Luminescent polymethyl methacrylate composite films ...

Luminescent polymethyl methacrylate composite films activated under infrared radiation, useful for engineering applications Karen Jaqueline Martínez Ávila Salvador Carmona Téllez Jesús Uriel Balderas Aguilar Rafael Martínez Matínez Luis Mariscal Becerra Ciro Falcony Guajardo

Karen Jaqueline Martínez Ávila, Salvador Carmona Téllez, Jesús Uriel Balderas Aguilar, Rafael Martínez Matínez, Luis Mariscal Becerra, Ciro Falcony Guajardo, “Luminescent polymethyl methacrylate composite films activated under infrared radiation, useful for engineering applications,” J. Photon. Energy 8(1), 014001 (2018), doi: 10.1117/1.JPE.8.014001.

Luminescent polymethyl methacrylate composite films activated under infrared radiation, useful for engineering applications Karen Jaqueline Martínez Ávila,a Salvador Carmona Téllez,b,* Jesús Uriel Balderas Aguilar,c Rafael Martínez Matínez,d Luis Mariscal Becerra,b and Ciro Falcony Guajardob a

Universidad Autónoma Metropolitana Campus Azcapotzalco, Azcapotzalco, Reynosa Tamaulipas, México b Departamento de Física del Centro de Investigación y de Estudios Avanzados del IPN. Av. Instituto Politécnico Nacional, San Pedro Zacatenco, Gustavo A. Madero, México c Centro de Investigación y de Estudios Avanzados del IPN, Programa de Doctorado en Nanociencias y Nanotecnología, Av. Instituto Politécnico Nacional, San Pedro Zacatenco, Gustavo A. Madero, México d Instituto de Física y Matemáticas de la Universidad Tecnológica de la Mixteca, Acatlima, Huajuapan de León, Oaxaca, México

Abstract. Upconversion (UC) luminescence characteristics of Ho3þ -, Yb3þ -, and Liþ -doped Gd2 O3 phosphors and composite films of these phosphors into polymethyl methacrylate (PMMA) under continuous wave excitation with 980-nm laser light are reported. The phosphors were synthesized by the simple solvent evaporation technique and characterized by x-ray diffraction and scanning electron microscopy techniques (SEM); SEM observations reveal that these phosphors have a layered structure. The UC emission of these phosphors was the characteristic green UC emission from the Ho3þ ions, and the effect of Liþ and Yb3þ codoping was twofold; (1) lithium’s presence provokes a slight change into Gd2 O3 structure, and (2) there is an energy transfer from the Yb3þ to Ho3þ ions. The PMMA composite films were synthesized by spin coating technique; they have excellent transparency even after the incorporation of the phosphor content. The UC luminescence CIE coordinates for the phosphors and the PMMA films are at the middle of the “green area” (0.3, 0.6). Phosphors like those are useful for both biological and engineering applications as biomarkers and for solar cell enhancers, respectively. © 2018 Society of Photo-Optical Instrumentation Engineers (SPIE) [DOI: 10.1117/1.JPE.8.014001]

Keywords: PMMA composite films; infrared activation; laminar shape phosphors. Paper 17110 received Nov. 6, 2017; accepted for publication Jan. 29, 2018; published online Feb. 20, 2018.

1 Introduction Rare earths (RE3þ )-doped luminescent materials have been extensively studied due to their exceptional optical characteristics and their thermal and chemical stabilities.1 In particular, RE3þ -doped upconversion (UC) luminescence materials have many potential applications in photonics, bifacial silicon solar cells, and advanced optical amplifiers.2–4 Among the UC luminescent RE ions, Ho3þ is an important activated ion, which produces green and red visible emissions when excited by a 980-nm diode laser,5 on the other hand, Yb3þ ion is an excellent sensitizer for Ho3þ ions, which enhances UC luminescent efficiency through energy transfer due to its strong absorption in the region around 980 nm. Hence, the Yb3þ − Ho3þ array is commonly used to produce visible green or red UC emission in a great variety of host materials

*Address all correspondence to: Salvador Carmona Téllez, E-mail: [email protected] 1947-7988/2018/$25.00 © 2018 SPIE

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Martínez-Avila et al.: Luminescent polymethyl methacrylate composite films activated. . .

