Carbon-Promoted in Situ Evolution of Cu Nanoclusters Influencing

Article pubs.acs.org/JPCC

Carbon-Promoted in Situ Evolution of Cu Nanoclusters Influencing Eu3+ Photoluminescence in Glass: Bidirectional Energy Transfer José A. Jiménez* Department of Chemistry, University of North Florida, Jacksonville, Florida 32224, United States ABSTRACT: The present work explores the use of carbon powder, recently proposed for producing plasmonic metal nanocomposites, as a means to obtain Eu3+ photoluminescence (PL) enhancements via Cu nanoclusters in glass. Phosphate glasses containing Eu2O3 and CuO were prepared by melting in ambient atmosphere with graphite powder added to the batch materials for the chemical reduction of copper(II). Optical absorption and PL spectroscopy characterizations, including emission decay dynamics, were performed. The data show consistently the effective reduction of Cu2+ ions via carbon during melting which ultimately leads to thermally induced copper particle precipitation. Further, the novel in situ concurrent PL and absorption microspectroscopy technique was employed for the real-time monitoring of the optical properties of the codoped glasses during thermal processing from 470 to 490 °C. Bidirectional energy transfer between europium ions and copper nanoclusters has been manifested through enhancement and quenching regimes of Eu3+ PL. These periods were observed well separated in time, favorable for the optical tuning of the solid-state luminescent material. Relating simultaneously the luminescence with the time evolution in optical absorption allowed for discriminating the effects of Cu preplasmonic clusters as energy donors and Cu nanoparticles as acceptors, to and from Eu3+ ions, respectively.

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INTRODUCTION Copper nanoclusters have generated considerable interest in various fields of research such as catalysis,1 surface-enhanced Raman scattering,2 nonlinear materials in optical media,3 and photonic applications as luminescent sensitizers.4 With respect to the latter, the presence of nonplasmonic clusters (nPCs) of the noble metal is realized in a robust transparent inorganic material together with a rare-earth (RE) metal of practical interest for optical applications such as Er3+.5 However, material preparation presents a challenge given that copper can acquire various oxidation states (Cu2+, Cu +, Cu 0), where the predominant stability is for the Cu2+ ion especially for syntheses carried out under ambient atmosphere.6,7 Hence, studies concerning the influence of Cu nPCs on other RE ions of interest coembedded in a durable dielectric matrix such as glass are currently lacking in the literature. One of the most attractive RE metals for optical applications is europium, since the emission properties of Eu3+ ions make them suitable for elaborating materials for color displays, lasers, and solid-state lighting.8,9 Relating it with copper, Reisfeld et al.10 were recently able to incorporate Cu nanoparticles (NPs) with a luminescent Eu3+ complex in a polymeric matrix. The authors observed improved luminescent properties ascribed to local field effects from surface plasmons in the copper particles. On the other hand, codoping of Eu3+-containing sodium silicate glasses with (Cu+)2 species has been reported by Guo et al.11 The system was proposed for white-light-emitting devices based on existent energy-transfer processes and the simultaneous presence of red, green, and blue emissions in the glasses. Nevertheless, the influence of Cu nPCs on Eu3+ ions in glass remains unexplored to the best of the author’s knowledge. © 2016 American Chemical Society

