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Enhanced photoluminescence properties of Sm3+ ions in Cu+ and Sn2+ co-doped P2O5:BaO glass
Journal of Physics and Chemistry of Solids 75 (2014) 1334–1339
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Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Enhanced photoluminescence properties of Sm3 þ ions in Cu þ and Sn2 þ co-doped P2O5:BaO glass José A. Jiménez n Department of Chemistry, University of North Florida, Jacksonville, FL 32224, USA
art ic l e i nf o
a b s t r a c t
Article history: Received 7 April 2014 Received in revised form 5 June 2014 Accepted 4 July 2014 Available online 14 July 2014
Luminescent glasses activated with Sm3 þ ions are of current interest given their potential for a wide range of photonic applications. In this work, Sm3 þ -containing P2O5:BaO glasses are prepared by a simple melt-quench method, and the influence of CuO and SnO co-doping on Sm3 þ photoluminescence (PL) is investigated. Optical absorption, solid-state 31P nuclear magnetic resonance spectroscopy, and PL spectroscopy are employed in the assessment of material optical and structural properties. The data indicates that monovalent copper ions and twofold-coordinated Sn centers are successfully stabilized in the matrix and both species can enhance the orange–red emission of Sm3 þ ions. The optical properties of the material after heat treatment have been also assessed. Results indicate the chemical reduction of ionic copper via Sn2 þ ultimately producing Cu nanoparticles as evidenced by the surface plasmon resonance. As a result, Sm3 þ PL diminishes consistent with an excitation energy transfer to plasmonic Cu particles, i.e. the “plasmonic diluent” effect prevails. & 2014 Elsevier Ltd. All rights reserved.
Keywords: A. Glasses A. Optical materials D. Luminescence D. Nuclear magnetic resonance (NMR)
1. Introduction Noble metal and rare earth (RE) doped luminescent materials are of current interest for efficient photonic conversion in solar cells and solid-state lighting applications [1,2]. In particular, glasses doped with Sm3 þ ions have received considerable attention recently given their distinctive properties making them especially valuable for color displays, lasers, and solid-state lighting [3–8]. Nevertheless, since f–f transitions in Sm3 þ (4f5) possess low absorption cross-sections, identifying suitable sensitizers for use with the Sm3 þ ions as activators is highly desirable for developing the materials for photonic applications. Accordingly, strategies have been reported for enhancing the orange–red emission of Sm3 þ in dielectrics, for instance, by co-doping with another RE ion [8], monovalent metal ions as charge compensators [9], and noble metals in different oxidation and aggregation states [4,5,7,10]. Concerning the noble metals, reports on the influence of copper species on Sm-containing glasses are scarce. Wei et al. [5] and Yang et al. [11] have investigated on Cu þ -Sm3 þ energy transfer and the generation of varied hues in Cu þ and Sm3 þ co-doped silicate-based glasses. However, the effects of various copper species such as the single Cu þ ions and Cu nanoparticles (NPs) on Sm3 þ photoluminescence (PL) in phosphate-based glass matrices remain unexplored to the best of the author’s knowledge. Phosphate glasses are exceptionally attractive matrices due to low cost, ease of preparation (e.g. by
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http://dx.doi.org/10.1016/j.jpcs.2014.07.005 0022-3697/& 2014 Elsevier Ltd. All rights reserved.
melting), and high metal solubility which allows for the tuning of material optical properties based on the dopants concentration [12,13]. Recently, the efficient stabilization of relatively large amounts of Cu þ ions in a P2O5:BaO glass matrix with relevance to the field of photonics has been demonstrated by the author [14]. Monovalent copper is successfully incorporated using redox chemicals copper(II) oxide and tin(II) oxide, by simply melting batch materials under ambient atmosphere with sucrose as antioxidant agent protecting tin. Pertinent to the field of plasmonic materials, this approach also allows for the precipitation of Cu nanoparticles during subsequent thermal treatments [15,16]. In this work, the author reports on the spectroscopic properties of the barium–phosphate matrix with added CuO, SnO, and Sm2O3 as prepared by the single-step meltquench method. Material characterization is carried out by optical absorption, solid-state 31P nuclear magnetic resonance (NMR) spectroscopy, and PL spectroscopy. The data indicates that monovalent copper ions and twofold-coordinated Sn centers are successfully stabilized in the glass matrix. Consequently, the influence of such on Sm3 þ PL is evaluated. Further, as material heat treatment (HT) results in the formation of the plasmonic Cu NPs, the effect of the nanoparticulate phase on Sm3 þ emission is also examined.
2. Materials and methods Glasses were made with a 50P2O5:50BaO (mol%) composition from high purity compounds (P2O5 and BaCO3) by the meltquenching technique as described elsewhere [14,15]. Copper, tin,
J.A. Jiménez / Journal of Physics and Chemistry of Solids 75 (2014) 1334–1339
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Table 1 Matrix composition and additive concentrations (mol%) of CuO, SnO and Sm2O3 in the studied glasses. Glass
P2O5
BaO
CuO
SnO
Sm2O3
Host CuSn CuSn–Sm Sm
50 50 50 50
50 50 50 50
– 10 10 –
– 10 10 –
– – 2 2
and/or samarium doping was done by adding CuO, SnO, and/or Sm2O3 quantities in mol%, in relation to network former P2O5. Samples prepared for the present study are (referred to as): glass host (undoped glass system); CuSn glass (10% of each CuO and SnO added); CuSn–Sm (10% of each CuO and SnO, and 2% Sm2O3 added); and Sm glass (2% Sm2O3 added). The compositions of the glasses are summarized in Table 1. All glasses were cut and polished in order to produce glass slabs for optical measurements with final thicknesses of about 1.0 mm. A CuSn–Sm glass sample was further subjected to HT at 500 1C for 40 min. Optical absorption measurements were performed using a Perkin-Elmer 35 ultraviolet (UV)/visible double-beam spectrophotometer. All absorption spectra were recorded with air as reference. Photoluminescence spectra and emission decay curves were obtained with a Photon Technology International QuantaMaster 30 spectrofluorometer equipped with a Xenon flash lamp having a pulse width of about 2 ms (L4633, Hamamatsu Photonics K.K.) and a photomultiplier tube (R1527P, Hamamatsu Photonics K. K.). Emission and excitation spectra were acquired with the flash lamp operating at a frequency of 125 Hz with the total period of data collection set to 8 ms. The step size used for all spectral acquisitions was 1 nm. All PL measurements were recorded with samples mounted in a solid sample holder at an angle of 401 with particular attention given to keep conditions constant during experiments. Solid-state 31P NMR analyses were performed with an Agilent VNMRS 500 MHz NMR spectrometer equipped with a 3.2 mm T3 NB HXY probe. The spin rate was 15 kHz and the Larmor frequency 202.28 MHz. Kel-F rotors were used for sample preparation. Chemical shifts are relative to a 85% H3PO4 solution at 0 ppm. All measurements were carried out at room temperature.
