A. Jime´nez).
0038-1098/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ssc.2012.06.017
The extent to which a PL enhancement (or quenching) is expressed depends not only on material chemical composition, but on further processing, due to the influence of processing parameters on physico-chemical transformations involving the metal. This is for instance the case of a thermal treatment of codoped glasses which modifies the chemical states of the metal and induces processes such as particle nucleation and growth [11,15,16]. As a consequence, the proper choice of heat treatment conditions can effectively result in enhanced optical properties in the material in connection with the appropriate generation of the ‘enhancer’ or sensitizer inside the dielectric medium [15]. In this respect, a real-time monitoring of material optical properties during isothermal treatment can allow for their tuning for specific applications, while providing insights on the underlying mechanisms. In such context, we have recently proposed in situ optical microspectroscopy for the study of metal transport and NP transformations in nanocomposite films [17,18] and glasses [19,20], given that the technique allows for monitoring optical absorption in real time during thermal processing. Further, PL spectra collected in situ together with optical absorption data on a silver-doped phosphate glass during isothermal treatment proved effective in providing insights into the nucleation of Ag particles [20]. In this communication, we report the first application of an in situ concurrent photoluminescence and absorption microspectroscopy technique for the real-time monitoring of the optical properties of noble metal and RE co-doped dielectrics during thermal processing. A study on a previously well-characterized
´nez, M. Sendova / Solid State Communications 152 (2012) 1786–1790 J.A. Jime
[21] Ag and Sm co-doped phosphate glass is presented, for which the time evolution of both PL and optical absorption is followed during isothermal treatment. The technique has revealed enhancement and quenching regimes of Sm3 þ ions luminescence, in connection to the time periods related to the formation of Ag non-plasmonic clusters (nPCs) and their subsequent growth into the plasmonic regime. Thus, the in situ technique is proved valuable for understanding the optical interactions that result in either enhancement or quenching of RE PL. This aspect being of fundamental nature is especially important in view of how controversial the subject has been in the past years regarding the conditions and mechanisms by which PL enhancements are attained in RE and metal co-doped dielectrics [3,5,7,9,11]. Moreover, such basic knowledge can become valuable for the tuning of material optical properties in step with applications. It is indeed relevant to help meet global energy demands [22,23] by advancing the development of more efficient alternatives to be used for instance in telecommunications [3], light-emitting devices [7], and solar cell technologies [23]. Furthermore, a RE PL enhancement/quenching monitored in situ may become useful as an indicator for non-plasmonic/plasmonic particle dynamics, or in a well-designed system, developed as an additional tool for studying particle nucleation and growth. Phosphate-based glasses of the P2O5:Al2O3:CaO:SrO:BaO type were prepared by the melt-quenching technique [21,24]. Silver, tin and/or samarium doping was done by adding Ag2O, SnO and/ or Sm2O3 quantities in mol% in relation to network former P2O5. SnO is added as a reducing agent in order to promote Ag particle formation in the host [24]. Samples were studied as glass slabs about 1.0 mm thick. The glasses are referred to as (amounts in mol%) Ag/Sn–Sm glass (containing 4% of each Ag2O and SnO, and 2% Sm2O3), and Sm glass (2% Sm2O3), the latter being used for reference purposes. A CRAIC Technologies QDI 2010 microspectrophotometer equipped with a Xe short-arc lamp and a heating stage was used to conduct in situ steady-state PL and optical absorption measurements jointly during thermal processing. Samples were heated to the desired temperature at a rate of 50 1C/min. Measurements were performed with a 10 objective on 50 mm 50 mm sample areas with particular attention given to keep the sampled area and all other conditions unaltered during experiments. The in situ optical data was collected sequentially by obtaining an absorption profile immediately after the emission spectrum, both spectra being collected not more than 15 s apart. The emission spectra were obtained under excitation at 420 nm (2.95 eV), where wavelength selection was achieved by a band pass filter with a full-width at half-maximum (FWHM) of about 45 nm. The rationale for the selection of the excitation wavelength is the following. It is known that a surface plasmon resonance (SPR) of Ag NPs in the studied Ag/Sn phosphate glass matrix is achievable around 420 nm [24] and it is in tune with Sm3 þ excitation peaks [21]. Thus, having the excitation in resonance with optical absorption in both Sm3 þ ions and Ag NPs would allow for assessing whether a so-called local-field enhancement [14,15], or else a quenching effect [6,11], of Sm3 þ PL is manifested in the presence of Ag NPs. Moreover, several Ag nPCs (e.g., Ag2, Ag3, Ag4) are also expected to bring about optical absorption features near 3.0 eV [25,26]. Therefore, the influence of such type of molecule-like Ag clusters on the PL of Sm3 þ ions can be investigated as well during the process of Ag particle development, prior to cluster transition into the plasmonic regime via diffusion-based growth [19,20]. In this way, unambiguous evidence can be obtained in situ regarding the influence of various metallic species on the luminescence of the RE ions in the dielectric host. It is worth stressing that this is a key advantage of the in situ approach employed herein over traditional PL measurements.