Admittedly, further difficulties arise for Eu−Cu codoped inorganic glasses if Cu2+ impurities are present which are detrimental to Eu3+ emission for being effective photoluminescence (PL) quenchers.12,13 Thus, exploring the effects of different copper species with potential for enhanced PL properties, whether these are ionic, molecule-like clusters, or plasmonic NPs, dictates significant reduction, if not complete removal, of divalent copper. Carbon has been documented as valuable for the inhibition of Cu nanoclusters oxidation, for instance as graphene shells.2 It has been also proposed for ionic copper reduction in a controlled atmosphere as carbon monoxide for preparing Cu NP doped glasses.14 More recently, incorporating graphite as part of batch materials was demonstrated effective by the author for the controlled fabrication of Cu nanocomposite glasses by melting in ambient atmosphere.15 In this work, the latter approach employing carbon powder is utilized for exploring the influence of copper nanoclusters on Eu3+ PL in a P2O5:BaO (50:50) glass matrix. The selected glass system is desirable for optical applications, e.g., all-optical signal processing and ultrafast switching, given its high metal solubility.16 Further, the low-melting character, relatively low glass transition temperature, high thermal stability, and adequate mechanical properties make it attractive for photonic devices.17,18 Herein, remarkable reduction of Cu2+ via graphite during glass preparation by melting is consistently demonstrated whereby the copper dopant is added as copper(II) Received: December 9, 2015 Revised: January 24, 2016 Published: January 26, 2016 3557

DOI: 10.1021/acs.jpcc.5b12051 J. Phys. Chem. C 2016, 120, 3557−3563

Article

The Journal of Physical Chemistry C

with a 10× objective on 97.4 μm × 97.4 μm sample areas with particular attention given to keep the sampled area and all other conditions unaltered during experiments. The PL 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. Accordingly, the selected light from the continuous source can populate excited states in Eu3+ (e.g., 5D3) which subsequently decay nonradiatively to the 5D0 emitting state of interest.13 The in situ optical data were collected sequentially by obtaining an absorption profile immediately after the emission spectrum.20 Under the excitation conditions employed, several Cu nPCs (e.g., Cun, n = 2−9) are expected to bring about optical absorption near 3.0 eV.23 Therefore, the influence of such type of molecule-like Cu clusters on the PL of Eu3+ ions can be investigated during the process of Cu particle development, prior to cluster transition into the plasmonic regime.15 In this way, unequivocal proof can be obtained in situ regarding the influence of copper metallic species on the luminescence of the Eu3+ ions in the dielectric material.

oxide. Further, in situ concurrent PL and absorption microspectroscopy19−21 is put to use for the real-time monitoring of the optical properties of the copper and Eu3+ codoped glasses during thermal processing. The technique reveals the occurrence of bidirectional energy transfer between copper nanoclusters and europium in connection with enhancement and quenching regimes of Eu3+ ions luminescence. Linked to the contrasting PL manifestations are the optically distinct periods related to the development of Cu nPCs and their subsequent growth into the plasmonic regime.

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EXPERIMENTAL SECTION Materials. Glasses with a 50P 2O 5:50BaO (mol %) composition were prepared from high-purity Alfa Aesar chemicals (P2O5, ≥98% and BaCO3, 99.8%) by the meltquenching technique. Batch materials (about 25 g batches) were thoroughly mixed and melted at 1150 °C between 15 and 25 min under normal atmospheric conditions and immediately quenched.15 Copper, europium, and carbon were added as CuO (Alfa Aesar, 97%), Eu2O3 (Alfa Aesar, 99.99%), and graphite powder (Alfa Aesar, crystalline, −300 mesh, 99%), respectively. Copper(II) oxide concentration was held constant in the corresponding samples at 0.5 mol % whereas Eu2O3 was fixed at 1.0 mol % (in relation to network former P2O5). Glasses containing merely the 0.5% CuO (labeled Cu glass), or the 1% Eu2O3 (labeled Eu glass), were made as references. As the main object of this study, a glass was prepared to contain the prescribed 0.5% CuO and 1% Eu2O3 along with graphite as part of the batch materials, added at 1.5 wt % (referred to as EuCuC glass) following the recent study where this was found to be the smallest amount of carbon significantly reducing CuO.15 The glasses were cut and polished in order to produce glass slabs with final thicknesses of about 1.0 mm. The asprepared Cu glass presented a light blue appearance whereas the Eu and EuCuC glasses were colorless; the latter develops an intense ruby color upon thermal processing for extended time periods under conditions such as those specified below. Spectroscopic Measurements. Room temperature (RT) optical absorption measurements were performed using a PerkinElmer 35 UV/vis double-beam spectrophotometer. The absorption spectra were recorded with air as reference. PL emission and excitation spectra were obtained with a Photon Technology International QuantaMaster 30 spectrofluorometer equipped with a xenon flash lamp having a pulse width of about 2 μs and a photomultiplier tube. The flash lamp was kept operating at a frequency of 125 Hz with the total period of data collection set to 8 ms. The step size used for the spectral acquisitions was 1 nm. Emission decay data were obtained under 532 nm (2.33 eV) excitation (7F1 → 5D1 transition in Eu3+); the emission was monitored at 615 nm (2.02 eV) corresponding to the 5D0 → 7F2 transition in Eu3+ as the most prominent peak. These PL measurements were recorded at RT with samples mounted in a solid sample holder at an angle of 40° with particular attention given to keep conditions constant during experiments. A CRAIC Technologies FLEX microspectrophotometer (MSP) equipped with mercury and xenon lamps and a Linkam THMS600 temperature control stage13,22 was used to conduct steady-state PL and optical absorption measurements jointly during heat treatment (HT) in situ within the 470−490 °C range (above the glass transition temperature of the glass matrix17). Heated samples were taken to the desired temperature at a rate of 50 K/min. The measurements were performed