such as: Re2 TeO6 (Re = La, Gd, and Lu),6 Y2 O3 ,7 and ZnO.8 Among them, cubic Gd2 O3 is recognized as a suitable host material due to its adequate properties: low phonon energy (around 600 cm−1 ), easy doping with other REs ions because of similarity in ionic radius and charge, chemical and thermal stability.9 Its high thermal stability makes Gd2 O3 one of the best matrixes for production of highly luminescent phosphors with great luminescent properties.10 Recently, Gd2 O3 is used both as fluorescent microscopy marker and as contrast enhancer for magnetic resonance imaging due to its magnetic properties.11 One way to enhance the luminescent intensity of some materials is by use of lithium as a codopant.12–14 The improvement of the RE3þ emission by Liþ codoping is commonly associated with the modification of the RE─O bond lengths, which modify the crystal field around the RE3þ ions, allowing the otherwise forbidden intra-4f transitions of some Res.15 Another previously reported way to explain the Liþ effect is its possible role as charge compensator.13 Although, Gd2 O3 has been extensively studied, there are only few reports on UC Gd2 O3 phosphors; these phosphors are commonly synthetized by methods such as combustion and sol–gel.16–18 The simple solvent evaporation technique is an inexpensive method in which the precursor is dissolved in appropriated solvent and then it is subjected to evaporation of the solvent at an appropriate temperature, this method allows to obtain large amount of powder with excellent photoluminescent characteristics that could be scalable to industrial applications. Phosphors like those are useful for engineering and biological applications such as sensing and heating applications and biomarkers in the detection and treatment of cancer cells.19–21 On the other hand, a perennial challenge for all powder shape phosphors is to achieve its incorporation in large and thin areas while their optical properties are retained. Polymer films offer extraordinary properties such as mechanical flexibility, high transparency, and low manufacturing cost, and they may be synthesized in presence of some phosphors as composites, gaining its luminescent properties, and at the same time solving the challenge mentioned above. Polymer films have been sensitized by many techniques such as spin-coating, sol–gel, spray pyrolysis, and MAPLE (matrix-assisted pulsed laser evaporation) among others. Polymethyl methacrylate (PMMA) is a transparent thermoplastic often used in sheet form as a lightweight or shatter-resistant alternative to glass; it has excellent transparency (92% to 99%T, between 380 and 1500 nm), high flexibility, and good chemical and thermal stabilities. These characteristics make PMMA an excellent candidate to be used as a coating on solar cell devices.22 In this work, the UC luminescent, optical, and structural characteristics of Gd2 O3 phosphors synthesized by simple evaporation method as well of those for PMMA films incorporating these phosphors are presented. The roles that the relative concentration of Ho3þ , Yb3þ , and Liþ ions play on these characteristics were investigated in detail. The PMMA composite films were obtained by the spin-coating technique.

2 Experimental Details 2.1 Phosphors Synthesis Gd2 O3 phosphors doped with Ho, Ho-Yb, and Ho-Yb-Li were prepared by a simple solvent evaporation method; this technique has been used widely to obtain powders of different materials, mainly metal oxides,23,24 since this technique is used under atmospheric pressure conditions. The simple evaporation technique is an inexpensive and scalable technique to obtain powders by the solvent evaporation of a solution containing the precursor reactants, this process guarantees that the achieved mixture of the precursors is homogeneous at molecular level in the starting powders and ready for their thermal sintering at atmospheric pressure conditions to achieve the proper chemical composition and crystalline structure. The difference with regular thermal annealing method is that the starting powders, in this last case, are usually a physical mixture of the reactant powders, which could have an impact on the uniformity of the final powders produced. For this work, appropriated amounts of GdðNO3 Þ3 6H2 O, YbCl3 6H2 O, HoCl3 6H2 O, and LiCl supplied by Sigma Aldrich were dissolved in deionized water (18 MΩ-cm) and heated at 200°C on a heating grid until the solvent was evaporated and a powder is achieved; then, this powder was put into a furnace at 1100°C during 2 h. The RE3þ and Li contents were varied from 0.1 to 1.5 at. % for Ho, 0 to 12 at. % for Yb and from 0 to 6 at. % for Li; all of them, Journal of Photonics for Energy

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in relation to the gadolinium content. The maximum intensity of the characteristic green light emission from Ho3þ under UC conditions was obtained for the phosphors doped with 0.4 at. % of Ho3þ , 3 at. % of Yb3þ , and 4.5 at. % of Li. Most of the powders obtained with this technique have particles in the size range from 100 nm to a few microns.