3. Results and discussion 3.1. Optical absorption and
31
P NMR spectroscopy evaluation
Fig. 1 shows optical absorption for the CuSn–Sm glass together with spectra for the host and CuSn glasses for comparison. Apart from the absorption peaks characteristic of f–f electronic transitions in Sm3 þ (e.g. absorption peak around 400 nm), the CuSn–Sm glass exhibits a significant redshift in glass absorption edge relative to the host, similarly observed for the CuSn glass. While some contribution is expected from twofold-coordinated Sn centers (referred to as ¼Sndd where¼ represents the bonds with two oxygen atoms and dd represents the two paired electrons) [16,17], the main species accountable for the near-UV absorption are the Cu þ ions incorporated into the matrix [14–16]. Monovalent copper ions absorb UV light by virtue of the 3d10-3d94s1 electronic transitions, which although parity-forbidden in nature, become partially allowed in solids due to ion-lattice interactions. The broad character of the absorption results from the fact that Cu þ ions occupy a wide variety of sites in the amorphous host [18]. This is further connected to the structural properties of the material as revealed by NMR spectroscopy (vide infra). More into the visible, an absorption feature rises after 600 nm for the CuSn–Sm and CuSn glasses. Such band is due to the presence of Cu2 þ ions which
Fig. 1. Optical absorption spectra of the phosphate host, CuSn and CuSn–Sm glasses. The inset shows the plot of peak optical density of the Cu2 þ band vs. concentration of CuO added to the reference glasses (squares) reported in Ref. [14]; the solid line is the linear fit to the data. The triangle is the data point obtained for the CuSn–Sm ( 0.2 mol% residual CuO) glass as estimated from its Cu2 þ peak optical absorption value and the use of the equation generated from the linear regression.
absorb broadly owing to 2E-2T2 intra-configurational (d–d) transitions [14,19]. It can be employed for analytical purposes by use of absorption spectra of CuO-containing reference glasses as reported in Ref. [14]. In such spectroscopic approach of chemical analysis, a plot of the peak optical density of the Cu2 þ band around 850 nm vs. the amount of CuO added to the reference glasses is constructed. Such plot, based on reported data [14] is shown in the inset of Fig. 1, which can be treated as a calibration curve following Beer’s Law. Thus, from the equation of the line, the residual amount of copper(II) in the CuSn–Sm glass can be estimated. The assessment has been applied to the CuSn glass [14], yielding an amount of copper(II) after the melting corresponding to 0.20 mol% copper(II) oxide. As observed in Fig. 1, the Cu2 þ absorption band of the CuSn–Sm glass appears similar in intensity to that of the CuSn glass. Herein, by means of the equation of the line (Fig. 1, inset) resulting from the regression analysis and the experimental Cu2 þ optical density at 850 nm, a corresponding amount of 0.18 mol% copper(II) oxide is estimated for the CuSn–Sm glass. The value is plotted together with the calibration curve in the inset of Fig. 1. Remarkably, the residual CuO quantities estimated for both the CuSn–Sm and CuSn glasses represent about 2% of the nominal amount of CuO added initially, consistent with a high reduction efficiency during the melting process [14]. Hence, the effective stabilization of monovalent copper ions in the CuSn–Sm glass is suggested, consistent with the substantial redshift in UV-edge presented. Presented in Fig. 2 is the 31P NMR spectrum obtained for the CuSn–Sm glass along with spectra for the host and CuSn glasses as reference. The spectrum for the glass host shows a main peak around 24.1 ppm, with a second peak of lower intensity around 9.1 ppm, characteristic of the Q2 (PO4 tetrahedra with two bridging oxygens) and Q1 species (PO4 tetrahedra with one bridging oxygen), respectively [14,20]. The spectra obtained for the CuSn and CuSn–Sm glasses also display the two-peak feature, where an increase in the intensity of the small peak at higher chemical shift is noticeable with addition of dopants. The characteristic Q2 and Q1 bands which appear overlapping can be deconvoluted in order to obtain information about the relative amount of the tetrahedral species in the glasses [20]. Such a deconvolution into Gaussian components has been performed for the three glasses for comparison. The parameters extracted from decomposing the spectra are given in Table 2. The resulting
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Fig. 2.
J.A. Jiménez / Journal of Physics and Chemistry of Solids 75 (2014) 1334–1339
31
P NMR spectra of the phosphate host, CuSn and CuSn–Sm glasses.
Table 2 Chemical shifts (δ), full-width at half-maximum (FWHM), and relative amounts of Qn sites in the host, CuSn and CuSn–Sm glasses as estimated from Gaussian deconvolutions to the 31P NMR spectra (Fig. 2). Glass
Host CuSn CuSn–Sm
Q2
Q1
δ (ppm)
FWHM
%
δ (ppm)
FWHM
%
24.4 24.1 23.8
10.7 9.8 11.7
87.9 73.3 70.3
9.6 10.3 10.2
7.8 11.1 9.7
12.1 26.7 29.7
relative percent of Q1 sites for the host is 12.1%. Addition of 10 mol % CuO and SnO produces a significant increase in the relative percent of Q1 sites in the CuSn glass, rising to 26.7%. This behavior is indicative of a shortening of the phosphate chains induced primarily by copper in agreement with a network modifier role of the metal cations [21]. Further, the relative percent of Q1 sites increases to 29.7% for the CuSn–Sm glass. This is consistent with a minor role of Sm3 þ ions as network modifiers given their lower concentration relative to copper. In regard to bandwidths, some broadening is reflected for the band associated to the Q2 species for the CuSn–Sm glass, likely related to a wider distribution of the corresponding chain lengths. In addition, increased full-width at half-maximum (FWHM) values are reflected in relation to the Q1 sites for both the CuSn and CuSn–Sm glasses. Such broadening is in agreement with the association of cations with the Q1 chain-end groups producing an increase in the chemical shift range. However, a somewhat larger FWHM is reflected for the CuSn glass in relation to the Q1 sites, suggesting a slightly less uniform distribution of the corresponding chain lengths. Still, the chemical shifts extracted in association with both Q2 and Q1 species remain relatively stable in both the CuSn and CuSn–Sm glasses. Accordingly, an effective dispersion of copper ions via the increase in non-bridging oxygen terminals is indicated to be realized in the P2O5:BaO matrix and it containing Sm3 þ as well, consistent with a high metal solubility of the glass system. Hence, the monovalent copper ions are stabilized in the CuSn–Sm glass in concurrence with the optical absorption data (vide supra), which allows for the assessment of their influence on Sm3 þ PL. 3.2. Photoluminescence properties of melt-quenched CuSn–Sm glass Concerning PL properties, the recent investigation on the CuSn glass has revealed two distinct emitting centers, namely¼ Sndd and single Cu þ ions [14]. The excitation-wavelength dependence of the PL is such that under short-wavelength excitation (e.g.