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Optical and structural properties of the melt-quenched Ag/Sn– Sm glass have been studied recently by optical absorption, PL spectroscopy, Raman scattering, and transmission electron microscopy [21]. Ag þ ions and Sn centers were established to be at the origin of an absorption band around 273 nm. The Ag þ ions were associated with the contribution to the absorption band mainly around 273 nm owing to the 4d10-4d95s1 parity-forbidden transitions [21]. The Sn centers were confirmed to contribute to absorption around 255 nm due to electronic transitions from the ground singlet state to the first excited singlet state (S0-S1) [20]. Such assignments were in fact consistent with glasses of similar composition having europium as the RE instead of samarium [6]. Hence, significant excitation of the luminescent Ag þ ions and Sn centers by the optical source used herein centered around 420 nm can be neglected. Other complex luminescent silver species such as the (Ag2) þ and (Ag þ )2 pairs typically present at higher silver concentration [11] were not observed for the as-prepared Ag/Sn– Sm glass. Fig. 1 shows PL spectra obtained for the Ag/Sn–Sm glass during isothermal treatment at 580 1C within 10–120 min of holding time. Three-band emission from Sm3 þ ions corresponding to 4G5/ 6 6 6 2- H9/2, H7/2 and H5/2 transitions around 643, 597 and 562 nm, respectively, is consistently observed to increase during the first 50 min; thereafter the PL intensity decreases. Fig. 2 shows the corresponding time evolution of the three-band integrated intensity for each of the spectra in Fig. 1. Relative to the spectrum collected after 10 min, a maximum increase in the integrated PL intensity of about 16% occurred after 50 min at 580 1C. The quenching effect observed afterwards resulted in an intensity drop of about 18% after 120 min relative to the 50-min maximum. On the other hand, an analogous experiment was carried out for the Sm glass at 580 1C for reference purposes. The PL spectra for such glass which are shown in Fig. 3 are characterized by lack of significant time variation; no distinguishable trend of the PL intensity of the Sm glass such as that established for the Ag/Sn– Sm glass is observed throughout. Therefore, it is clear that both the enhancement and quenching of Sm3 þ PL observed for the Ag/
Fig. 1. PL spectra of Ag/Sn–Sm glass recorded in real time during isothermal treatment at 580 1C; emission corresponds to 4G5/2-6H9/2, 6H7/2 and 6H5/2 transitions of Sm3 þ ions. The first spectrum was collected after 10 min, and time intervals of 5 min were used up to 50 min where a maximum in the emission intensity was observed. Thereafter, periods of data collection were increased to 10 min for a total holding time of 120 min (total of 16 spectra).
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J.A. Jime´nez, M. Sendova / Solid State Communications 152 (2012) 1786–1790
Fig. 2. Time evolution in the integrated PL intensity for the three 4G5/2-6H9/2, 6 H7/2, 6H5/2 emission bands in spectra shown in Fig. 1.
Fig. 4. Optical absorption spectra of the Ag/Sn–Sm glass recorded during the isothermal treatment at 580 1C; the data was collected in situ together with the PL spectra shown in Fig. 1 for the same holding times within the 10–120 min range.
Fig. 3. PL spectra for the Sm glass (Reference) recorded in real time during isothermal treatment at 580 1C; spectra were collected for the same holding times within the 10–120 min range as the data shown in Fig. 1. A lack of significant time variation for the emission of 4G5/2-6H9/2, 6H7/2 and 6H5/2 transitions in Sm3 þ ions is exhibited.