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RESULTS AND DISCUSSION Spectroscopic Characterization of Melt-Quenched Glass. Shown in Figure 1 are visible-range absorption profiles

Figure 1. Optical absorption at RT for the Eu, Cu, and EuCuC glasses.

obtained at RT for the melt-quenched Eu, Cu, and EuCuC glasses. The Cu glass exhibits the broad absorption feature peaking around 1.46 eV (850 nm) ascribed to 2E → 2T2 intraconfigurational (d−d) transitions in Cu2+ ions.13,24 The Eu glass shows no significant absorption in the spectral region as expected, for no CuO was added to it. However, a remarkable resemblance is observed between this and the EuCuC glass prepared with both the copper(II) oxide and the carbon powder. The Cu2+ absorption band vanished as a consequence of adding the 1.5 wt % graphite, indicating that the reduction of copper(II) via carbon occurred effectively during the melting. This is consistent with the recent report by the author where the use of carbon was proposed for the controlled production of plasmonic metal nanocomposite glasses by melting and HT processes.15 Based on the spectroscopic assessment, the formation of P−O−C bonds25 is considered linked to the creation of highly reactive oxygen radicals (e.g., C−O· and C− O−O·) upon melt quenching,15 in analogy to those observed in amorphous silicon dioxide.26 Likewise, the chemical reduction of divalent copper indicated herein in Figure 1 by lack of Cu2+ absorption can result concomitantly in the creation of such 3558


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The Journal of Physical Chemistry C carbon-induced defects, to be exploited for further reduction of ionic copper during the in situ HT (vide inf ra). PL data provide additional evidence in support of divalent copper reduction during the melting. The spectrum for the EuCuC glass in Figure 2 collected at RT under excitation at

Figure 3. Emission decay curve for the EuCuC glass obtained at RT under excitation at 2.33 eV (532 nm) by monitoring emission of 5D0 → 7F2 transition in Eu3+ at 2.02 eV (615 nm). The solid line is a firstorder exponential fit to the data.

with Cu2+ concentration in the Eu−Cu containing phosphate glass system.13 Accordingly, the Eu3+−Cu2+ energy transfer interaction can be proposed as a probe for residual copper(II) in the EuCuC glass treated as “unknown”, similar to the Sm3+ decay rate−Cu2+ concentration correlation recently established.31 Herein, the total emission decay rate, τ−1, for the Eu3+ ions can be written as

Figure 2. PL emission spectra collected at RT under excitation at 4.28 eV (290 nm) for the Eu and EuCuC glasses. The inset shows the excitation spectrum obtained for the EuCuC glass by monitoring emission at 2.67 eV (465 nm).