Fig. 3. PL emission spectra obtained for the CuSn–Sm glass under excitation at displayed wavelengths. The inset shows the PL excitation spectrum for the glass obtained by monitoring emission at 540 nm; the spectrum is deconvoluted into two Gaussian components (dotted curves).
260 nm), broadband emission is observed towards the blue having a significant contribution from¼Sndd owing to the triplet-tosinglet (T1-S0) transitions [14]. However, with increasing excitation wavelength, emission shifts to the red, which is indicative of increasing contributions from 3d94s1-3d10 transitions in Cu þ ions to band emission. Ultimately, under excitation at 360 nm and longer wavelengths, the emission spectra for the CuSn glass display single-band behavior in agreement with emission arising merely from the Cu þ ions [14]. Shown in Fig. 3 are PL spectra for the CuSn–Sm glass obtained under excitation at 260, 280, 300, and 360 nm. Besides the observed emission from the 4G5/2-6H5/2, 6 H7/2, 6H9/2 transitions in Sm3 þ ions, the evolution in the broad PL feature in the spectra resembles aforementioned behavior. A contribution from¼ Sndd to the blue of the emission bands resulting from excitation at 260, 280 and 300 nm is evident, while excitation at 360 nm produces the single-band behavior associated with Cu þ . Further, the two luminescent species become patent in the excitation spectrum shown in the inset of Fig. 3, recorded by monitoring emission at the intermediate wavelength of 540 nm. The spectrum displays a maximum around 295 nm with a shoulder around 350 nm, indicative of the two distinct emitting centers. In fact, the spectrum is effectively deconvoluted into two Gaussian components with peaks around 290 and 345 nm having FWHM values of 50 and 85 nm, respectively. These two peaks match well with the expected contributions of ¼Sndd (S0-S1 transitions) and Cu þ ions (3d10-3d94s1 transitions) at high and low energy, respectively [14,16]. Consistently, excitation spectra collected (not shown) by monitoring band emission at 500/ 580 nm resulted in a relative increase/decrease in the ¼Sndd contribution to shorter wavelengths and decrease/increase in the Cu þ contribution to longer wavelengths. Being elucidated the nature of the tin- and copper-related luminescent centers in the CuSn–Sm glass as¼ Sndd and single Cu þ ions, let us now turn our attention to assessing the influence of these on Sm3 þ PL. As inferred from Fig. 3, Sm3 þ emission observed under non-resonant excitation at 260 and 280 nm suggests the presence of an energy transfer process taking place from ¼ Sndd and/or Cu þ ions which results in populating the 4G5/2 state in Sm3 þ ions. Fig. 4 shows excitation spectra collected for the CuSn–Sm and Sm (reference) glasses obtained by monitoring emission from the 4G5/2-6H7/2 transition in Sm3 þ at 600 nm. Certainly, besides the expected Sm3 þ peaks [22], the spectrum for the CuSn–Sm glass shows an additional broad excitation feature covering from about 250 to 400 nm with a peak around 295 nm. Interestingly, it resembles the convoluted excitation characteristic
J.A. Jiménez / Journal of Physics and Chemistry of Solids 75 (2014) 1334–1339
Fig. 4. Excitation spectra of Sm and CuSn–Sm glasses obtained by monitoring emission of 4G5/2-6H7/2 transition in Sm3 þ at 600 nm.
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discriminate the contribution to the Sm3 þ PL enhancement from¼Sndd largely at 290 nm (lies near the excitation maximum associated to¼Sndd, Fig. 3, inset), and exclusively that of Cu þ at 353 nm (excitation of ¼Sndd not expected, Fig. 3, inset). Since excitation at 290 nm is completely out of resonance with Sm3 þ excitation peaks (Fig. 4), no significant Sm3 þ emission is observed from the Sm glass in Fig. 5(a). Conversely, the spectrum recorded for the CuSn–Sm glass under excitation at 290, displays significant Sm3 þ PL from 4G5/2-6H5/2, 6H7/2, 6H9/2 transitions along with the tin/copper-related broad emission background. Herein, considerable excitation of¼ Sndd is expected, suggesting a UV-sensitized Sm3 þ PL in association to the presence of¼ Sndd, as similarly reported for Dy3 þ in the barium–phosphate matrix [17]. Accordingly, Sm3 þ emission observed under non-resonant excitation at 260 and 280 nm in Fig. 3 seems mostly induced by energy transfer from¼Sndd. These results are also in agreement with the previously suggested role of the tin centers to the improved PL of Sm3 þ in Ag2O/SnO co-doped aluminophosphate glass [22]. Yet, the influence of Cu þ ions under short-wavelength excitation cannot be wholly excluded in accord with the excitation properties indicated by the deconvolution of the spectrum in the inset of Fig. 3. On the other hand, under the excitation at 353 nm as shown in Fig. 5(b), the influence of Cu þ ions can be isolated. Evidently, along with the background emission arising from the Cu þ ions, an enhanced Sm3 þ PL (about three times) is still noticeable. Thus, it becomes patent that excitation of either ¼Sndd or Cu þ ions leads to an enhanced Sm3 þ emission in the phosphate matrix. However, a more significant effect from¼Sndd is suggested by the PL emission data considered in Fig. 3 and Fig. 5(a), together with the fact that the new excitation feature in Fig. 4 displays a maximum around 295 nm connected to tin. This is conceivable given that a greater spectral overlap exists between Sm3 þ absorption and ¼Sndd emission than with Cu þ emission within the 400 to 500 nm range as inferred by comparison of Fig. 3 with Fig. 4.
3.3. Influence of heat treatment on optical properties of CuSn–Sm glass In evaluating the effect of thermal processing on the CuSn–Sm glass, we first consider optical absorption spectra for the glass before and after HT as shown in Fig. 6. An intense absorption band is observed around 565 nm after HT, which certainly is the characteristic surface plasmon resonance (SPR) of Cu NPs [15,16,23]. This clearly indicates their effective precipitation in the matrix. The rise in absorption exhibited at about 530 nm towards shorter wavelengths is due to the interband transitions in the metallic particles, as the threshold for the excitation of 3d-band electrons into the 4sp
Fig. 5. PL emission spectra for the Sm and CuSn–Sm glasses obtained under excitation at: (a) 290 nm; and (b) 353 nm.
of ¼Sndd and Cu þ ions in the inset of Fig. 3. Thus, the data suggests a UV-sensitized Sm3 þ PL in association to the presence of both ¼Sndd and Cu þ ions. This is further verified by the emission spectra presented in Fig. 5(a) and (b), recorded for the CuSn–Sm and Sm glasses under excitation at 290 and 353 nm, respectively. These wavelengths are chosen first because they are away from Sm3 þ excitation peaks, so emission from the Sm glass reference would be minimized allowing for a marked distinction of PL enhancements in connection to energy transfer. And second, because these wavelengths provide the opportunity to
Fig. 6. Optical absorption spectra of CuSn–Sm glass before and after the HT.