Sn–Sm glass are due to the presence of silver and tin and the effects of the thermal treatment on the dopants. Insights on the origin of the time evolution of the PL in the Ag/ Sn–Sm glass can be obtained from the optical absorption spectra collected jointly in situ, which are presented in Fig. 4. No significant variation in the spectra is observed during the first 50 min where the enhanced PL was manifested, suggesting a period of Ag nPCs development [27]. These spectra essentially display Sm3 þ ions absorption characteristic of the material [21]; e.g. absorption features are observed around 360, 372, 400, 420, 440 and 468 nm. However, the absorption band characteristic of the SPR of Ag NPs [24] develops around 420 nm after 50 min, timed with the onset of the PL quenching regime in Fig. 1. Thus, the in situ approach reveals that the enhanced PL is not due to the presence of the plasmonic NPs, since the emission increase takes place during the time period characterized by the absence of plasmonic NPs. The data then suggests that the enhancement occurs due to a sensitizing effect, i.e. via energy transfer from
sub-nanometric Ag clusters produced early during heat treatment through the chemical reduction of silver ions as 2Ag þ þSn2 þ 2Ag0 þSn4 þ [21,24,27]. Certainly, such small Ag clusters exhibiting quantum confinement effects have been recognized before in this type of heat-treated phosphate glass (with 4 mol% of both Ag2O and SnO) by X-ray photoelectron spectroscopy (XPS) in connection to Ag particle nucleation [27]. Further, the current results are consistent with the recent reports from Eichelbaum and Rademann [7], Maurizio et al. [9], and also Trave et al. [12], all of whom have reported PL enhancements of RE ions in noble metal co-doped dielectrics in connection to a classical energy transfer mechanism originating at molecule-like metallic clusters. It therefore appears that an effective excitation of the Ag clusters is achieved by the optical source employed herein, which is in agreement with studies from other groups [7,25,26]. Regarding the nature of the nPCs indicated as sensitizers, a neutral character is suggested by the presence of tin acting as a reducing agent during the isothermal processing. Although cluster size cannot be unambiguously assigned, some candidates can be proposed based on experimental and theoretical works on the optical absorption of small Ag clusters in different matrices [25,26]. Herein, Ag2, Ag3 and Ag4 are reasonable candidates, since these clusters have shown optical absorption features around 3.0 eV [25,26]. In particular, the Ag2 dimer has shown experimental absorption peaks at energies marginally below 3.0 eV (e.g. at about 2.96 eV [26]), which is consistent with our excitation source having a peak at 2.95 eV (420 nm). However, the simultaneous excitation of different nPCs cannot be excluded due to: (i) the dynamic process of the formation and growth of the clusters during thermal treatment leading to a range of cluster sizes and therefore energy levels; (ii) the FWHM (45 nm) of the excitation source centered at 2.95 eV (420 nm), which may produce significant excitation in the 2.8–3.1 eV range; and (iii) matrix effects which may result in an energy shift of the optical transitions in the clusters. The evidence is unequivocal in that no local-field enhancement effect of the Sm3 þ luminescence takes place once the nonplasmonic Ag particle nuclei become plasmonic. An effective energy transfer from the metallic NPs to the luminescent ions [28] leading to an increased emission intensity can be also excluded. Indeed, the appearance of the Ag NPs supporting the
´nez, M. Sendova / Solid State Communications 152 (2012) 1786–1790 J.A. Jime
Fig. 5. (color online) Simplified schematic illustrating resonant energy transfer processes leading to the enhanced Sm3 þ ions PL (ET-E) via Ag nPCs (e.g. Ag2), and the PL quenching effect (ET-Q) via the SPR of Ag NPs. Radiative transitions are indicated by solid arrows. Direct optical excitation of Sm3 þ ions to the 6P5/2 state is also illustrated.
SPR shows with the PL quenching, which becomes patent in the comparison of Fig. 1 with Fig. 4. This result is in agreement with several reports where the presence of NPs resulted in the quenching of the PL of Sm3 þ [29,30] Eu3 þ [6,31] and Dy3 þ ions [11] in Ag nanocomposites. A decrease in the density of sensitizers, i.e. the number of nPCs, could lead to a reduction in the degree of enhancement in connection to the maximum reached at 50 min of holding time. However, in the absence of the generation of a PL quencher, a PL decrease should not occur. In addition, the presence of the reducing agent is known to result in continuous Ag cluster generation in parallel with NP growth [21,27], thus implying a sustained production of the energy donor species. Moreover, significant Sm3 þ -Sm2 þ reduction (Sm3 þ /Sm2 þ , E1¼ 1.55 V [32]) can be also neglected based on the fact that an XPS study carried out for europium (higher Eu3 þ /Eu2 þ reduction potential, E1 ¼ 0.35 V [32]) did not show Eu2 þ formation as a result of thermal treatment of the glass having reducing agent tin at the same concentration [31]. We also take into account that, whereas the peak excitation employed at 420 nm is resonant with both Sm3 þ and Ag NPs absorption, Sm3 þ emission bands around 643, 597 and 562 nm, are out of resonance with the Ag NP plasmon absorption. Thus, an ion-to-particle excitation energy transfer is indicated to be the main cause of the PL quenching effect, where the NPs appear to effectively provide paths for the nonradiative loss of excitation energy in Sm3 þ ions through coupling with the plasmon resonance [6,11,13]. The schematic shown in Fig. 5 synthesizes the indicated energy transfer processes related to the time-dependent PL enhancement and quenching. The energy level diagram for the Ag nPCs is simplified by including only an excited state in resonance with the employed peak excitation (420 nm), e.g. an excited state in Ag2 [26]. Also illustrated is optical absorption resulting in direct excitation of Sm3 þ ions to the 6P5/2 state (also relevant to excitation at 420 nm) within the Sm3 þ energy level structure [23]. Herein, optical excitation of the small Ag nPCs, the energy donors, and a subsequent resonant energy transfer to Sm3 þ ions (e.g. to 6P5/2 state), the acceptors, can result in populating the 4G5/2 emitting state in addition to the direct optical excitation. This is the situation likely dominating in the absence of the plasmonic NPs and therefore results in the PL enhancement early during thermal treatment. The tendency in PL increase observed before 50 min is likely due to the increase in