4.28 eV (290 nm) shows a broad emission band absent in the Eu reference. This luminescent behavior can be ascribed to 3d94s1 → 3d10 transitions in Cu+ ions,27 in agreement with significant Cu2+ → Cu+ chemical reduction in the melt.15 Observed at higher energy relative to that reported for glasses highly doped with monovalent copper,7,28 it is suggested according to Debnath and Das27 to be associated with a cubic coordination as expected for an oxygen-rich environment.15,24 The inset of Figure 2 shows an excitation spectrum for the EuCuC glass recorded by monitoring emission at 2.67 eV (465 nm). An excitation peak is observed around 4.28 eV (290 nm) attributable to Cu+ ions as similarly observed for merely Cu−C containing glasses.15 Accordingly, both absorption and PL results related to copper species point to the effective reduction of copper(II) via carbon producing Cu+ ions largely in the EuCuC glass. Europium(III) ions are known to be highly sensitive to the presence of copper(II) ions and its compounds.12,13,29,30 Divalent copper significantly quenches Eu3+ emission through energy transfer, which is well reflected in an excited state (e.g., 5 D0) lifetime decrease.13 Hence, an additional approach in characterizing the EuCuC glass is utilized herein based on the Eu3+ emission decay dynamics. Presented in Figure 3 is the PL decay curve obtained for the glass at RT under excitation at 2.33 eV (532 nm) by monitoring emission of 5D0 → 7F2 transition in Eu3+ at 2.02 eV (615 nm). A first-order decay is evident. Thus, consistent with reported data13 obtained for Eu−Cu containing glasses under the same excitation and emission conditions, the decay curve was fit by a singleexponential function ⎛ −t ⎞ I(t ) = A exp⎜ ⎟ ⎝ τ ⎠

τ −1 = γ[Cu 2 +] + τ0−1

τ0−1

(2) 3+

where is the decay rate of the Eu ions in the absence of Cu2+, and γ can be considered an apparent quenching constant.13 Hence, a linear plot of the decay rates τ−1 vs Cu2+ concentration can be constructed from the reported data,13 to be utilized as a calibration curve. Such a plot is shown in Figure 4, where a linear increase in the 5D0 emission decay

Figure 4. Plot of decay rate of 5D0 state in Eu3+ ions vs Cu2+ concentration constructed with data reported in ref 13 for Eu and Cu codoped glasses (squares). The solid line is the linear fit to the data (equation displayed). The asterisk is the data point obtained for the EuCuC glass from the corresponding experimental decay curve (Figure 3).

rate in Eu3+ with increasing Cu2+ concentration is manifest (correlation coefficient r of 0.989). From the equation of the line (displayed in Figure 4) and the experimental decay rate obtained for the EuCuC glass (from fit in Figure 3), the corresponding amount of copper(II) in the sample is estimated at 0.08 mol %. This value, plotted together with the graph in Figure 4, is fairly low. Furthermore, it lies below the limit of detection (LOD) calculated from the calibration graph32 (constant k = 3) at 0.18 mol % of Cu2+. Thus, the assessment

(1)

where I(t) is the time-dependent luminescence intensity, A is a pre-exponential weight factor, and τ the decay time. The fit yields a lifetime of 1897 μs for the EuCuC glass, which is relatively close to that reported for the Eu glass of 1984 μs.13 Moreover, the Eu3+ decay rates have been shown to correlate 3559