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J.A. Jiménez / Journal of Physics and Chemistry of Solids 75 (2014) 1334–1339
conduction band in copper is about 2.1 eV ( 590 nm) [24]. Hence, copper ions were reduced to the neutral state during HT and the Cu0 atoms produced were aggregated into NPs. An estimate for the mean particle radius, R, can be obtained from the spectrum assuming a sharp size distribution within the dipole regime of Mie theory as R ¼ νf =Δω1=2
ð1Þ
where νf is the Fermi velocity of the metal (1.57 108 cm/s for Cu) and Δω1=2 is the FWHM of the absorption band in units of angular frequency [15,16]. The equation yields a particle radius of 1.1 nm from the absorption data. The PL properties after the HT can be assessed from the spectra presented in Fig. 7(a)–(c) obtained for the excitation wavelengths
considered previously in Fig. 5, i.e. 290 and 353 nm, and in addition for 485 nm. The latter was selected as it can excite Sm3 þ ions away from the excitation of Sn centers and Cu þ ions (Fig. 4), and thus, can provide a more direct way for evaluating the effect of the plasmonic NPs. For the excitation at 290 nm, the broad band background appears suppressed to the low energy side, consistent with a decrease in the concentration of the luminescent Cu þ ions. This is further confirmed exciting at 353 nm where the broad emission which arises merely from monovalent copper is uniformly suppressed. This is consistent with the optical absorption data in Fig. 6, and supports that the following reaction succeeds upon HT 2Cu þ þ Sn2 þ -2Cu0 þ Sn4 þ
ð2Þ
with the consequence that the Cu0 atoms aggregate and form Cu NPs. In addition, the PL spectra of the CuSn–Sm glass obtained under excitation at 290 and 353 nm present diminished Sm3 þ emission bands after HT. Herein, a smaller degree of enhancement associated to excitation of species participating in energy transfer is expected in correspondence to their depletion (e.g. Cu þ ). However, a quenching effect induced by the plasmonic Cu particles is also likely [15]. This possibility is further tested by exciting at 485 nm as shown in Fig. 7 (c), where it is clearly observed that the Sm3 þ PL in the heat-treated glass is weakened at least two times. Thus, even though there is a reasonable matching between the 4G5/2-6H5/2, 6H7/2 transitions in Sm3 þ around 565 and 600 nm, respectively, and the broad SPR band of Cu NPs around 565 nm, no plasmonic enhancement of Sm3 þ emission is exhibited. The PL weakening manifested in its place is likely connected to the recently established “plasmonic diluent” effect [25]. Such effect denotes that substantial inclusion of metallic NPs in a RE co-doped matrix has an outcome analogous to the decrease in concentration of the luminescent RE ions by deactivating the ions via excitation energy transfer [15,25]. It is herein indicated to operate via interband transitions in the nanoscale copper particles as inferred from Fig. 6, consistent with the analogous result reported for Dy3 þ in Cu nanocomposite glasses [15]. Thus, overall the HT acts against the enhancement of Sm3 þ PL induced by copper and tin co-doping in the melt-quenched CuSn–Sm glass.
4. Conclusion
Fig. 7. PL emission spectra for the CuSn–Sm glass before and after the HT obtained under excitation at: (a) 290 nm; (b) 353 nm; and (c) 485 nm.
An investigation of the optical and structural properties of a barium–phosphate glass matrix co-doped with CuO, SnO and Sm2O3 as prepared by the melt-quenching technique was carried out. The spectroscopic assessment indicated the effective incorporation of single Cu þ ions and twofold-coordinated Sn centers as the copperand tin-related luminescent species. A remarkable emission from Sm3 þ ions was realized under wide-range UV excitation as a consequence of the co-doping. It can be concluded that both Cu þ ions and Sn centers act as sensitizers of Sm3 þ activator ions. A major role from the Sn centers has been suggested under deep-UV excitation. However, the Cu þ ions were shown to produce the enhancement of Sm3 þ emission exclusively under near-UV excitation. Thermal processing was shown to result in the chemical reduction of ionic copper species via Sn2 þ , resulting in the formation of plasmonic Cu particles and decreasing the contribution of Cu þ ions to material luminescence. On the other hand, Sm3 þ luminescence was quenched after HT, indicative of the Cu NPs effectively providing paths for the nonradiative loss of excitation energy in Sm3 þ , i.e. the “plasmonic diluent” effect supersedes. Accordingly, the optimum luminescent properties in the Cu þ /Sn2 þ /Sm3 þ co-doped glass were attained for the material as melted. The current results may be useful for the development of luminescent materials for efficient photonic conversion relevant to solar cells and lighting applications.
J.A. Jiménez / Journal of Physics and Chemistry of Solids 75 (2014) 1334–1339
Acknowledgements The author is grateful to Dr. Chunqing Zhao, Manager of the Analytical Instrumentation Facility in the Chemistry Department at UNF for solid-state 31P NMR spectroscopy experiments. References [1] X. Huang, S. Han, W. Huang, X. Liu, Chem. Soc. Rev. 42 (2013) 173. [2] A.S. Kuznetsov, V.K. Tikhomirov, M.V. Shestakov, V.V. Moshchalkov, Nanoscale 5 (2013) 10065. [3] V. Singh, S. Watanabe, T.K. Gundu Rao, J.F.D. Chubaci, H.-Y. Kwak, J. Non-Cryst. Solids 356 (2010) 1185. [4] J.A. Jiménez, M. Sendova, Solid State Commun. 152 (2012) 1786. [5] R.F. Wei, C.G. Ma, Y.L. Wei, J.Y. Gao, H. Guo, Opt. Express 20 (2012) 29743. [6] P. Kaur, S. Kaur, G. Pal Singh, D.P. Singh, Solid State Commun. 171 (2013) 22. [7] L. Li, Y. Yang, D. Zhou, Z. Yang, X. Xu, J. Qiu, J. Appl. Phys. 113 (2013) 193103. [8] P. Kaur, S. Kaur, G. Pal Singh, D.P. Singh, J. Alloys Compd. 588 (2014) 394. [9] Z.-w. Zhang, Y.-n. Wu, X.-h. Shen, Y.-j. Ren, W.-g. Zhang, D.-j. Wang, Opt. Laser Technol. 62 (2014) 63.