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the density of the small silver clusters acting as energy donors. On the other hand, excited states in Sm3 þ ions in resonance with the Ag NPs absorption (e.g. 6P5/2) can be depopulated by coupling with surface plasmons in the NPs [11,13], which subsequently decay nonradiatively. Thus, in this scenario, Sm3 þ ions assume the role of energy donors, with NPs as acceptors. This becomes dominant with the progression of the Ag cluster growth into the plasmonic regime. Such quenching effect can take place for Sm3 þ ions excited directly by the optical source or else via energy transfer from the nPCs. A dynamic situation can be envisioned, involving an enhancement vs. quenching competition among the co-existent non-plasmonic and plasmonic Ag particles, alongside with the direct optical excitation of the Sm3 þ ions. It should be pointed out that the diagram in Fig. 3 is tentatively showing excitation of presumably an excited state in Ag2 and the 6P5/2 state in Sm3 þ in connection to the peak excitation wavelength employed at 420 nm (2.95 eV). However, other possibilities may arise due to the range of wavelengths provided by the filter bandwidth (FWHM of 45 nm), the different energy levels carried by varied Ag cluster sizes, and the different energy levels of Sm3 þ ions. Summarizing, a time-dependent enhancement and quenching of Sm3 þ ions PL in Ag-doped phosphate glass has been revealed by the in situ optical microspectroscopy technique, practical for an ‘optical tuning’ of multi-component nanocomposite materials. Further on, the capability of the proposed technique for the concurrent collection of photoluminescence and optical absorption data provided insights into the underlying mechanisms, based on energy transfer processes. The fact that the PL enhancement and quenching appeared in connection to the development of small Ag clusters and their subsequent growth into the plasmonic regime, further suggests the use of RE ions as luminescent probes for the study of metal particle dynamics in dielectrics. Indeed, the current study opens an array of possibilities for studying a variety hosts, e.g. glasses and films of different compositions, and with different noble metal and RE dopants and their concentrations. The proposed concurrent isothermal technique is appropriate for the quest of materials/conditions for the so-called plasmonic luminescence enhancement to be realized. This could shed light into the tailoring of materials’ optical properties for a wide range of applications.
Acknowledgment M. Sendova acknowledges financial support from ARLW911NF-09-2-0004. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
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Contents lists available at SciVerse ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
In situ isothermal monitoring of the enhancement and quenching of Sm3 þ photoluminescence in Ag co-doped glass Jose´ A. Jime´nez a,n, Mariana Sendova b a b
Department of Chemistry, University of North Florida, Jacksonville, FL 32224, USA Optical Spectroscopy & Nano-Materials Lab, New College of Florida, Sarasota, FL 34243, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 May 2012 Received in revised form 15 May 2012 Accepted 14 June 2012 by R. Phillips Available online 19 June 2012
A novel in situ concurrent photoluminescence and absorption microspectroscopy technique is proposed for the real-time monitoring of the optical properties of rare-earth and noble metal co-doped dielectrics during isothermal processing. The technique has been successfully applied to an Ag and Sm co-doped phosphate glass, where both enhancement and quenching regimes of Sm3 þ luminescence have been observed well separated in time, practical for an ‘optical tuning’ of the solid-state luminescent material. Further, relating simultaneously the luminescence with the time evolution in optical absorption allowed for discerning the effects of Ag non-plasmonic clusters as enhancers and nanoparticles as quenchers. Opportunities come forward for studying a variety of systems with relevance to a wide range of applications. & 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Glasses A. Nanostructures D. Luminescence D. Optical properties
Following the works of Malta et al. [1], Hayakawa et al. [2], and ¨ Strohhofer and Polman [3], numerous investigations have been carried out on the effect of noble metals on the photoluminescence (PL) of rare-earth (RE) ions in dielectric hosts [4–10]. The aim of obtaining an enhanced light emission from the REs has been reported as feasible by having the metal in different physico-chemical states such as ions [3,6,11], sub-nanometric clusters [5,7,9,12], and plasmonic nanoparticles (NPs) [1,2,4,8,10]. Nevertheless, the influence of the plasmonic NPs has been a subject of discussion [3,5–7,11] due to the fact that the they are effective as PL quenchers [3,6,11,13]. The situation becomes rather complicated when several oxidation and aggregation states of the metal are co-existent in a matrix, such as the metal NP precursors (e.g. single ions, dimers, etc.) and the NPs themselves, all of which may influence RE luminescence in different ways [5,7,11]. As a consequence, an energy transfer [14] mechanism proceeding via atomic or molecule-like metallic species could be mistakenly attributed to so-called local-field enhancements from plasmonic metal particles. It is then essential to distinguish between the possible contributors to the increase in RE luminescence in order to be able to arrive at proper conclusions about the mechanisms operating, and as a result be effective in tailoring material properties for applications [7].