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presence of the plasmonic NPs. In fact, the surface plasmon resonance (SPR) of Cu NPs28,33 develops afterward and is particularly synchronized with the significantly quenched Eu3+ PL in Figure 5a. The plasmonic development is consistent with the presence of Cu NPs within the dipole regime of Mie theory as observed by transmission electron microscopy for analogous absorption in the glass matrix.33 Similarly, correlating trends were also observed for other temperatures studied. Figures 6a

shows good agreement with the optical data considered in relation to the effective reduction of copper(II) during melting (vide supra). This allows for the subsequent assessment of Eu3+ PL in the material lacking the detrimental effect of divalent copper impurities. Since the influence of monovalent copper species on Eu3+ PL was reported by Guo et al.,11 the focus of this work is henceforth on the real-time assessment of the effect of Cu nanoclusters developed during the thermal processing in situ. Real-Time Optical Assessment during Material Thermal Processing. Figures 5a and 5b show PL and optical

Figure 6. Time dependencies of (a) peak PL intensity of 5D0 → 7F2 transition in Eu3+ and (b) the optical density at 2.2 eV for the EuCuC glass collected during thermal treatments in the MSP at 470, 480, and 490 °C within a 60 min period (5 min intervals).

Figure 5. Real-time evolution of (a) PL and (b) optical absorption for the EuCuC glass collected during thermal treatment at 480 °C in the MSP within a 60 min period (5 min intervals).

and 6b summarize the time dependencies obtained for Eu3+ PL and Cu absorption, respectively, at the three isotherms studied within 470−490 °C. The most prominent 5D0 → 7F2 transition intensity is plotted for the Eu3+ PL, whereas the optical density at 2.2 eV is followed in connection to the progression of the Cu NPs SPR peak. The enhancement of Eu3+ PL is observed in Figure 6a at the three temperatures, where the maximum emission intensification appears after 25, 15, and 10 min for HT at 470, 480, and 490 °C, respectively. Thereafter, the PL starts to decrease; yet, it is still considerably enhanced relative to the PL at t = 0 min up to holding times of 50, 25, and 20 min for the HT at 470, 480, and 490 °C, respectively. On the other hand, a correlation is observed with respect to the evolution in optical absorption in Figure 6b, where aforementioned time periods are characterized by lack of significant absorption growth as expected in connection to formation of Cu nPCs. Actual Eu3+ PL quenching occurs relative to the t = 0 min emission only after 60, 35, and 30 min for the HT at 470, 480, and 490 °C, respectively. This can be clearly observed from Figure 7 where the relative PL change (%) is plotted as a

absorption spectra, respectively, recorded for the EuCuC glass during the in situ thermal treatment at 480 °C at time intervals of 5 min for a total holding time of 60 min. Three-band emission from Eu3+ ions in Figure 5a corresponding to 5D0 → 7 FJ (J = 1, 2, 3) transitions is observed to increase consistently during the early stage of HT (e.g., maximum PL recorded after 15 min). Thereafter the PL intensity decreases until it shows severe quenching (e.g., after 35 min) relative to the spectrum collected at time t = 0 min. Comparing the time evolution in the Eu3+ PL with the optical spectra in Figure 5b, it is clear that no significant variation in absorption is observed early during the HT where the enhancement occurs. Instead, a period of development of preplasmonic Cu clusters33 is suggested, which can occur as a consequence of the reduction of Cu+ ions by carbon-induced defects (e.g., via oxygen radicals).15 Accordingly, the in situ approach exposes that the enhanced PL is due to a sensitizing effect, i.e., via energy transfer from subnanometric preplasmonic Cu clusters, and not to the 3560


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concentrations of metal NPs.21,36−40 In such case, a role of the metallic particles as a plasmonic diluent36 is interpreted as a result of an effective excitation energy transfer from the RE ions to the metal NPs overcoming any local field enhancements. The effect was indicated previously to occur not only through the SPR absorption in the metallic NPs but also through interband transitions as in the case of copper.36,37 It is in this latter case that the current results fit into, since the employed optical excitation source lies outside the SPR of the Cu NPs but within the interband transitions in the metal (threshold at about 2.1 eV).43 Such ion-to-particle excitation energy transfer is then indicated to be the main cause of the PL quenching effect, where the NPs provide the paths for the nonradiative loss of excitation energy in Eu3+ ions.36,44,45 Still, a quenching through Eu3+ emitting transitions (e.g., 5D0 → 7F1) in resonance with the SPR of Cu NPs cannot be ruled out at present. Shown in Figure 8 is a schematic intended to summarize the indicated bidirectional energy transfer processes related to the

