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Photoluminescent Nanocrystals in a Multicomponent Aluminoborosilicate Glass Andreia Ruivo, Marta C. Ferro, Suzana M Andrade, Joao Rocha, Fernando Pina, and Cesar Antonio Tonicha Laia J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b04552 • Publication Date (Web): 07 Oct 2016 Downloaded from http://pubs.acs.org on October 9, 2016

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Photoluminescent Nanocrystals in a Multicomponent Aluminoborosilicate Glass

Andreia Ruivo1,2, Marta Ferro3, Suzana M. Andrade4, João Rocha5, Fernando Pina1, César A.T. Laia1*

1

LAQV-REQUIMTE, Chemistry Department, Faculty of Science and Technology,

Universidade NOVA de Lisboa, 2829-516 Caparica, Portugal. Tel: + 351 212 948 300; 2

Research Unit VICARTE, Vidro e Cerâmica para as Artes, Faculty of Science and

Technology, Universidade NOVA de Lisboa, 2829-516 Caparica, Portugal. Tel: +351 212947893; 3

Department of Materials and Ceramic Engineering, CICECO-Aveiro Institute of

Materials, University of Aveiro 3810-193 Aveiro, Portugal. 4

Centro de Química Estrutural, Complexo 1, Instituto Superior Técnico, Universidade

de Lisboa, 1049-001 Lisboa, Portugal. 5

Department of Chemistry, CICECO-Aveiro Institute of Materials, University of Aveiro

3810-193 Aveiro, Portugal. *Corresponding author, email: [email protected] and Tel: + 351 212948310; Fax: + 351 212948550

1 ACS Paragon Plus Environment


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ABSTRACT: In this study, stable and non-expensive aluminoborosilicate glasses with different photoluminescence colors were synthesized by doping with Pb(II) and sodium halides. While glasses with NaF and NaCl exhibit no (or very low) luminescence, glasses doped with NaBr and NaI display room temperature photoluminescence at 435 nm and 530 nm, respectively. The observed room-temperature photoluminescence is attributed to nanocrystals whose presence is revealed by transmission electron microscopy. The crystalline nature of the particles, which are pointed out as barium-lead halides, is also revealed by anisotropy measurements for Br and I doped samples. Timeresolved luminescence measurements show a second-order kinetics component combined with a first-order nonradiative rate constant. The photoluminescence properties here described are important for the future design of new optical materials or devices based on lead halide nanocrystals.


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1. INTRODUCTION New luminescent and chromic devices are continuously being investigated due to their wide-scope of applications in, e.g., chemistry, biology and electronics. Nowadays, these devices frequently require organic compounds prone to degradation or expensive and/or toxic raw materials such as lanthanides, indium, ruthenium or iridium. Doping of glass materials is a suitable strategy to achieve photoluminescence and may be used in several applications, such as in biomaterials and in lighting.1,2 Although luminescent glasses are often obtained by doping the base composition with lanthanide oxides,3,4,5 their high cost motivates the investigation of other alternatives. The latter include the formation of luminescent nanoparticles in the glass matrix (e.g., ZnSe, ZnS, CdTe Cd1-xMnxS, PbS and Ag nanoparticles)6,7,8,9,10,11 or the glass synthesis with transition metals in certain oxidation states (e.g., Cu+ and Mn2+).12,13,14 An essential feature to achieve this goal is the use of multicomponent glasses, usually synthesized including alkali metals and property modifiers such as Ca or Al.15 Silver photochromic glasses are one example of multicomponent glasses that allow the segregation of AgCl nanocolloids, which lead to the photochemical formation of silver nanoparticles.16 Multicomponent glasses are, therefore, very promising for the development of luminescent materials by forming luminescent centers, such as lead halide particles. Lead halides materials, through charge transfer mechanisms, also lead to light emission and even photochemical reactions17,18,19 (e.g., scintillator detectors of γ and X-rays).17,20 Organic lead halides perovskite solar cells have also attracted much attention because they are excellent sensitizers and their power conversion efficiencies are improving considerably in a short time.21,22,23 More recently, in order to achieve more thermally stable materials, some work is focusing on the synthesis of completely inorganic lead halide perovskite solar cells.24