n
Corresponding author. Tel.: þ1 904 620 1963; fax: þ1 904 620 3535. E-mail address: [email protected] (J.A. Jime´nez).
0038-1098/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ssc.2012.06.017
The extent to which a PL enhancement (or quenching) is expressed depends not only on material chemical composition, but on further processing, due to the influence of processing parameters on physico-chemical transformations involving the metal. This is for instance the case of a thermal treatment of codoped glasses which modifies the chemical states of the metal and induces processes such as particle nucleation and growth [11,15,16]. As a consequence, the proper choice of heat treatment conditions can effectively result in enhanced optical properties in the material in connection with the appropriate generation of the ‘enhancer’ or sensitizer inside the dielectric medium [15]. In this respect, a real-time monitoring of material optical properties during isothermal treatment can allow for their tuning for specific applications, while providing insights on the underlying mechanisms. In such context, we have recently proposed in situ optical microspectroscopy for the study of metal transport and NP transformations in nanocomposite films [17,18] and glasses [19,20], given that the technique allows for monitoring optical absorption in real time during thermal processing. Further, PL spectra collected in situ together with optical absorption data on a silver-doped phosphate glass during isothermal treatment proved effective in providing insights into the nucleation of Ag particles [20]. In this communication, we report the first application of an in situ concurrent photoluminescence and absorption microspectroscopy technique for the real-time monitoring of the optical properties of noble metal and RE co-doped dielectrics during thermal processing. A study on a previously well-characterized
´nez, M. Sendova / Solid State Communications 152 (2012) 1786–1790 J.A. Jime
[21] Ag and Sm co-doped phosphate glass is presented, for which the time evolution of both PL and optical absorption is followed during isothermal treatment. The technique has revealed enhancement and quenching regimes of Sm3 þ ions luminescence, in connection to the time periods related to the formation of Ag non-plasmonic clusters (nPCs) and their subsequent growth into the plasmonic regime. Thus, the in situ technique is proved valuable for understanding the optical interactions that result in either enhancement or quenching of RE PL. This aspect being of fundamental nature is especially important in view of how controversial the subject has been in the past years regarding the conditions and mechanisms by which PL enhancements are attained in RE and metal co-doped dielectrics [3,5,7,9,11]. Moreover, such basic knowledge can become valuable for the tuning of material optical properties in step with applications. It is indeed relevant to help meet global energy demands [22,23] by advancing the development of more efficient alternatives to be used for instance in telecommunications [3], light-emitting devices [7], and solar cell technologies [23]. Furthermore, a RE PL enhancement/quenching monitored in situ may become useful as an indicator for non-plasmonic/plasmonic particle dynamics, or in a well-designed system, developed as an additional tool for studying particle nucleation and growth. Phosphate-based glasses of the P2O5:Al2O3:CaO:SrO:BaO type were prepared by the melt-quenching technique [21,24]. Silver, tin and/or samarium doping was done by adding Ag2O, SnO and/ or Sm2O3 quantities in mol% in relation to network former P2O5. SnO is added as a reducing agent in order to promote Ag particle formation in the host [24]. Samples were studied as glass slabs about 1.0 mm thick. The glasses are referred to as (amounts in mol%) Ag/Sn–Sm glass (containing 4% of each Ag2O and SnO, and 2% Sm2O3), and Sm glass (2% Sm2O3), the latter being used for reference purposes. A CRAIC Technologies QDI 2010 microspectrophotometer equipped with a Xe short-arc lamp and a heating stage was used to conduct in situ steady-state PL and optical absorption measurements jointly during thermal processing. Samples were heated to the desired temperature at a rate of 50 1C/min. Measurements were performed with a 10 objective on 50 mm 50 mm sample areas with particular attention given to keep the sampled area and all other conditions unaltered during experiments. The in situ optical data was collected sequentially by obtaining an