Figure 7. Time dependencies of % change in PL intensity of 5D0 → 7 F2 transition in Eu3+ collected during thermal treatments in the MSP at 470, 480, and 490 °C within a 60 min period (5 min intervals).

function of time for the three isotherms considered. It is particularly after these Cu incubation periods that the SPR of Cu NPs grows gradually. The current results are in agreement with the reports from Trave et al.4 and Cattaruzza et al.5 on the sensitizing effects of Cu nPCs on Er3+ emission. Further, an analogous PL vs absorption correlation as that observed in the present work was previously reported regarding Sm3+ PL enhancement and quenching regimes timed with the transition of Ag nPCs into NPs.20 Similarly, Eichelbaum and Rademann34 and Maurizio et al.35 have reported PL enhancements of RE ions in noble metal codoped dielectrics in connection to a classical energy transfer mechanism originating at molecule-like clusters. It is then suggested that an effective excitation of the Cu clusters is achieved herein with the optical source employed in accord with optical absorption studies from Lecoultre et al.23 on Cu clusters. Thus, the enhancement in Eu3+ PL appears connected to the sensitizing effect of the Cu nPCs. Consistent with this, the maximum Eu3+ emission in this work occurred for thermal treatment at 470 °C appearing as the more subtle temperature for generating the Cu clusters responsible for the enhancement. With respect to the nature of the nPCs, a neutral character is suggested in accord with the reducing properties of the preparation method employing carbon powder.15 Even though Cu cluster size cannot be unambiguously assigned, Cun clusters with n = 2−9 seem suitable candidates since these have shown optical absorption features around 3.0 eV.23 Yet, it is not possible to exclude the simultaneous excitation of different nPCs. This is because the dynamic process of the formation and growth of the clusters during thermal treatment may lead to a range of cluster sizes and therefore energy levels, while at the same time the fwhm (45 nm) of the excitation source centered at 2.95 eV (420 nm) may produce significant excitation in the 2.8−3.1 eV range. Regarding the quenching effect, a decrease in the number of sensitizers, i.e., the Cu nPCs, could lead to a reduction in the degree of enhancement.20 This becomes patent in the observed results (e.g., Figure 7) where the Eu3+ PL is quenched relative to the maximum enhancement but still enhanced relative to the initial PL. Still, in the absence of the production of a quencher,36−40 further PL decrease is unlikely. Significant Eu3+ → Eu2+ reduction does not seem probable during HT given the negative Eu3+/Eu2+ standard reduction potential E° = −0.35 V.41,42 On the other hand, various glass systems have consistently shown RE emission quenching linked to high

Figure 8. Simplified schematic illustrating energy transfer processes leading to the enhanced Eu3+ ions PL (ET-E) through Cu nPCs and the PL quenching effect (ET-Q) through Cu NPs. Vertical solid and dotted arrows represent radiative and nonradiative transitions, respectively. Direct optical excitation of Eu3+ ions to the 5D3 state is also illustrated.

time-dependent PL enhancement and quenching. Similar to the case reported for the interaction between Sm3+ and Ag,20 the energy level diagram for Cu nPCs is simplified by merely showing an excited state in resonance with the employed peak excitation (2.95 eV). Optical absorption resulting in direct excitation of Eu3+ ions to the 5D3 state (also relevant to 2.95 eV excitation) within the Eu3+ energy level structure is also illustrated. In this scheme, optical excitation of the Cu nPCs as the energy donors, and a subsequent resonant energy transfer to Eu3+ ions (e.g., to 5D3 state) as the acceptors, can result in populating the 5D0 metastable state besides direct optical excitation. This Cu (nPCs) → Eu3+ energy transfer channel is indicated to succeed in the absence of the plasmonic NPs, resulting in the PL enhancements observed early during the in situ HT. Conversely, energy transfer can go in the opposite direction, i.e., Eu3+ → Cu (NPs), as excited states in Eu3+ ions can be depopulated by coupling with interband transitions in the metal particles37 which then decay nonradiatively. Herein, 3561