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Light emission mechanism from lead halides involves charge-transfer from halide to lead, closely related with other systems such as AgCl.19,25,26 Like silver halides that form silver nanoparticles when irradiated with light of suitable intensity and wavelength, lead halides can also give rise to lead metal nanoparticles. However, in many cases a transient Pb23+ species, which exists as a self-trapped electron center (STEL), in pair with the halide, X2-, leads to a self-trapped hole (STH). Afterwards, the reversed process generates an emissive self-trapped exciton (STE).25,26,27,28 Lead halides photoluminescence has already been reported in different studies, however to the best of our knowledge, the majority of these studies were performed at cryogenic temperatures,18,25,26,28,29,30,31,32 because these crystals have limited stability at room temperature.17 In a previous study, where a multicomponent aluminoborosilicate photoluminescent glass was synthesized by doping with PbO and NaBr,33 it was reported that a photoluminescent glass matrix is obtained at room temperature. The preparation of glasses doped with nanocrystalline lead halides,34,35 PbS 36 or PbWO434,37 was already described, normally with the aim to obtain scintillation materials. In this context, here aluminoborosilicate glasses doped with lead and different halogens, were synthesized and photoluminescent nanocrystals formed. The particles formation is discussed by analyzing the structure and photoluminescence of the materials.


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2. EXPERIMENTAL DETAILS 2.1. Glass synthesis The synthesis protocol is similar to the one described elsewhere.33 The main difference is the presence of different halogens (X), such as F-, Cl-, Br- and I-, with a molar ratio (r) of X/Pb=1.33 in the glass composition. In the case of I-, a glass with a different ratio was also produced, I/Pb=3.

Reagent grade SiO2 (p.a., Fluka), B2O3 (99%, Acros

Organics), Al2O3 (p.a., Merk), Pb3O4 (Panreac), BaO (>98%, Aldrich), Li2CO3 (p.a., Fluka), Na2CO3 (p.a., Riedel de Haen), K2CO3 (p.a., Panreak), NaF (Merk), NaCl (p.a., Panreak), NaBr (99.5%, Merck) and NaI (Merk) were used as raw materials. The full synthesis protocol is available in the previous article33 and in the supplementary information. 2.2. Glass samples characterization Structural Measurements – Transmission electron microscopy (TEM), Energy dispersive X-ray spectroscopy (EDS), Microenergy Dispersive X-ray Fluorescence (µEDXRF) and Powder X-ray Diffraction (XRD) measurements were obtained with equipment and methods described elsewhere.33 In the case of the sample doped with iodine a JEOL JEM-2200FS, 200kV, Oxford INCA Energy TEM 250 was also used for TEM measurements. Optical Measurements - UV/Vis absorbance spectra were recorded on a Varian Cary5000 UV/VIS/NIR spectrophotometer over the 300-1800 nm wavelength range with a 1 nm resolution. Luminescence spectra, Time-resolved luminescence spectra and Single photon counting (SPC) measurements were described elsewhere.33 Fluorescence quantum yields were measured using an F-3018 Integrating Sphere accessory (Horiba Jobin Yvon).



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3. RESULTS AND DISCUSSION Colorless transparent glasses were obtained using the procedure described in the Experimental section. Major differences are observed when the glass samples are under a 364 nm UV light. While glass A, containing no halogen in its composition, does not exhibit photoluminescence, when certain amounts of halogens are added to the base glass a significant visible emission is observed, as in the case of the samples doped with NaBr and NaI, r=1.33, with chromaticity coordinates x = 0.17; y =0.14 and x = 0.33 ; y = 0.39, respectively (Figure 1). A preliminary study of this multicomponent glasses, using a mixed halide system, show that this luminescence is only observed in the presence of lead and is much more intense when barium is also added, see supporting information SI 1.

Figure 1. (A) CIE (Commission Internationale de l’ Eclairage) diagram with colour coordinates of the luminescent aluminoborosilicate glasses doped with NaBr and NaI (r=1.33). (B) Glass samples doped with NaBr and NaI under a 364 nm UV light.

3.1. TEM, XRD and XRF measurements Multicomponent glasses are used in several studies to obtain different optical effects. They are used for example in photochromic glasses16 and also photoluminescent glasses


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based in silver clusters38 or in lead halides.33 Nanoparticles are frequently formed in these cases and can be studied by TEM and XRD. The XRD pattern of all samples show a broad band ascribed to the amorphous glass (Supporting information SI2). Only small peaks with low intensity appear in the halides doped samples diffractograms that can be attributed to the formation of low temperature quartz, which crystalizes in a hexagonal space group P6222.39 Since XRD provides no evidence for the presence of any nanoparticles we have resorted to TEM experiments. While in the glass doped with NaF no nanocrystals could be found, these were observed in the glasses doped with NaCl, NaBr and NaI, with diameters ranging from 1 to 13 nm (Figure 2). More TEM images may be found in the supporting information SI 3.