absorption profile immediately after the emission spectrum, both spectra being collected not more than 15 s apart. The emission spectra were obtained under excitation at 420 nm (2.95 eV), where wavelength selection was achieved by a band pass filter with a full-width at half-maximum (FWHM) of about 45 nm. The rationale for the selection of the excitation wavelength is the following. It is known that a surface plasmon resonance (SPR) of Ag NPs in the studied Ag/Sn phosphate glass matrix is achievable around 420 nm [24] and it is in tune with Sm3 þ excitation peaks [21]. Thus, having the excitation in resonance with optical absorption in both Sm3 þ ions and Ag NPs would allow for assessing whether a so-called local-field enhancement [14,15], or else a quenching effect [6,11], of Sm3 þ PL is manifested in the presence of Ag NPs. Moreover, several Ag nPCs (e.g., Ag2, Ag3, Ag4) are also expected to bring about optical absorption features near 3.0 eV [25,26]. Therefore, the influence of such type of molecule-like Ag clusters on the PL of Sm3 þ ions can be investigated as well during the process of Ag particle development, prior to cluster transition into the plasmonic regime via diffusion-based growth [19,20]. In this way, unambiguous evidence can be obtained in situ regarding the influence of various metallic species on the luminescence of the RE ions in the dielectric host. It is worth stressing that this is a key advantage of the in situ approach employed herein over traditional PL measurements.
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Optical and structural properties of the melt-quenched Ag/Sn– Sm glass have been studied recently by optical absorption, PL spectroscopy, Raman scattering, and transmission electron microscopy [21]. Ag þ ions and Sn centers were established to be at the origin of an absorption band around 273 nm. The Ag þ ions were associated with the contribution to the absorption band mainly around 273 nm owing to the 4d10-4d95s1 parity-forbidden transitions [21]. The Sn centers were confirmed to contribute to absorption around 255 nm due to electronic transitions from the ground singlet state to the first excited singlet state (S0-S1) [20]. Such assignments were in fact consistent with glasses of similar composition having europium as the RE instead of samarium [6]. Hence, significant excitation of the luminescent Ag þ ions and Sn centers by the optical source used herein centered around 420 nm can be neglected. Other complex luminescent silver species such as the (Ag2) þ and (Ag þ )2 pairs typically present at higher silver concentration [11] were not observed for the as-prepared Ag/Sn– Sm glass. Fig. 1 shows PL spectra obtained for the Ag/Sn–Sm glass during isothermal treatment at 580 1C within 10–120 min of holding time. Three-band emission from Sm3 þ ions corresponding to 4G5/ 6 6 6 2- H9/2, H7/2 and H5/2 transitions around 643, 597 and 562 nm, respectively, is consistently observed to increase during the first 50 min; thereafter the PL intensity decreases. Fig. 2 shows the corresponding time evolution of the three-band integrated intensity for each of the spectra in Fig. 1. Relative to the spectrum collected after 10 min, a maximum increase in the integrated PL intensity of about 16% occurred after 50 min at 580 1C. The quenching effect observed afterwards resulted in an intensity drop of about 18% after 120 min relative to the 50-min maximum. On the other hand, an analogous experiment was carried out for the Sm glass at 580 1C for reference purposes. The PL spectra for such glass which are shown in Fig. 3 are characterized by lack of significant time variation; no distinguishable trend of the PL intensity of the Sm glass such as that established for the Ag/Sn– Sm glass is observed throughout. Therefore, it is clear that both the enhancement and quenching of Sm3 þ PL observed for the Ag/
Fig. 1. PL spectra of Ag/Sn–Sm glass recorded in real time during isothermal treatment at 580 1C; emission corresponds to 4G5/2-6H9/2, 6H7/2 and 6H5/2 transitions of Sm3 þ ions. The first spectrum was collected after 10 min, and time intervals of 5 min were used up to 50 min where a maximum in the emission intensity was observed. Thereafter, periods of data collection were increased to 10 min for a total holding time of 120 min (total of 16 spectra).
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Fig. 2. Time evolution in the integrated PL intensity for the three 4G5/2-6H9/2, 6 H7/2, 6H5/2 emission bands in spectra shown in Fig. 1.