2.2 Synthesis of PMMA 20 ml of methyl methacrylate monomer were dissolved in 80 ml of toluene and 0.048 g of benzoyl peroxide was then added. Polymerization reaction was carried out in a 500-ml three-neck flask connected to a high purity nitrogen inlet, a condenser, and a thermometer. The solution was heated at 70°C under magnetic stirring for 10 h. The reaction mixture was then precipitated in methanol and the resulting PMMA polymer was dried under vacuum at 60°C for 24 h. According to measurements of average molecular weight, PMMA has a MW of 123 27 kDa.

2.3 PMMA Films Synthesis PMMA films were prepared by spin-coating technique using a Laurell Technologies Corp. spin coater model WS-200-4NPD. This technique has been widely used to obtain films or coatings of different materials, mainly polymers. In the spin-coating process, a small drop of a liquid resin is placed onto the center of a substrate and then the substrate spins at high speed (between 3000 and 6500 rpm). Centripetal acceleration will cause the resin to spread and eventually to cover the substrate leaving a film of resin on the substrate surface. The final film thickness and other properties will depend on the nature of the resin (viscosity, drying rate, percent solids, surface tension, etc.) and the parameters chosen for the spin process. In this case, PMMA films were spin coated onto Corning glass substrates. The composite films were processed using Gd2 O3 -based phosphors, with the doping concentrations corresponding to the best UC luminescence emission, functionalized with TOPO to insure good incorporation into the PMMA matrix in which it was mixed as follows: the functionalized phosphors were mixed with the achieved PMMA (as described above) and dissolved in chloroform (10% of the phosphors in weight with 0.35 g of PMMA into 7 ml of chloroform), a drop of this mix (0.5 ml approx.) was put on the substrate surface, and spun at 500, 1000, 1500, and 2000 rpm for 150 s. The films were then immediately dried for 10 min at 200°C on a preheated hot plate. The kinds of substrates used in this experiment are quartz (for nondoped films) and glass (for Gd2 O3 phosphors-doped films).

2.4 Characterization Instruments X-ray Diffraction (XRD) measurements were made in a Siemens D-500 equipment Cu Kα (α ¼ 1.541 Å). The morphology of the obtained phosphors was observed using a scanning electron microscope JEOL using an acceleration voltage of 20 KV and ×5000 as magnification, this equipment is capable to determinate the chemical composition analysis by energy dispersion spectroscopy (EDS). IR spectroscopy was used to determinate any kind of bond that are present in Gd2 O3 phosphors, these measurements were made in a Fourier transform infrared spectrometer (FT-IR) Mod 6700-FT-IR NICOLET. The average molecular weight of the PMMA was determined with a Malvern Zatasizer Nano SZ using toluene as solvent. Atomic force microscopy measurements were carried out in an AFM: Bruker, Bioscope Catalyst ScanAsyst. Photoluminescence (PL) measurements were carried out using an Edinburgh Instruments 980S spectrophotometer equipped with an integrating sphere to solve quantum yield efficiency. All measurements were carried out at room temperature.

3 Results and Discussion 3.1 Luminescent Phosphors The structural characteristics and phase purity of the as-prepared phosphors were examined by XRD. Figure 1(a) shows the XRD pattern for triple doped Gd2 O3 phosphors and the data from JCPDS 12-0797 diffraction card25 for the cubic structure of Gd2 O3 ; it can be observed that Journal of Photonics for Energy

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Fig. 1 XRD measurements from Gd2 O3 phosphors.