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[10] G. Kaur, R.K. Verma, D.K. Rai, S.B. Rai, J. Lumin. 132 (2012) 1683. [11] J.W. Yang, H. Guo, Y.L. Wei, H.M. Noh, J.H. Jeong, Opt. Mater. Express 4 (2014) 315. [12] M. Yamane, Y. Asahara, Glasses for Photonics, Cambridge University Press, UK, 2000. [13] J.A. Jiménez, M. Sendova, H. Liu, F.E. Fernández, Plasmonics 6 (2011) 399. [14] J.A. Jiménez, J. Mater. Sci. 49 (2014) 4387. [15] J.A. Jiménez, J.B. Hockenbury, J. Mater. Sci. 48 (2013) 6921. [16] J.A. Jiménez, Appl. Phys. A114 (2014) 1369. [17] J.A. Jiménez, J. Non-Cryst. Solids 387 (2014) 124. [18] H. Liu, F. Gan, J. Non-Cryst. Solids 80 (1986) 447. [19] T. Murata, K. Morinaga, Proc. SPIE 4102 (2000) 316. [20] L. Koudelka, I. Rösslerová, J. Holubová, P. Mošner, L. Montagne, B. Revel, J. NonCryst. Solids 357 (2011) 2816. [21] J.A. Jiménez, C. Zhao, Mater. Chem. Phys. 147 (3) (2014) 469. [22] J.A. Jiménez, S. Lysenko, H. Liu, M. Sendova, Opt. Mater. 33 (2011) 1215. [23] K. Uchida, S. Kaneko, S. Omi, C. Hata, H. Tanji, Y. Asahara, A.J. Ikushima, T. Tokisaki, A. Nakamura, J. Opt. Soc. Am. B: Opt. Phys. 11 (1994) 1236. [24] U. Kreibig, M. Vollmer, Optical Properties of Metal Clusters, Springer, Berlin, 1995. [25] J.A. Jiménez, Phys. Chem. Chem. Phys. 15 (2013) 17587.
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Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Enhanced photoluminescence properties of Sm3 þ ions in Cu þ and Sn2 þ co-doped P2O5:BaO glass José A. Jiménez n Department of Chemistry, University of North Florida, Jacksonville, FL 32224, USA
art ic l e i nf o
a b s t r a c t
Article history: Received 7 April 2014 Received in revised form 5 June 2014 Accepted 4 July 2014 Available online 14 July 2014
Luminescent glasses activated with Sm3 þ ions are of current interest given their potential for a wide range of photonic applications. In this work, Sm3 þ -containing P2O5:BaO glasses are prepared by a simple melt-quench method, and the influence of CuO and SnO co-doping on Sm3 þ photoluminescence (PL) is investigated. Optical absorption, solid-state 31P nuclear magnetic resonance spectroscopy, and PL spectroscopy are employed in the assessment of material optical and structural properties. The data indicates that monovalent copper ions and twofold-coordinated Sn centers are successfully stabilized in the matrix and both species can enhance the orange–red emission of Sm3 þ ions. The optical properties of the material after heat treatment have been also assessed. Results indicate the chemical reduction of ionic copper via Sn2 þ ultimately producing Cu nanoparticles as evidenced by the surface plasmon resonance. As a result, Sm3 þ PL diminishes consistent with an excitation energy transfer to plasmonic Cu particles, i.e. the “plasmonic diluent” effect prevails. & 2014 Elsevier Ltd. All rights reserved.
Keywords: A. Glasses A. Optical materials D. Luminescence D. Nuclear magnetic resonance (NMR)
1. Introduction Noble metal and rare earth (RE) doped luminescent materials are of current interest for efficient photonic conversion in solar cells and solid-state lighting applications [1,2]. In particular, glasses doped with Sm3 þ ions have received considerable attention recently given their distinctive properties making them especially valuable for color displays, lasers, and solid-state lighting [3–8]. Nevertheless, since f–f transitions in Sm3 þ (4f5) possess low absorption cross-sections, identifying suitable sensitizers for use with the Sm3 þ ions as activators is highly desirable for developing the materials for photonic applications. Accordingly, strategies have been reported for enhancing the orange–red emission of Sm3 þ in dielectrics, for instance, by co-doping with another RE ion [8], monovalent metal ions as charge compensators [9], and noble metals in different oxidation and aggregation states [4,5,7,10]. Concerning the noble metals, reports on the influence of copper species on Sm-containing glasses are scarce. Wei et al. [5] and Yang et al. [11] have investigated on Cu þ -Sm3 þ energy transfer and the generation of varied hues in Cu þ and Sm3 þ co-doped silicate-based glasses. However, the effects of various copper species such as the single Cu þ ions and Cu nanoparticles (NPs) on Sm3 þ photoluminescence (PL) in phosphate-based glass matrices remain unexplored to the best of the author’s knowledge. Phosphate glasses are exceptionally attractive matrices due to low cost, ease of preparation (e.g. by
n
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http://dx.doi.org/10.1016/j.jpcs.2014.07.005 0022-3697/& 2014 Elsevier Ltd. All rights reserved.
melting), and high metal solubility which allows for the tuning of material optical properties based on the dopants concentration [12,13]. Recently, the efficient stabilization of relatively large amounts of Cu þ ions in a P2O5:BaO glass matrix with relevance to the field of photonics has been demonstrated by the author [14]. Monovalent copper is successfully incorporated using redox chemicals copper(II) oxide and tin(II) oxide, by simply melting batch materials under ambient atmosphere with sucrose as antioxidant agent protecting tin. Pertinent to the field of plasmonic materials, this approach also allows for the precipitation of Cu nanoparticles during subsequent thermal treatments [15,16]. In this work, the author reports on the spectroscopic properties of the barium–phosphate matrix with added CuO, SnO, and Sm2O3 as prepared by the single-step meltquench method. Material characterization is carried out by optical absorption, solid-state 31P nuclear magnetic resonance (NMR) spectroscopy, and PL spectroscopy. The data indicates that monovalent copper ions and twofold-coordinated Sn centers are successfully stabilized in the glass matrix. Consequently, the influence of such on Sm3 þ PL is evaluated. Further, as material heat treatment (HT) results in the formation of the plasmonic Cu NPs, the effect of the nanoparticulate phase on Sm3 þ emission is also examined.