Fig. 4. Optical absorption spectra of the Ag/Sn–Sm glass recorded during the isothermal treatment at 580 1C; the data was collected in situ together with the PL spectra shown in Fig. 1 for the same holding times within the 10–120 min range.
Fig. 3. PL spectra for the Sm glass (Reference) recorded in real time during isothermal treatment at 580 1C; spectra were collected for the same holding times within the 10–120 min range as the data shown in Fig. 1. A lack of significant time variation for the emission of 4G5/2-6H9/2, 6H7/2 and 6H5/2 transitions in Sm3 þ ions is exhibited.
Sn–Sm glass are due to the presence of silver and tin and the effects of the thermal treatment on the dopants. Insights on the origin of the time evolution of the PL in the Ag/ Sn–Sm glass can be obtained from the optical absorption spectra collected jointly in situ, which are presented in Fig. 4. No significant variation in the spectra is observed during the first 50 min where the enhanced PL was manifested, suggesting a period of Ag nPCs development [27]. These spectra essentially display Sm3 þ ions absorption characteristic of the material [21]; e.g. absorption features are observed around 360, 372, 400, 420, 440 and 468 nm. However, the absorption band characteristic of the SPR of Ag NPs [24] develops around 420 nm after 50 min, timed with the onset of the PL quenching regime in Fig. 1. Thus, the in situ approach reveals that the enhanced PL is not due to the presence of the plasmonic NPs, since the emission increase takes place during the time period characterized by the absence of plasmonic NPs. The data then suggests that the enhancement occurs due to a sensitizing effect, i.e. via energy transfer from
sub-nanometric Ag clusters produced early during heat treatment through the chemical reduction of silver ions as 2Ag þ þSn2 þ 2Ag0 þSn4 þ [21,24,27]. Certainly, such small Ag clusters exhibiting quantum confinement effects have been recognized before in this type of heat-treated phosphate glass (with 4 mol% of both Ag2O and SnO) by X-ray photoelectron spectroscopy (XPS) in connection to Ag particle nucleation [27]. Further, the current results are consistent with the recent reports from Eichelbaum and Rademann [7], Maurizio et al. [9], and also Trave et al. [12], all of whom have reported PL enhancements of RE ions in noble metal co-doped dielectrics in connection to a classical energy transfer mechanism originating at molecule-like metallic clusters. It therefore appears that an effective excitation of the Ag clusters is achieved by the optical source employed herein, which is in agreement with studies from other groups [7,25,26]. Regarding the nature of the nPCs indicated as sensitizers, a neutral character is suggested by the presence of tin acting as a reducing agent during the isothermal processing. Although cluster size cannot be unambiguously assigned, some candidates can be proposed based on experimental and theoretical works on the optical absorption of small Ag clusters in different matrices [25,26]. Herein, Ag2, Ag3 and Ag4 are reasonable candidates, since these clusters have shown optical absorption features around 3.0 eV [25,26]. In particular, the Ag2 dimer has shown experimental absorption peaks at energies marginally below 3.0 eV (e.g. at about 2.96 eV [26]), which is consistent with our excitation source having a peak at 2.95 eV (420 nm). However, the simultaneous excitation of different nPCs cannot be excluded due to: (i) the dynamic process of the formation and growth of the clusters during thermal treatment leading to a range of cluster sizes and therefore energy levels; (ii) the FWHM (45 nm) of the excitation source centered at 2.95 eV (420 nm), which may produce significant excitation in the 2.8–3.1 eV range; and (iii) matrix effects which may result in an energy shift of the optical transitions in the clusters. The evidence is unequivocal in that no local-field enhancement effect of the Sm3 þ luminescence takes place once the nonplasmonic Ag particle nuclei become plasmonic. An effective energy transfer from the metallic NPs to the luminescent ions [28] leading to an increased emission intensity can be also excluded. Indeed, the appearance of the Ag NPs supporting the
´nez, M. Sendova / Solid State Communications 152 (2012) 1786–1790 J.A. Jime
Fig. 5. (color online) Simplified schematic illustrating resonant energy transfer processes leading to the enhanced Sm3 þ ions PL (ET-E) via Ag nPCs (e.g. Ag2), and the PL quenching effect (ET-Q) via the SPR of Ag NPs. Radiative transitions are indicated by solid arrows. Direct optical excitation of Sm3 þ ions to the 6P5/2 state is also illustrated.