the peaks in the XRD graph can be associated nicely with those expected for the cubic phase of Gd2 O3 . The diffraction pattern shows reflections centered at values of 2θ of 28.57 deg, 33.1 deg, 47.52 deg, and 56.4 deg mainly, corresponding to diffracting planes (2 2 2), (4 0 0), (4 4 0), and (6 2 2), respectively, among others; where the strongest reflection is related to the plane (2 2 2) of the cubic structure. This behavior is similar in doped and nondoped samples. The cubic triple doped Gd2 O3 primitive cell has a lattice parameter of a ¼ 10.7019 Å, β ¼ 90.00 deg, and volume ¼ 1225.72 Å3 .25 The crystallite size was estimated by the Scherrer’s formula;26 T ¼ ð0.9λÞ∕B cos θB , where T represents the crystallite size, λ is the wavelength of CuKα (1.5406 Å) radiation, B is the full-width in radians at half-maximum (FWHM) of the peak at 2θ ¼ 28.5 and θB is the Bragg’s angle of the XRD peak. According to this formula, the average crystallite size was estimated to be about 44.2 6.8 nm for both doped and nondoped; commonly, the Gd2 O3 phase has particle sizes between 40 and 60 nm for samples synthesized at 1100°C.27 In addition, Fig. 1(b) shows that the incorporation of holmium and ytterbium improves the crystallinity of the phosphors reflected by the overall diffraction peaks intensity enhancement. However, the lithium incorporation produces a reduction of the crystallinity, this behavior caused by the lithium presence has been previously reported.28 An extra behavior observed at the same image is that the main diffraction peak (222) shifts toward large angles when holmium, ytterbium, and lithium are incorporated, this fact suggests that RE3þ and Liþ ions induce a shrinking of the lattice as they replace Gd3þ ions or as they are introduced at interstitial sites. Hence, the incorporation of RE3þ and Liþ ions affects the crystalline structure of the host, and thus, the crystalline field felt by the RE ions. By means of UnitCell software, the lattice parameter “a” as well as volume were calculated, it is observable that they suffer a contraction as the dopant materials are incorporated into Gd2 O3 matrix with values of a ¼ 10.7212, 10.7204, 10.7153, and 10.7019 Å, for Gd2 O3, Gd2 O3 ∶Ho, Gd2 O3 ∶Ho-Yb, and Gd2 O3 ∶Ho-Yb-Li samples, respectively. In the case of volume cell, the values of vary as follows, v ¼ 1232.3638, 1232.0851, 1230.2120, and 1225.7279 Å3 , also respectively. A contraction like this is due to the difference of the atomic radii between Gd and the dopant materials, mainly in the case of lithium ions, which are very small in comparison with the other present atoms, this kind of contraction could provoke an approach of the different atoms species, which could led to an enhancement of luminescence. This mechanism will be described with a major detail in PL section. This phenomenon has also been observed with the incorporation of Er3þ and Liþ ions into Y2 O3 thin films producing similar type of effect.28 The FTIR absorbance spectra for the phosphors and the undoped Gd2 O3 powder are shown in Fig. 2. These spectra reveal the presence of absorption bands in the ranges of 450 to 540 cm−1 , 1360 to 1630 cm−1 , and 3400 to 3500 cm−1 . The bands appearing at 450 to 540 cm−1 correspond to Gd–O vibration modes of the Gd2 O3 cubic phase as previously reported,29 the signal at 1360 to 1630 cm−1 is associated with the molecular H2 O (H─O─H) bending frequency and the broad adsorption band about 3400 to 3500 cm−1 corresponds to hydrated and physically Journal of Photonics for Energy

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Fig. 2 Infrared spectroscopy measurements from Gd2 O3 phosphors.

adsorbed water in the sample,30 specific features related to Liþ presence could be associated with small absorption features between 800 and 1200 cm−1 (inset) observed in the phosphors spectrum, according to the literature this bands could be originated by a superposition of absorption peaks related to vibrations of Li─C─O bonds such as those of Li2 CO3 (860 and 1090 cm−1 ), Li2 CO2 (984 cm−1 ), and LiC2 O4 (1160 cm−1 ).31,32 The formation of those kind of bonds could be due to the presence of lithium in a carried out reaction under atmospheric pressure with no vacuum conditions. Scanning electron microscopy (SEM) micrographs for powder phosphors are shown in Fig. 3 (a. for nondoped Gd2 O3 , b. for Gd2 O3 ∶Ho3þ, c. for Gd2 O3 ∶Ho3þ -Yb3þ, and d. for Gd2 O3 ∶Ho3þ -Yb3þ -Liþ ) all of them measured using 20 KV as acceleration voltage and magnification ×5000. In the case of nondoped powders as well as Ho3þ - and Ho3þ -Yb3þ -doped phosphors (without lithium presence), it is appreciated that powders grow in big laminar-shaped particles, which have small pores, forming large clusters of around 0.1 to 1 μm; the smallest particle size of the Gd2 O3 powders is found to be near 250 nm after annealing at 1100°C.

Fig. 3 SEM images for, (a) Gd2 O3 , (b) Gd2 O3 ∶Ho3þ , (c) Gd2 O3 ∶Ho3þ -Yb3þ , and (d) Gd2 O3 ∶Ho3þ -Yb3þ -Liþ powders. Journal of Photonics for Energy

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Table 1 EDS measurements Gd2 O3 ∶Ho3þ -Yb3þ -Liþ powders.