Figure 2. TEM images of undoped and halogen (Cl, Br and I) doped glasses; halogen/Pb molar ratio 1.33 for Cl and Br, and 3 for I. Examples of nanocrystals formed in the chloride and bromide doped samples are pointed out with white and black circles, respectively.



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The size histograms of the nanocrystals observed in the TEM images are shown in Figure 3, and Table 1 collects the results. For the glass doped with NaI, with an I/Pb molar ratio of 1.33, it was not possible to adequately image the nanocrystals due to their instability in the electron beam. However for r=3 nanocrystals images could be obtained, see Figure 2, even if after some time the nanocrystals disappeared when the electron beam is focused on the nanocrystals, showing that these crystals are sensitive to beam damage that possible may give rise to iodine volatilization. TEM images of the nanocrystals formed using NaCl, NaBr and NaI in the glass synthesis show that they are essentially with a spherical form. The histograms in Figure 3 present the size distribution of the nanocrystals observed in the TEM images. This histograms show that particles are polydispersed, with diameters ranging from, 2-7, 2-13 and 3-12 nm, for Cl, Br and I, respectively (average diameter = 4.2, 5.1 and 6.4), see Figure 3 and Table 1. The nanocrystals size slightly increase with the halogen ionic radius.

Figure 3. TEM size distribution analysis fitted with a log normal of the nanocrystals embedded in aluminoborosilicate glass samples doped with NaCl, NaBr and NaI, with molar ratios of halogen/Pb=1.33, 1.33 and 3, respectively.

Table 1. Number of nanoparticles measured (N), their average diameter and standard deviation (log normal distribution, σ) for several glass samples doped with different halogens (X).


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X/Pb molar N Diameter σ ratio (nm) Cl/Pb=1.33 30a 4.2 0.25 b Br/Pb=1.33 53 5.1 0.38 c I/Pb=3 132 6.4 0.32 a 2 b 2 N counted in these areas: 0.06 µm , 0.47 µm , and c 0.78 µm2

To study the crystal structure of the nanoparticles, high resolution TEM was performed on the sample with I/Pb=3, which featured more nanocrystals with a better contrast. High-resolution TEM was only performed for the I-doped samples that presented larger nanocrystals. In the glass samples, with bromide and chloride, the nanocrystals changed their morphology in the presence of the beam preventing similar measurements. Figure 4(A) shows nanocrystals exhibiting lattice fringes, indicating well defined crystalline structure, and allowing measuring a lattice planes distance of 0.31 and 0.34 nm. This distances can be attributed to two crystalline phases of PbI2 (JCPDS 00-007-0235 and 01-071-6148, respectively). Besides the TEM information EDS analysis show that the nanoparticles contain mainly barium and lead, Figure 4 (B), with no iodine being detected. However, given that the samples are only luminescent and the nanoparticles only form when halogens are present, we presume that the absence of iodine from the EDS spectra is probably due to its low concentration in the glass.



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Figure 4. (A) TEM image of the glass sample doped with NaI (I/Pb molar ratio 3); (B) EDS analysis of the nanoparticles (np) and of the surrounding area (out), normalized to the peak of Cu at 8.03 keV.

The EDS of all samples did not detect the presence of Cl, Br or I, in the nanocrystals or in the glass matrix due to intrinsic overlap with other peaks from elements such as Pb or large background. The small size of the nanocrystals also prevent an accurate EDS mapping of the system. However, XRF qualitative analysis provided the evidence of the presence of these three halogens (Figure 5). Iodine was the most difficult to detect because its peaks appear close to the two barium peaks (Figure 5 C). We conclude that the halogen’s concentration in all glasses, after their synthesis process at 1300ºC, is below the EDS limits of detection. In the TEM images the lattice distances points out to the formation of lead iodine in the synthesized samples, however this do not exclude the presence of barium on the observed nanoparticles, as can be observed in the EDS analysis. Therefore the present data indicate the formation of nanocrystals in the luminescent halogen doped glasses that are probably composed by a barium, lead (that have a close ionic radii) and iodine mixture.


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Figure 5. XRF spectra of glass A, without halogens (black line), that is represented in all figures (A), (B) and (C). XRF spectra of glasses doped with (A) NaCl (pink line), (B) NaBr (blue line) and (C) NaI (orange line), with molar ratios of halogen/Pb=1.33, normalized in the barium peak at 4.83 keV. A more detailed spectra is presented in the region were chloride and iodine peaks can be observed.