2. Materials and methods Glasses were made with a 50P2O5:50BaO (mol%) composition from high purity compounds (P2O5 and BaCO3) by the meltquenching technique as described elsewhere [14,15]. Copper, tin,
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Table 1 Matrix composition and additive concentrations (mol%) of CuO, SnO and Sm2O3 in the studied glasses. Glass
P2O5
BaO
CuO
SnO
Sm2O3
Host CuSn CuSn–Sm Sm
50 50 50 50
50 50 50 50
– 10 10 –
– 10 10 –
– – 2 2
and/or samarium doping was done by adding CuO, SnO, and/or Sm2O3 quantities in mol%, in relation to network former P2O5. Samples prepared for the present study are (referred to as): glass host (undoped glass system); CuSn glass (10% of each CuO and SnO added); CuSn–Sm (10% of each CuO and SnO, and 2% Sm2O3 added); and Sm glass (2% Sm2O3 added). The compositions of the glasses are summarized in Table 1. All glasses were cut and polished in order to produce glass slabs for optical measurements with final thicknesses of about 1.0 mm. A CuSn–Sm glass sample was further subjected to HT at 500 1C for 40 min. Optical absorption measurements were performed using a Perkin-Elmer 35 ultraviolet (UV)/visible double-beam spectrophotometer. All absorption spectra were recorded with air as reference. Photoluminescence spectra and emission decay curves were obtained with a Photon Technology International QuantaMaster 30 spectrofluorometer equipped with a Xenon flash lamp having a pulse width of about 2 ms (L4633, Hamamatsu Photonics K.K.) and a photomultiplier tube (R1527P, Hamamatsu Photonics K. K.). Emission and excitation spectra were acquired with the flash lamp operating at a frequency of 125 Hz with the total period of data collection set to 8 ms. The step size used for all spectral acquisitions was 1 nm. All PL measurements were recorded with samples mounted in a solid sample holder at an angle of 401 with particular attention given to keep conditions constant during experiments. Solid-state 31P NMR analyses were performed with an Agilent VNMRS 500 MHz NMR spectrometer equipped with a 3.2 mm T3 NB HXY probe. The spin rate was 15 kHz and the Larmor frequency 202.28 MHz. Kel-F rotors were used for sample preparation. Chemical shifts are relative to a 85% H3PO4 solution at 0 ppm. All measurements were carried out at room temperature.
3. Results and discussion 3.1. Optical absorption and
31
P NMR spectroscopy evaluation
Fig. 1 shows optical absorption for the CuSn–Sm glass together with spectra for the host and CuSn glasses for comparison. Apart from the absorption peaks characteristic of f–f electronic transitions in Sm3 þ (e.g. absorption peak around 400 nm), the CuSn–Sm glass exhibits a significant redshift in glass absorption edge relative to the host, similarly observed for the CuSn glass. While some contribution is expected from twofold-coordinated Sn centers (referred to as ¼Sndd where¼ represents the bonds with two oxygen atoms and dd represents the two paired electrons) [16,17], the main species accountable for the near-UV absorption are the Cu þ ions incorporated into the matrix [14–16]. Monovalent copper ions absorb UV light by virtue of the 3d10-3d94s1 electronic transitions, which although parity-forbidden in nature, become partially allowed in solids due to ion-lattice interactions. The broad character of the absorption results from the fact that Cu þ ions occupy a wide variety of sites in the amorphous host [18]. This is further connected to the structural properties of the material as revealed by NMR spectroscopy (vide infra). More into the visible, an absorption feature rises after 600 nm for the CuSn–Sm and CuSn glasses. Such band is due to the presence of Cu2 þ ions which
Fig. 1. Optical absorption spectra of the phosphate host, CuSn and CuSn–Sm glasses. The inset shows the plot of peak optical density of the Cu2 þ band vs. concentration of CuO added to the reference glasses (squares) reported in Ref. [14]; the solid line is the linear fit to the data. The triangle is the data point obtained for the CuSn–Sm ( 0.2 mol% residual CuO) glass as estimated from its Cu2 þ peak optical absorption value and the use of the equation generated from the linear regression.
absorb broadly owing to 2E-2T2 intra-configurational (d–d) transitions [14,19]. It can be employed for analytical purposes by use of absorption spectra of CuO-containing reference glasses as reported in Ref. [14]. In such spectroscopic approach of chemical analysis, a plot of the peak optical density of the Cu2 þ band around 850 nm vs. the amount of CuO added to the reference glasses is constructed. Such plot, based on reported data [14] is shown in the inset of Fig. 1, which can be treated as a calibration curve following Beer’s Law. Thus, from the equation of the line, the residual amount of copper(II) in the CuSn–Sm glass can be estimated. The assessment has been applied to the CuSn glass [14], yielding an amount of copper(II) after the melting corresponding to 0.20 mol% copper(II) oxide. As observed in Fig. 1, the Cu2 þ absorption band of the CuSn–Sm glass appears similar in intensity to that of the CuSn glass. Herein, by means of the equation of the line (Fig. 1, inset) resulting from the regression analysis and the experimental Cu2 þ optical density at 850 nm, a corresponding amount of 0.18 mol% copper(II) oxide is estimated for the CuSn–Sm glass. The value is plotted together with the calibration curve in the inset of Fig. 1. Remarkably, the residual CuO quantities estimated for both the CuSn–Sm and CuSn glasses represent about 2% of the nominal amount of CuO added initially, consistent with a high reduction efficiency during the melting process [14]. Hence, the effective stabilization of monovalent copper ions in the CuSn–Sm glass is suggested, consistent with the substantial redshift in UV-edge presented. Presented in Fig. 2 is the 31P NMR spectrum obtained for the CuSn–Sm glass along with spectra for the host and CuSn glasses as reference. The spectrum for the glass host shows a main peak around 24.1 ppm, with a second peak of lower intensity around 9.1 ppm, characteristic of the Q2 (PO4 tetrahedra with two bridging oxygens) and Q1 species (PO4 tetrahedra with one bridging oxygen), respectively [14,20]. The spectra obtained for the CuSn and CuSn–Sm glasses also display the two-peak feature, where an increase in the intensity of the small peak at higher chemical shift is noticeable with addition of dopants. The characteristic Q2 and Q1 bands which appear overlapping can be deconvoluted in order to obtain information about the relative amount of the tetrahedral species in the glasses [20]. Such a deconvolution into Gaussian components has been performed for the three glasses for comparison. The parameters extracted from decomposing the spectra are given in Table 2. The resulting
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Fig. 2.
J.A. Jiménez / Journal of Physics and Chemistry of Solids 75 (2014) 1334–1339
31
P NMR spectra of the phosphate host, CuSn and CuSn–Sm glasses.