SPR shows with the PL quenching, which becomes patent in the comparison of Fig. 1 with Fig. 4. This result is in agreement with several reports where the presence of NPs resulted in the quenching of the PL of Sm3 þ [29,30] Eu3 þ [6,31] and Dy3 þ ions [11] in Ag nanocomposites. A decrease in the density of sensitizers, i.e. the number of nPCs, could lead to a reduction in the degree of enhancement in connection to the maximum reached at 50 min of holding time. However, in the absence of the generation of a PL quencher, a PL decrease should not occur. In addition, the presence of the reducing agent is known to result in continuous Ag cluster generation in parallel with NP growth [21,27], thus implying a sustained production of the energy donor species. Moreover, significant Sm3 þ -Sm2 þ reduction (Sm3 þ /Sm2 þ , E1¼ 1.55 V [32]) can be also neglected based on the fact that an XPS study carried out for europium (higher Eu3 þ /Eu2 þ reduction potential, E1 ¼ 0.35 V [32]) did not show Eu2 þ formation as a result of thermal treatment of the glass having reducing agent tin at the same concentration [31]. We also take into account that, whereas the peak excitation employed at 420 nm is resonant with both Sm3 þ and Ag NPs absorption, Sm3 þ emission bands around 643, 597 and 562 nm, are out of resonance with the Ag NP plasmon absorption. Thus, an ion-to-particle excitation energy transfer is indicated to be the main cause of the PL quenching effect, where the NPs appear to effectively provide paths for the nonradiative loss of excitation energy in Sm3 þ ions through coupling with the plasmon resonance [6,11,13]. The schematic shown in Fig. 5 synthesizes the indicated energy transfer processes related to the time-dependent PL enhancement and quenching. The energy level diagram for the Ag nPCs is simplified by including only an excited state in resonance with the employed peak excitation (420 nm), e.g. an excited state in Ag2 [26]. Also illustrated is optical absorption resulting in direct excitation of Sm3 þ ions to the 6P5/2 state (also relevant to excitation at 420 nm) within the Sm3 þ energy level structure [23]. Herein, optical excitation of the small Ag nPCs, the energy donors, and a subsequent resonant energy transfer to Sm3 þ ions (e.g. to 6P5/2 state), the acceptors, can result in populating the 4G5/2 emitting state in addition to the direct optical excitation. This is the situation likely dominating in the absence of the plasmonic NPs and therefore results in the PL enhancement early during thermal treatment. The tendency in PL increase observed before 50 min is likely due to the increase in
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the density of the small silver clusters acting as energy donors. On the other hand, excited states in Sm3 þ ions in resonance with the Ag NPs absorption (e.g. 6P5/2) can be depopulated by coupling with surface plasmons in the NPs [11,13], which subsequently decay nonradiatively. Thus, in this scenario, Sm3 þ ions assume the role of energy donors, with NPs as acceptors. This becomes dominant with the progression of the Ag cluster growth into the plasmonic regime. Such quenching effect can take place for Sm3 þ ions excited directly by the optical source or else via energy transfer from the nPCs. A dynamic situation can be envisioned, involving an enhancement vs. quenching competition among the co-existent non-plasmonic and plasmonic Ag particles, alongside with the direct optical excitation of the Sm3 þ ions. It should be pointed out that the diagram in Fig. 3 is tentatively showing excitation of presumably an excited state in Ag2 and the 6P5/2 state in Sm3 þ in connection to the peak excitation wavelength employed at 420 nm (2.95 eV). However, other possibilities may arise due to the range of wavelengths provided by the filter bandwidth (FWHM of 45 nm), the different energy levels carried by varied Ag cluster sizes, and the different energy levels of Sm3 þ ions. Summarizing, a time-dependent enhancement and quenching of Sm3 þ ions PL in Ag-doped phosphate glass has been revealed by the in situ optical microspectroscopy technique, practical for an ‘optical tuning’ of multi-component nanocomposite materials. Further on, the capability of the proposed technique for the concurrent collection of photoluminescence and optical absorption data provided insights into the underlying mechanisms, based on energy transfer processes. The fact that the PL enhancement and quenching appeared in connection to the development of small Ag clusters and their subsequent growth into the plasmonic regime, further suggests the use of RE ions as luminescent probes for the study of metal particle dynamics in dielectrics. Indeed, the current study opens an array of possibilities for studying a variety hosts, e.g. glasses and films of different compositions, and with different noble metal and RE dopants and their concentrations. The proposed concurrent isothermal technique is appropriate for the quest of materials/conditions for the so-called plasmonic luminescence enhancement to be realized. This could shed light into the tailoring of materials’ optical properties for a wide range of applications.
Acknowledgment M. Sendova acknowledges financial support from ARLW911NF-09-2-0004. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
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