3.2 Characterization of the Optical Properties Exciting the samples at 350 nm at room temperature results in blue luminescence for the NaBr doped glasses, due to large photoluminescence centered at 435 nm (2.85 eV) already reported,33 and yellow luminescence for NaI due to a broad band at 530 nm (2.34 eV), Figure 6 (A). These glass samples present fluorescence quantum yields of 3.7% and 4.1%, respectively. In the case of PbCl2 the luminescence has a very low intensity and no emission was observed for PbF2 (Figure 6).



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Figure 6. (A) Emission spectra of aluminoborosilicate glasses doped with NaF, NaCl, NaBr and NaI, r=1.33 (λexc=350 nm); (B) Absorption spectra of undoped glasses and of glasses doped with the same sodium halides. Extrapolation of the linear part of the represented spectra to obtain the energy gap, Egap.

The absorption in the UV region (Figure 6 (B)) is commonly observed in glasses and is attributed to the band gap between the valence and conduction bands.40 This absorption edge that goes into the gap states is called the Urbach edge and is described by the Urbach formula.31,40

(1) where α is the absorption coefficient, σ is the steepness parameter, E0 is the Urbach energy and E is the photon energy. A linear dependence between ln(α) and the photon energy of the exciton band is predicted, as observed in Figure 6, where a gradual shift to lower energies within the halide series is seen. This shift is ascribed to the increasing polarizability and decreasing electron affinity and, as a result, the excitation energy of the electronic transition decreases.41 The absorption spectra show features typical of semiconductors, namely an energy gap (Egap), i.e., the energy difference between the valence and conduction bands, (0 to 4 eV for semiconductors).42 A gradual shift across 12 ACS Paragon Plus Environment

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the halide series is observed in the absorption and emission spectra (Figures 6 and 7). Figure 7 illustrates the relation between the absorption and emission bands energy with the halogens electron affinity, showing that both bands shift towards lower energies when the electron affinity decreases.

Figure 7. Electron affinity of the different halogens used to dope the aluminoborosilicate glass and corresponding Egap (absorption) and Eemission (energy at the emission maximum).

Luminescence decays determined by Flash Photolysis are in the sub-microsecond scale, non-exponential and strongly dependent on the anion (supporting information SI 4). Luminescence decays with nanosecond resolution of glasses doped with Br and I, r=1.33, were also measured at room temperature and 77K. As previously reported, the decay of the NaBr doped glass is highly non-exponential and the luminescence anisotropy is time-independent, with an average value of 0.19.33 The decay of the NaI doped glass has the same characteristics (Figure 8), and the time-resolved luminescence anisotropy presents an average value of 0.2.



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Figure 8. (A) Luminescence decays of glass A doped with iodine, r=1.33, at room temperature and 77K at 520 nm fitted with eq. 2 (λex = 372 nm). (B) Anisotropy at 520 nm with λex = 372 nm.

The nature of this non-exponential decay was studied. As observed for NaBr doped glasses33, the decay of the sample doped with NaI was fitted using equation (2), with five adjustable parameters, including a mixed-order kinetics (first- and second-order) and an additional exponential fast decay (nanosecond timescale).

I (t ) =

a1 exp(− k1t )  t + a3 exp −  1 − a 2 exp(− k1t )  τ

(2)

in which

a1 = I 0

k1 [X ]0 k1 + k 2 [ X ]0

(2.1)

and

a2 =

k 2 [X ]0 k1 + k 2 [ X ]0

(2.2)

k1 is a first-order non-radiative rate constant and k2 is the second-order STH/STEL recombination rate constant. The results of the fitting are depicted in Table 2.


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Table 2. Fitting results of the luminescence decays (Figure 8A) using equation (2) Halogen

I0[X]0

a2

k1/µs−1

k2[X]0/µs−1

a3

τ/ns

Br 298K(a)

0.62

0.51 ± 0.03

1.90 ± 0.05

2.0 ± 0.3

0.38 ± 0.3

23.4 ± 9.3

77K

1.00

0.97 ± 0.06

0.24 ± 0.01

6.9 ± 1





I 298K

0.67

0.77 ± 0.05

0.58 ± 0.02

2.0 ± 0.3

0.32 ± 0.2

39.7 ± 10

77K

0.54

0.83 ± 0.05

0.61 ± 0.02

2.9 ± 0.5

0.46 ± 0.3

16.9 ± 5

(a)