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Eu3+ ions now assume the role of energy donors with the NPs as acceptors, a scenario that prevails at the elevated temperatures with the transitioning of the Cu clusters into the plasmonic regime. This quenching effect can take place for Eu3+ ions excited directly by the optical source or else via energy transfer from the Cu nPCs. Like in the case of Sm3+ and Ag,20 a dynamic situation can be envisioned involving the enhancement vs quenching competition among the coexistent nonplasmonic and plasmonic Cu particles, together with the direct optical excitation of the Eu3+ ions. Practical PL tuning is however possible owing to the real-time monitoring capability of the MSP, where the time window for direct Cu (nPCs) → Eu3+ transfer can be selected in order to successfully reach the enhancement culmination.

REFERENCES

(1) Oliver-Messeguer, J.; Liu, L.; García-García, S.; Canós-Giménez, C.; Domínguez, I.; Gavara, R.; Doménech-Carbó, A.; Concepción, P.; Leyva-Pérez, A.; Corma, A. Stabilized naked sub-nanometric Cu clusters within a polymeric film catalyze C−N, C−C, C−O, C−S, and C−P bond-forming reactions. J. Am. Chem. Soc. 2015, 137, 3894− 3900. (2) Qiu, H.; Huo, Y.; Li, Z.; Zhang, C.; Chen, P.; Jiang, S.; Xu, S.; Ma, Y.; Wang, S.; Li, H. Surface-enhanced Raman scattering based on controllable-layer graphene shells directly synthesized on Cu nanoparticles for molecular detection. ChemPhysChem 2015, 16, 2953− 2960. (3) Stepanov, A. L. Nonlinear optical properties of implanted metal nanoparticles in various transparent matrixes: a review. Rev. Adv. Mater. Sci. 2011, 27, 115−145. (4) Trave, E.; Cattaruzza, E.; Riello, P. Er and Cu codoped SiO2 films obtained by sputtering deposition: enhancement of the rare earth emission at 1.54 μm mediated by metal sensitizers. Opt. Mater. 2013, 35, 2018−2022. (5) Cattaruzza, E.; Battaglin, G.; Visentin, F.; Trave, E.; Aquilanti, G.; Mariotto, G. Enhanced photoluminescence at λ = 1.54 μm in the Cudoped Er:SiO2 system. J. Phys. Chem. C 2012, 116, 21001−21011. (6) Zhang, Q.; Chen, G.; Dong, G.; Zhang, G.; Liu, X.; Qiu, J.; Zhou, Q.; Chen, Q.; Chen, D. The reduction of Cu2+ to Cu+ and optical properties of Cu+ ions in Cu-doped and Cu/Al-codoped high silica glasses sintered in an air atmosphere. Chem. Phys. Lett. 2009, 482, 228−233. (7) Jiménez, J. A.; Zhao, C. Optical absorption, 31P NMR, and photoluminescence spectroscopy study of copper and tin co-doped barium−phosphate glasses. Mater. Chem. Phys. 2014, 147, 469−475. (8) Kesavulu, C. R.; Kumar, K. K.; Vijaya, N.; Lim, K.-S.; Jayasankar, C. K. Thermal, vibrational and optical properties of Eu3+ -doped lead fluorophosphate glasses for red laser applications. Mater. Chem. Phys. 2013, 141, 903−911. (9) Dillip, G. R.; Dhoble, S. J.; Manoj, L.; Reddy, C. M.; Raju, B. P. D. A potential red emitting K4Ca(PO4)2:Eu3+ phosphor for white light emitting diodes. J. Lumin. 2012, 132, 3072−3076. (10) Reisfeld, R.; Levchenko, V.; Piccinelli, F.; Bettinelli, M. Amplification of light emission of chiral pyridine Eu(III) complex by copper nanoparticles. J. Lumin. 2016, 170, 820−824. (11) Guo, H.; Wei, R. F.; Liu, X. Y. Tunable white luminescence and energy transfer in (Cu+)2, Eu3+ codoped sodium silicate glasses. Opt. Lett. 2012, 37, 1670−1672. (12) Batyaev, A. I.; Tinus, A. M. Transport of electronic excitation energy in solid-state glassy phosphors activated with europium (III) and copper (II). Technol. Phys. Lett. 1998, 24, 26−27. (13) Jiménez, J. A. Photoluminescence of Eu3+-doped glasses with Cu2+ impurities. Spectrochim. Acta, Part A 2015, 145, 482−486. (14) Schreiber, H. D.; Stone, M. A.; Swink, A. M. Novel red-blue dichroic glass containing copper nanocrystals. J. Non-Cryst. Solids 2006, 352, 534−538. (15) Jiménez, J. A. Carbon as reducing agent for the precipitation of plasmonic Cu particles in glass. J. Alloys Compd. 2016, 656, 685−688. (16) Yamane, M.; Asahara, Y. Glasses for Photonics; Cambridge University Press: UK, 2000. (17) Narayanan, M. K.; Shashikala, H. D. Thermal and optical properties of BaO-CaF2-P2O5 glasses. J. Non-Cryst. Solids 2015, 422, 6−11. (18) Narayanan, M. K.; Shashikala, H. D. Physical, mechanical and structural properties of BaO-CaF2-P2O5 glasses. J. Non-Cryst. Solids 2015, 430, 79−86. (19) Jiménez, J. A.; Sendova, M.; Liu, H. Evolution of the optical properties of a silver-doped phosphate glass during thermal treatment. J. Lumin. 2011, 131, 535−538. (20) Jiménez, J. A.; Sendova, M. In situ isothermal monitoring of the enhancement and quenching of Sm3+ photoluminescence in Ag codoped glass. Solid State Commun. 2012, 152, 1786−1790.