for

Gd2 O3 ,

Gd2 O3 ∶Ho3þ ,

Gd2 O3 ∶Ho3þ -Yb3þ ,

and

Kind of sample Gd2 O3

Gd2 O3 ∶Ho

Gd2 O3 ∶Ho∕Yb

Gd2 O3 ∶Ho∕Yb∕Li

Oxygen

59.5 1.8

62.7 1.9

65.6 1.9

60.2 1.8

Gadolinium

40.5 1.2

37.1 1.1

33.2 1.0

38.2 1.1

Holmium

X

0.2 0.1

0.3 0.1

0.3 0.1

Ytterbium

X

X

0.9 0.1

1.3 0.1

Lithium

X

X

X

X

Element (at. %)

It has been previously reported that due to the growing chemical dynamics, typical of the evaporation technique, and the subsequent heat treatment, the powders synthesized by this technique are commonly formed by micrometric particles.33,34 The estimated crystallite size of 44.2 6.8 nm was determined from the XRD data by means of Scherrer’s formula; in the case of SEM images the observed morphology shows the formation of crystallite clusters that sometimes could form big structures as ribbons, tubes, spheres, and others. In our specific case, we could observe the formation of laminar phosphors, which in one of its dimensions (thickness) has similar dimensions to those achieved from XRD. In the case of Li codoped powders (Gd2 O3 ∶Ho3þ -Yb3þ and Liþ ), the morphology is pretty different, although powders grow up in big clusters of similar sizes as those of nondoped samples (or doped with Ho and Yb); these clusters have no pores and acquire very large interconnected areas (kebab shape), which have sizes between 300 nm and 5 μm; the introduction of RE ions and, mainly, lithium ions favors the formation “kebab shape” particles, perhaps because of the flux properties of lithium.35 Although nanometric powder sizes are of interest in luminescence particularly in biological applications,36–38 the micrometric size powders are particularly good for some technological applications such as solid-state lighting devices,39,40 and particularly, a laminar or kebab shape could be especially used in the synthesis of materials to be used into field effect displays where the area is important in order to take advantage of the electron beam at maximum.41 Even though EDS is not a quantitative technique, it is useful to estimate the chemical composition of samples. EDS measurements on doped and nondoped phosphors were performed, and the results are listed in Table 1. In the case of doped samples, they were classified according to the type of doping (Ho3þ , Ho3þ -Yb3þ , and Ho3þ -Yb3þ -Liþ ). As it will be shown below, samples that have the best green light emission were synthesized using an initial solution: Ho (0.4%), Yb (3%), and Li (4.5%). According to the expected stoichiometry for gadolinium oxide (a relation of 3 to 2 of the oxygen to gadolinium), a presence close of 60% of oxygen and 40% of gadolinium is observed. In the case of doped samples, there is an apparent substitution of Gd ions by RE3þ and probably Liþ ions (although lithium cannot be measured by EDS technique). Figure 4(a) shows the PL emission spectra under continuous excitation at 980 nm for the best green light emission phosphors, synthesized using an initial solution: Ho (0.4%), Yb (3%), and Li (4.5%). The characteristic green UC emissions centered at 537, 551, and 667 nm, associated with interelectronic energy, state transitions of Ho3þ ions from 5 F4 → 5 I8 , 5 S2 → 5 I8 , and 5 F → 5 I , respectively. As it was mentioned before, doping concentrations of Ho3þ in the 5 8 range of 0.1 to 1.5 at. % were studied in this work, Fig. 4(c) shows the behavior of the green (551 nm peak) emission as a function of Ho3þ concentration (with constant 3 at. % of Yb3þ ). It can be observed that the optimal concentration of Ho3þ is 0.4 at. %, higher concentrations produce a quenching of the luminescence, this emission is intense enough that can be observed at naked eye. The presence of Yb3þ as a codopant is necessary, since without it, the UC emission from Ho3þ is not observed, but an adequate combination with Yb3þ ions enhances the green light emission. Figure 4(d) shows the behavior of emission intensity as a function of Yb3þ concentration which it has been varied from 0 to 12 at. %, (with constant 0.4 at. % of Ho3þ ), Journal of Photonics for Energy

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Fig. 4 Photoluminescent properties of Gd2 O3 ∶Ho3þ -Yb3þ -Liþ phosphors.