Table 2 Chemical shifts (δ), full-width at half-maximum (FWHM), and relative amounts of Qn sites in the host, CuSn and CuSn–Sm glasses as estimated from Gaussian deconvolutions to the 31P NMR spectra (Fig. 2). Glass
Host CuSn CuSn–Sm
Q2
Q1
δ (ppm)
FWHM
%
δ (ppm)
FWHM
%
24.4 24.1 23.8
10.7 9.8 11.7
87.9 73.3 70.3
9.6 10.3 10.2
7.8 11.1 9.7
12.1 26.7 29.7
relative percent of Q1 sites for the host is 12.1%. Addition of 10 mol % CuO and SnO produces a significant increase in the relative percent of Q1 sites in the CuSn glass, rising to 26.7%. This behavior is indicative of a shortening of the phosphate chains induced primarily by copper in agreement with a network modifier role of the metal cations [21]. Further, the relative percent of Q1 sites increases to 29.7% for the CuSn–Sm glass. This is consistent with a minor role of Sm3 þ ions as network modifiers given their lower concentration relative to copper. In regard to bandwidths, some broadening is reflected for the band associated to the Q2 species for the CuSn–Sm glass, likely related to a wider distribution of the corresponding chain lengths. In addition, increased full-width at half-maximum (FWHM) values are reflected in relation to the Q1 sites for both the CuSn and CuSn–Sm glasses. Such broadening is in agreement with the association of cations with the Q1 chain-end groups producing an increase in the chemical shift range. However, a somewhat larger FWHM is reflected for the CuSn glass in relation to the Q1 sites, suggesting a slightly less uniform distribution of the corresponding chain lengths. Still, the chemical shifts extracted in association with both Q2 and Q1 species remain relatively stable in both the CuSn and CuSn–Sm glasses. Accordingly, an effective dispersion of copper ions via the increase in non-bridging oxygen terminals is indicated to be realized in the P2O5:BaO matrix and it containing Sm3 þ as well, consistent with a high metal solubility of the glass system. Hence, the monovalent copper ions are stabilized in the CuSn–Sm glass in concurrence with the optical absorption data (vide supra), which allows for the assessment of their influence on Sm3 þ PL. 3.2. Photoluminescence properties of melt-quenched CuSn–Sm glass Concerning PL properties, the recent investigation on the CuSn glass has revealed two distinct emitting centers, namely¼ Sndd and single Cu þ ions [14]. The excitation-wavelength dependence of the PL is such that under short-wavelength excitation (e.g.
Fig. 3. PL emission spectra obtained for the CuSn–Sm glass under excitation at displayed wavelengths. The inset shows the PL excitation spectrum for the glass obtained by monitoring emission at 540 nm; the spectrum is deconvoluted into two Gaussian components (dotted curves).
260 nm), broadband emission is observed towards the blue having a significant contribution from¼Sndd owing to the triplet-tosinglet (T1-S0) transitions [14]. However, with increasing excitation wavelength, emission shifts to the red, which is indicative of increasing contributions from 3d94s1-3d10 transitions in Cu þ ions to band emission. Ultimately, under excitation at 360 nm and longer wavelengths, the emission spectra for the CuSn glass display single-band behavior in agreement with emission arising merely from the Cu þ ions [14]. Shown in Fig. 3 are PL spectra for the CuSn–Sm glass obtained under excitation at 260, 280, 300, and 360 nm. Besides the observed emission from the 4G5/2-6H5/2, 6 H7/2, 6H9/2 transitions in Sm3 þ ions, the evolution in the broad PL feature in the spectra resembles aforementioned behavior. A contribution from¼ Sndd to the blue of the emission bands resulting from excitation at 260, 280 and 300 nm is evident, while excitation at 360 nm produces the single-band behavior associated with Cu þ . Further, the two luminescent species become patent in the excitation spectrum shown in the inset of Fig. 3, recorded by monitoring emission at the intermediate wavelength of 540 nm. The spectrum displays a maximum around 295 nm with a shoulder around 350 nm, indicative of the two distinct emitting centers. In fact, the spectrum is effectively deconvoluted into two Gaussian components with peaks around 290 and 345 nm having FWHM values of 50 and 85 nm, respectively. These two peaks match well with the expected contributions of ¼Sndd (S0-S1 transitions) and Cu þ ions (3d10-3d94s1 transitions) at high and low energy, respectively [14,16]. Consistently, excitation spectra collected (not shown) by monitoring band emission at 500/ 580 nm resulted in a relative increase/decrease in the ¼Sndd contribution to shorter wavelengths and decrease/increase in the Cu þ contribution to longer wavelengths. Being elucidated the nature of the tin- and copper-related luminescent centers in the CuSn–Sm glass as¼ Sndd and single Cu þ ions, let us now turn our attention to assessing the influence of these on Sm3 þ PL. As inferred from Fig. 3, Sm3 þ emission observed under non-resonant excitation at 260 and 280 nm suggests the presence of an energy transfer process taking place from ¼ Sndd and/or Cu þ ions which results in populating the 4G5/2 state in Sm3 þ ions. Fig. 4 shows excitation spectra collected for the CuSn–Sm and Sm (reference) glasses obtained by monitoring emission from the 4G5/2-6H7/2 transition in Sm3 þ at 600 nm. Certainly, besides the expected Sm3 þ peaks [22], the spectrum for the CuSn–Sm glass shows an additional broad excitation feature covering from about 250 to 400 nm with a peak around 295 nm. Interestingly, it resembles the convoluted excitation characteristic
J.A. Jiménez / Journal of Physics and Chemistry of Solids 75 (2014) 1334–1339
Fig. 4. Excitation spectra of Sm and CuSn–Sm glasses obtained by monitoring emission of 4G5/2-6H7/2 transition in Sm3 þ at 600 nm.
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discriminate the contribution to the Sm3 þ PL enhancement from¼Sndd largely at 290 nm (lies near the excitation maximum associated to¼Sndd, Fig. 3, inset), and exclusively that of Cu þ at 353 nm (excitation of ¼Sndd not expected, Fig. 3, inset). Since excitation at 290 nm is completely out of resonance with Sm3 þ excitation peaks (Fig. 4), no significant Sm3 þ emission is observed from the Sm glass in Fig. 5(a). Conversely, the spectrum recorded for the CuSn–Sm glass under excitation at 290, displays significant Sm3 þ PL from 4G5/2-6H5/2, 6H7/2, 6H9/2 transitions along with the tin/copper-related broad emission background. Herein, considerable excitation of¼ Sndd is expected, suggesting a UV-sensitized Sm3 þ PL in association to the presence of¼ Sndd, as similarly reported for Dy3 þ in the barium–phosphate matrix [17]. Accordingly, Sm3 þ emission observed under non-resonant excitation at 260 and 280 nm in Fig. 3 seems mostly induced by energy transfer from¼Sndd. These results are also in agreement with the previously suggested role of the tin centers to the improved PL of Sm3 þ in Ag2O/SnO co-doped aluminophosphate glass [22]. Yet, the influence of Cu þ ions under short-wavelength excitation cannot be wholly excluded in accord with the excitation properties indicated by the deconvolution of the spectrum in the inset of Fig. 3. On the other hand, under the excitation at 353 nm as shown in Fig. 5(b), the influence of Cu þ ions can be isolated. Evidently, along with the background emission arising from the Cu þ ions, an enhanced Sm3 þ PL (about three times) is still noticeable. Thus, it becomes patent that excitation of either ¼Sndd or Cu þ ions leads to an enhanced Sm3 þ emission in the phosphate matrix. However, a more significant effect from¼Sndd is suggested by the PL emission data considered in Fig. 3 and Fig. 5(a), together with the fact that the new excitation feature in Fig. 4 displays a maximum around 295 nm connected to tin. This is conceivable given that a greater spectral overlap exists between Sm3 þ absorption and ¼Sndd emission than with Cu þ emission within the 400 to 500 nm range as inferred by comparison of Fig. 3 with Fig. 4.