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CONCLUSIONS Carbon powder in the form of graphite has been shown valuable for the significant chemical reduction of copper(II) in Eu3+ codoped melt-quenched glass, as required for a sensitive europium(III)-containing luminescent material. As a new material preparation approach, the exact underlying reduction mechanism and fate of carbon in the glass are still not well understood. Melting the glass with a certain amount of graphite is assumed to result in the creation of reactive oxygen radicals in the matrix which can be subsequently activated during thermal treatment to further reduce metal ions to the neutral state. The unambiguous outcome nevertheless allowed for employing the in situ concurrent PL and absorption microspectroscopy technique for the real-time monitoring of the optical properties of the codoped glasses during thermal processing which produces the metal aggregates. A timedependent enhancement and quenching evolution of Eu3+ ions PL in the Cu codoped phosphate glass was revealed for three different isotherms, practical for an optical tuning of the luminescent material. Distinctively, a bidirectional energy transfer between europium ions and copper nanoclusters has been manifested through the enhancement and quenching regimes of Eu3+ PL. This was shown to be connected to the development of small Cu clusters (energy donors) and their subsequent growth into the plasmonic (energy acceptor character) regime. Thus, the use of carbon consistently shows promise for producing solid-state materials of practical utility in the field of photonics. Further, combined with an in situ monitoring, the tuning of optical properties within an appropriate time window during thermal processing can be realized.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address

Functional Films Lab, BASF Corporation, 2655 Route 22 West, Union, NJ 07083. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The author is grateful for the experimental assistance of undergraduate student Joon Seok Oh from the Chemistry Department at UNF. 3562


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