the optimal concentration of Yb3þ is 3 at. %, again, the use of higher concentrations produces a new quenching of the luminescence. The incorporation of lithium ions as codopants into some matrices could increase the luminescence intensity of phosphors. However, this effect is not always present at every host material; incorporation of lithium ions in matrices such as Y2 O3 , SrTiO3 , and ZnO could increase 200 or even more times the luminescence intensity originated by codopants.28,42,43 In other materials such as LiAlO2 and LiLa6 O5 ðBO3 Þ3 , luminescence intensity could increase just 10% or 20% in comparison with matrices without Liþ codoping, and in some cases, the intensity decreases even below this level.44–46 In this work, doping concentration of Liþ was varied from 0 to 6 at. % (with constant 0.4 at. % of Ho3þ and 3 at. % of Yb3þ ), and the UC luminescence intensity behavior was monitored. Figure 4(e) shows the green luminescence intensity behavior for Gd2 O3 ∶0.4 at. % Ho3þ and 3 at. % Yb3þ , as a function of Liþ content, the green light emission has a maximum intensity when the samples were doped with 4.5 at. % of Li. It should be pointed out that there is an increment of intensities among single, double, and triple doped samples; this increment is of 9.18 and 11.09 times between Yb3þ -Ho3þ and Yb3þ -Ho3þ -Liþ samples, respectively; both in comparison with just Ho3þ -doped samples. From these results, it is simple to deduce that the presence of Yb3þ enhances dramatically the green (551 nm) emission of Ho3þ transitions, in addition the presence of lithium ions produces an intensity increment of the Yb3þ -Ho3þ phosphors of about 17.3%. According to previous reports for similar materials, it is probably that Ho3þ and Yb3þ ions enter in place of Gd atoms, whereas Liþ could be incorporated in either a Gd site or at interstitial places within the Gd2 O3 matrix, since Liþ is a small atom. It has been proposed47 that Liþ is incorporated in positions associated with Ho3þ and Yb3þ ions, in such a way that the amount of Liþ that will benefit the Ho3þ and Yb3þ ions as luminescent centers will depend on the Ho3þ and Yb3þ concentration. Larger Liþ concentration could either compete with the Ho3þ and Yb3þ ions incorporation in a Gd site, or it could generate other centers that might act as luminescence killers. As it was suggested before in XRD analysis, a contraction in the volume of the cell is expected, and it could help to enhance the luminescence of the phosphors. Figure 4(a) (inset) shows the UV–vis absorption spectrum of triply doped phosphors recorded in reflectance mode in a 200 to 900 nm range. Spectra consists of sharp peaks ascribed to several transitions corresponding to intraconfigurational (4f-4-f) transitions of Gd3þ , Ho3þ , Liþ , and Yb3þ ions. The intense broad absorption band centered at 223 nm is corresponding to the 8 S7∕2 to 6 Gj (j ¼ 13∕2, 11∕2, 9∕2, 7∕2, and 5∕2) transitions of Gd3þ ions. The absorption spectra of these samples consist of several absorption bands of Ho3þ ion peaked at 275, 289, 306, Journal of Photonics for Energy

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316, 344, 362, 382, 392, 441, 451, 463, 487, 537, 622, 639 644, and 721 nm corresponding to transitions from the ground state 5 I8 level to the various excited states 3 H4 þ 3 F2 , 3L þ 3M þ 5D þ 3G , 3H þ 3H , 5G þ 3K , 5G , 5G þ 5F , 5F þ 3K , 5F , 5S þ 5F , 8 10 4 3 5 6 4 7 5 6 1 2 8 3 2 4 5 F5 , and 5 I5 levels, respectively. Some extra peaks associated to the presence of Yb3þ are expected at 910, 932, 953, and 976 nm attributed to 2 F7∕2 to 2 F5∕2 transition of Yb3þ ion, which are not shown due to instrumental limitations. The plot has no peaks associated to Liþ , perhaps because it is not optically active in this region.48 Figure 4(b) shows a schematic picture of various possible UC mechanisms starting with the excitation of an electron from the Yb3þ ion ground-state 2 F7∕2 to the excited-state 2 F5∕2 followed by an energy transfer event (ET) from Yb3þ to Ho3þ populating the 5 I6 , then another excited Yb3þ ons promote Ho3þ ions in the energy transfer UC process: 2 F5∕2 ðYb3þ Þ þ 5 I6 ðHo3þ Þ → 2F 3þ 5 5 3þ 7∕2 ðYb Þ þ S2 and F4 ðHo Þ producing the 537 and 551 nm green emissions. Some of the 5 5 ions in S2 and F4 levels relax nonradiatively to the 5 F5 level producing the 667-nm red emission. Moreover, 5 S2 , 5 F4 , and 5 F5 levels could be populated through excited state absorption enhancing the 537 and 551 nm emissions and partially the 667 emission.49 It is evident 5 F4 → 5 I8 and 5 S2 → 5 I8 transitions are very efficient in Gd2 O3 host material and 5 F5 → 5 I8 is not preferred. Figure 4(f) shows a plot of lnðIÞ versus lnðPÞ, where I is the luminescence intensity monitored for the green (551nm) emission and P is the 980 nm laser power, for samples doped with Ho, Ho-Yb, and Ho-Yb-Li. This figure illustrates the behavior of the UC emission monitored as a function of the excitation power. The expected behavior in UC of I versus P is the following: I ∝ Pn ;