(a) values reported in the literature

33

The fast decay process,τ, is here related with the glass intrinsic luminescence

43

(evident, e.g., in the emission of glass without Pb and halides, see Figure SI 1), however since it changes with the halide, it may also be due to the lead halide nanocrystals. The mixed-order kinetics typical features, with a higher first-order kinetics (k1) that is responsible for the long tail represented in Figure 8 (A). The STH/STEL radiative recombination already mentioned25,26,27,28 can be observed in the present nanocrystals, leading to the second-order kinetics33, Table 2. The pre-exponential factor a2 indicates the magnitude of the second-order kinetics and is larger for the iodine doped sample. The first-order non-radiative process has an impact on the electron recombination between STH/STEL centers giving rise to fast decays (which also decreases luminescence quantum yield due to an extra non-radiative channel), and is more pronounced for the bromide doped sample. These differences between both samples can be due to the different halogen electron affinity. Surprisingly, temperature effects seem small for the PbI2 case, mainly because k1 has small changes. This indicates that to supress this mechanism even lower temperatures would be required. Together, the presented results indicate that lead and halogens segregate in the glass matrix as PbX2. As already reported, in many cases the formation of a self-trapped electron center (attributed to Pb23+ species) and a self-trapped hole (X2-) is followed by a back electron-transfer process leading to a self-trapped exciton (STE) which relaxes via 15 ACS Paragon Plus Environment


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a radiative pathway. The observed results are consistent with charge-transfer from the halide to Pb2+. However, barium also has an important role, not only in the nanoparticles formation but also in the emission intensity. Barium is responsible for the enhanced room-temperature photoluminescence, presumably because it delays exciton recombination turning its diffusion hindered, given that similar lead halide systems only emit at very low temperature.26,27,28,29,30,31,32 To the best of our knowledge barium-lead halide nanocrystals are not reported in the literature and further studies on their crystalline structure are extremely important in order to improve their photoluminescence properties.

4. CONCLUSIONS Photoluminescent aluminoborosilicate glasses were successfully produced by melting low cost raw materials at 1300 ºC. Upon doping it with lead and with several sodium halides, different luminescent colors such as blue (NaBr) and yellow (NaI) were obtained at room temperature. The samples were characterized by TEM, XRF and anisotropy measurements that revealed the presence in the glassy matrices of crystalline nanoparticles ranging from 1 to 13 nm, which are pointed out as barium-lead halide nanocrystals. An optical characterization was also performed showing several variations with the halogens in the absorption and emission spectra and in the photoluminescent decays, suggesting a charge-transfer from the halide to Pb (II), probably linked to the existence of self-trapped centers as previously reported.33

ACKNOWLEDGMENTS This work has been supported by the European project NMP4-SL-2012-310651 under


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FP7-NMP-2012-SMALL-6 and by Fundação para a Ciência e a Tecnologia through PTDC/QEQ-QIN/3007/2014,

UID/EAT/00729/2013,

UID/QUI/50006/2013

and

UID/CTM/50011/2013 co-financed by the ERDF under the PT2020 Partnership Agreement (POCI-01-0145-FEDER – 007265 and POCI-01-0145-FEDER – 007679). A. Ruivo would like to thank a grant by FCT (SFRH/BD/46659/2008).

SUPPORTING INFORMATION AVAILABLE Emission spectra of glass sample A doped with several halogens, with and without BaO and PbO, X-ray diffraction of the base glass without halogens and doped with different sodium halides, TEM images of glass samples doped with NaCl, NaBr and NaI and luminescence decays of aluminoborosilicate glasses doped with different sodium halides. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author E-mail: [email protected]. Tel: + 351 212948310. Fax: + 351 212948550. Notes The authors declare no competing financial interest.

References (1) Fan, Y.; Yang, P.; Huang, S.; Jiang, J.; Lian, H.; Lin, J. Luminescent and Mesoporous Europium-Doped Bioactive Glasses (MBG) as a Drug Carrier. J. Phys. Chem. C 2009, 113, 7826–7830. (2) Sun, X.Y.; Wu, S.; Liu, X.; Gao, P.; Huang, S. M. Intensive White Light Emission from Dy3+ -Doped Li2B4O7 Glasses. J. Non-Cryst. Solids 2013, 368, 51–54. (3) Ruivo, A.; Muralha, V. S. F.; Águas, H.; Pires de Matos, A.; Laia, C. A. T. TimeResolved Luminescence Studies of Eu3+ in Soda-lime Silicate Glasses. J. Quant. Spectrosc. Radiat. Transfer 2014, 134, 29-38. 17 ACS Paragon Plus Environment


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