3.3. Influence of heat treatment on optical properties of CuSn–Sm glass In evaluating the effect of thermal processing on the CuSn–Sm glass, we first consider optical absorption spectra for the glass before and after HT as shown in Fig. 6. An intense absorption band is observed around 565 nm after HT, which certainly is the characteristic surface plasmon resonance (SPR) of Cu NPs [15,16,23]. This clearly indicates their effective precipitation in the matrix. The rise in absorption exhibited at about 530 nm towards shorter wavelengths is due to the interband transitions in the metallic particles, as the threshold for the excitation of 3d-band electrons into the 4sp
Fig. 5. PL emission spectra for the Sm and CuSn–Sm glasses obtained under excitation at: (a) 290 nm; and (b) 353 nm.
of ¼Sndd and Cu þ ions in the inset of Fig. 3. Thus, the data suggests a UV-sensitized Sm3 þ PL in association to the presence of both ¼Sndd and Cu þ ions. This is further verified by the emission spectra presented in Fig. 5(a) and (b), recorded for the CuSn–Sm and Sm glasses under excitation at 290 and 353 nm, respectively. These wavelengths are chosen first because they are away from Sm3 þ excitation peaks, so emission from the Sm glass reference would be minimized allowing for a marked distinction of PL enhancements in connection to energy transfer. And second, because these wavelengths provide the opportunity to
Fig. 6. Optical absorption spectra of CuSn–Sm glass before and after the HT.
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conduction band in copper is about 2.1 eV ( 590 nm) [24]. Hence, copper ions were reduced to the neutral state during HT and the Cu0 atoms produced were aggregated into NPs. An estimate for the mean particle radius, R, can be obtained from the spectrum assuming a sharp size distribution within the dipole regime of Mie theory as R ¼ νf =Δω1=2
ð1Þ
where νf is the Fermi velocity of the metal (1.57 108 cm/s for Cu) and Δω1=2 is the FWHM of the absorption band in units of angular frequency [15,16]. The equation yields a particle radius of 1.1 nm from the absorption data. The PL properties after the HT can be assessed from the spectra presented in Fig. 7(a)–(c) obtained for the excitation wavelengths
considered previously in Fig. 5, i.e. 290 and 353 nm, and in addition for 485 nm. The latter was selected as it can excite Sm3 þ ions away from the excitation of Sn centers and Cu þ ions (Fig. 4), and thus, can provide a more direct way for evaluating the effect of the plasmonic NPs. For the excitation at 290 nm, the broad band background appears suppressed to the low energy side, consistent with a decrease in the concentration of the luminescent Cu þ ions. This is further confirmed exciting at 353 nm where the broad emission which arises merely from monovalent copper is uniformly suppressed. This is consistent with the optical absorption data in Fig. 6, and supports that the following reaction succeeds upon HT 2Cu þ þ Sn2 þ -2Cu0 þ Sn4 þ
ð2Þ
with the consequence that the Cu0 atoms aggregate and form Cu NPs. In addition, the PL spectra of the CuSn–Sm glass obtained under excitation at 290 and 353 nm present diminished Sm3 þ emission bands after HT. Herein, a smaller degree of enhancement associated to excitation of species participating in energy transfer is expected in correspondence to their depletion (e.g. Cu þ ). However, a quenching effect induced by the plasmonic Cu particles is also likely [15]. This possibility is further tested by exciting at 485 nm as shown in Fig. 7 (c), where it is clearly observed that the Sm3 þ PL in the heat-treated glass is weakened at least two times. Thus, even though there is a reasonable matching between the 4G5/2-6H5/2, 6H7/2 transitions in Sm3 þ around 565 and 600 nm, respectively, and the broad SPR band of Cu NPs around 565 nm, no plasmonic enhancement of Sm3 þ emission is exhibited. The PL weakening manifested in its place is likely connected to the recently established “plasmonic diluent” effect [25]. Such effect denotes that substantial inclusion of metallic NPs in a RE co-doped matrix has an outcome analogous to the decrease in concentration of the luminescent RE ions by deactivating the ions via excitation energy transfer [15,25]. It is herein indicated to operate via interband transitions in the nanoscale copper particles as inferred from Fig. 6, consistent with the analogous result reported for Dy3 þ in Cu nanocomposite glasses [15]. Thus, overall the HT acts against the enhancement of Sm3 þ PL induced by copper and tin co-doping in the melt-quenched CuSn–Sm glass.
4. Conclusion
Fig. 7. PL emission spectra for the CuSn–Sm glass before and after the HT obtained under excitation at: (a) 290 nm; (b) 353 nm; and (c) 485 nm.
An investigation of the optical and structural properties of a barium–phosphate glass matrix co-doped with CuO, SnO and Sm2O3 as prepared by the melt-quenching technique was carried out. The spectroscopic assessment indicated the effective incorporation of single Cu þ ions and twofold-coordinated Sn centers as the copperand tin-related luminescent species. A remarkable emission from Sm3 þ ions was realized under wide-range UV excitation as a consequence of the co-doping. It can be concluded that both Cu þ ions and Sn centers act as sensitizers of Sm3 þ activator ions. A major role from the Sn centers has been suggested under deep-UV excitation. However, the Cu þ ions were shown to produce the enhancement of Sm3 þ emission exclusively under near-UV excitation. Thermal processing was shown to result in the chemical reduction of ionic copper species via Sn2 þ , resulting in the formation of plasmonic Cu particles and decreasing the contribution of Cu þ ions to material luminescence. On the other hand, Sm3 þ luminescence was quenched after HT, indicative of the Cu NPs effectively providing paths for the nonradiative loss of excitation energy in Sm3 þ , i.e. the “plasmonic diluent” effect supersedes. Accordingly, the optimum luminescent properties in the Cu þ /Sn2 þ /Sm3 þ co-doped glass were attained for the material as melted. The current results may be useful for the development of luminescent materials for efficient photonic conversion relevant to solar cells and lighting applications.
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