EQ-TARGET;temp:intralink-;sec3.1;116;484

where n is the average number of photons involved in the UC process. In the case of the Ho-Ybdoped samples, it is observable a tendency to increase the slope value, the addition of Liþ shows also the same tendency but in less proportion, in comparison with the effect observed in the incorporation of Yb3þ to the system. Finally, the UC quantum yield (UCQY) expression derived in Ref. 50 can be used to infer the improvement of the UCQY as follows: QYF ¼

EQ-TARGET;temp:intralink-;sec3.1;116;393

K 1 Pn1 −1 ; K 2 Pn2 −1

where QYF is a ratio between the UCQY of just Ho-doped samples and double and triple doped samples, P is the laser power, n1 , n2 , and n3 are the slopes and K 1 , K 2 , and K 3 are the proportional constants obtained from the LnðIÞ versus LnðPÞ measurements for just holmium doping, double and triple doped samples, respectively. Hence, the UPQY of 551 nm emission at 240 mW for Ho-Yb and Ho-Yb-Li is enhanced by a factor (FQY) of 7.05 and 8.54, respectively, in comparison with just Ho3þ -doped samples, which reveals that the incorporation of Yb3þ and Liþ also makes more efficient the generation of upconverted photons. Finally, in order to achieve a nominal QY efficiency, measurements with an integrating sphere were performed using a 980-nm laser operated at 240 mW, for a Ho-Yb-Li sample, presenting a QY of 1.8. According to the above described analysis, Ho-Yb and just Ho-doped samples have QY values of 1.48 and 0.21, respectively; those values fall into previous reported values, and according to Ref. 51 the QY values for UC phosphors are always between 0.005% and 3%.

3.2 PMMA Phosphor Composite Films As mentioned above, the Gd2 O3 -based phosphors with the highest light emission intensity were incorporated into PMMA thin films by spin-coating technique, Fig. 5(a) shows the IR absorbance spectra for the nondoped polymeric films, the spectra show, basically, the same bands reported for infrared spectra of PMMA.52,53 Table 2 lists the wavenumber values as a function of the kind of bond vibration, in the polymeric films with phosphor particles embedded, the spectrum do not show specific features related to the phosphor particles embedded in the PMMA films, perhaps because the strong signals from the PMMA shadows them. Figure 5(c) shows the UV–vis-near IR transmittance characteristics of PMMA composite films. From this plot, it is observed that nondoped films are highly transparent (close to Journal of Photonics for Energy

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Fig. 5 (a) Infrared spectroscopy, (b) PL spectrum, (c) UV–vis–NIR spectroscopy, (d) luminescence and thickness versus spin velocity, and (e) thickness normalized emission intensity versus spin rate measurements from PMMA-doped films.

Table 2 Vibrational modes and wavenumbers exhibited by PMMA films. Descriptions of vibrations

Wavenumbers 754 cm−1

α-methyl group C─O─C and ─OCH3 stretching

1148 to 1275 cm−1

CH3 stretching

1439 to 1481 cm−1

Acrylate carboxyl group

1665 to 1734 cm−1

C═O stretching

1700 to 1744 cm−1

C─H bonds of ─CH3 group

2930 to 2990 cm−1

─OH stretching

3440 to 3613 cm−1

99% T) in UV region (260 to 400 nm), in all the whole visible range, and near-infrared (800 to 1100 nm) comparable to the best quality bulk PMMA.52,53 In the case gadolinium phosphors doped films, its transparency depends directly on the deposition spin velocity used of the spin coating equipment, at low velocities films are semitransparent between 65 and 80%T (as in the case of films deposited at 500 rpm); however, if the velocity is increased, those values change to reach ∼96%T as in the case of films deposited at 2000 rpm. This behavior [shown in Fig. 5(d)] is due to the fact that high velocity reduces the films’ thicknesses and provokes the expulsion of a part of the phosphors because of the centripetal force, therefore at high velocities the achieved films are thinner than those synthesized at low velocities, and its total phosphor contents are lower. According to profilometry measurements, the thickness of films for 500 rpm is 1.3 0.09 μm, for 1000 rpm 0.87 0.04 μm, for 1500 rpm 0.78 0.037 μm, and for 2000 rpm 0.73 0.04 μm. Figure 6 shows the AFM surface plots from the 500 (a) and 2000 (b) rpm films, it can be observed that, for both cases, the surface is composed by two main phases: thicker regions of the particles embedded in the polymer matrix (higher profiles) surrounded by thinner regions of pure PMMA (lower profiles). However, when the spin rate is as low as 500 rpm, the film presents particle agglomeration and a surface with higher roughness features. In contrast, the lower phosphor content and quicker solvent evaporation at higher spin Journal of Photonics for Energy

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Fig. 6 AFM images for (a) 500 rpm films and (b) 2000 rpm films.

rates (2000 rpm) produces far smoother films with better distribution of particles in the PMMA matrix. The polymeric films without Gd2 O3 -based phosphors embedded into them do not present any luminescent emission as expected. When the phosphors above described were embedded into the PMMA films, the luminescent properties are transferred to films, the emission luminescent spectrum for Gd2 O3 ∶0.4 at. % Ho3þ , 3 at. % Yb3þ , and 4.5 at. % Liþ phosphors embedded into PMMA films is shown in Fig. 5(b), this spectrum is pretty similar to the Gd2 O3 -doped phosphors [Fig. 4(a)], it presents the characteristic green UC emissions centered at 537, 551, and red emission at 667 nm, associated with interelectronic energy state transitions of Ho3þ ions from 5 F4 → 5 I8 , 5 S2 → 5 I8 , and 5 F5 → o 5 I8 , respectively. However, although the PMMA films have the same luminescent properties of powdered phosphors, the relative intensity light emission between them are clearly different, since the powdered phosphors’ intensity is two orders of magnitude higher than PMMA films. The light emission intensity depends directly on the relative phosphor content in the matrix, but excessive phosphors concentration can affect the transparency of the films, decreasing its effective conversion efficiency; as it was discussed before, if the deposition spin velocity changes, the relative phosphor content is directly modified [like is shown in Fig. 5(d)], which in turn, affects the film transparency features. Thus, for comparison purposes, the ratio of the area under the PL emission curve divided by the thickness was calculated and plotted as a function of the spin rates [Fig. 5(e)]. This value is a normalization of the luminescence intensity to the film thickness and was used as an indicator of the overall conversion efficiency for each film. It is evident that there is a linked effect between transparency, thickness, and light emission intensity, that is to say, a thick film has high light emission because it has more luminescent particles, but reduces the films transparency; on the other hand, a thinner film has low light emission but is highly transparent and has no coloration in the visible range. All those described characteristics could be controlled in order to make them very attractive to be used as thin-film gas sensors and solar cell enhancers.54 Finally, according to the diagram CIE (Commission international de l’éclairage),55 the global UC emission generated from PMMA composite films and Gd2 O3 -based phosphors has (0.2973, 0.6162) and (0.3029, 0.6097) coordinates, respectively (not shown diagram); these values are located at the “green area” of the CIE diagram, which is expected for holmium ions presence.

4 Conclusions The luminescent properties of Gd2 O3 doped with Ho3þ , Yb3þ , and Liþ phosphors are transferred to PMMA composite films. They have excellent UC luminescence properties since 980 nm continuous radiation was used in order to excite them. Composite films (and phosphors) exhibit a broad band luminescence emission centered at 551 nm, associated with the Ho3þ ions interelectronic energy state transition 5 S2 → 5 I8 . PMMA composite films present high transparency, which is closely linked to UC luminescence intensity, because high concentration of phosphors into PMMA composite films provokes high UC luminescence intensity, but it sacrifices films transparency and vice versa. At the same time, both depend on PMMA composite films thicknesses, which is determined by the spin deposition velocity (films have thicknesses between 0.7 and 1.4 μm). The above-mentioned properties make PMMA composite films an excellent candidate to be used as a coating on solar cell devices. Journal of Photonics for Energy

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Acknowledgments The authors wish to acknowledge the technical assistance of Z. Rivera and M. Guerrero from the Physics Department of CINVESTAV-IPN. The authors gratefully acknowledge Dr. Juan Méndez and Centro de Nanociencias-IPN for their support in the AFM measurements and to Dr. Rocio Casañas for his help in molecular weight calculations. The authors also thank Consejo Nacional de Ciencia y Tecnología (CONACyT) for financial support.

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