Polyethylene terephthalate thin films; a luminescence ...

Optical Materials 42 (2015) 99–105

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Polyethylene terephthalate thin films; a luminescence study S. Carmona-Téllez a,⇑, G. Alarcón-Flores c, A. Meza-Rocha b, E. Zaleta-Alejandre d, M. Aguilar-Futis c, H. Murrieta S a, C. Falcony b a

IF-UNAM, México D.F. 04510, Mexico CINVESTAV-IPN, México D.F. 07360, Mexico c CICATA-IPN, México D.F. 11500, Mexico d UAEH-ESAp, Apan, Hidalgo 43920, Mexico b

a r t i c l e

i n f o

Article history: Received 18 September 2014 Received in revised form 19 December 2014 Accepted 22 December 2014 Available online 7 January 2015 Keywords: Spray pyrolysis Polymeric films PET Light emission

a b s t r a c t Polyethylene Terephthalate (PET) films doped with Rare Earths (RE3+) have been deposited on glass by spray pyrolysis technique at 240 °C, using recycled PET and (RE3+) chlorides as precursors. Cerium, terbium, dysprosium and europium were used as dopants materials, these dopants normally produce luminescent emissions at 450, 545, 573 and 612 nm respectively; the doped films also have light emissions at blue, green, yellow and red respectively. All RE3+ characteristic emissions were observed at naked eyes. Every deposited films show a high transmission in the visible range (close 80% T), films surfaces are pretty soft and homogeneous. Films thickness is around 3 lm. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Over years the synthesis and studies about rare-earth ions (RE3+) doped materials have attained great importance for a wide variety of potential applications in optical technology, such as phosphors [1–3], lasers [4–6], fiber amplifiers [4,6], high-density optical storage [4,6], and electroluminescent display devices [2,3]. In particular, RE3+ doped thin film phosphors structures have drawn a special attention due to their promising applications in flat panel displays (FPDs) [2]. The development of flexible, thin displays is a much soughtafter goal, and has attracted a great deal of research effort [7– 11]. By integrating, for example, the high information content of a traditional flat-panel liquid–crystal display (LCD) into a thin, flexible sheet of plastic, one could obtain a durable, lightweight product suitable for many applications in the growing market of pagers, cell phones, and personal digital assistants (PDAs), as well as future ‘‘electronic paper’’. By eliminating the need to rely on thin and fragile glass substrates, flexible displays should also bring benefits in the form of improved manufacturing yield, large area display capability, and less material and lower production costs. Oxide-based phosphors such as yttrium, aluminum and hafnium [12–14] have been considered as potential thin film phosphors because of their high transparency, excellent chemical and ⇑ Corresponding author. E-mail address: s_carmona@fisica.unam.mx (S. Carmona-Téllez). http://dx.doi.org/10.1016/j.optmat.2014.12.026 0925-3467/Ó 2014 Elsevier B.V. All rights reserved.

thermal stabilities, and relatively high luminescence brightness performance [2–6,15] however, it is fair to say that it is necessary to use high reaction temperatures (above 500 °C) in order to achieve these characteristics. Nowadays, another kind of properties are also important like flexibility, environment friendly and low manufacturing cost; this properties are present in polymer films, since recycled materials could be employed to synthesize and low reaction temperatures are used. Polymeric thin films are common; some of them with luminescent properties [16–18] by the use of RE3+; many techniques are used to synthesize them such as: spin-coating, sol–gel, electrospinning and MAPLE (matrix assisted pulsed laser evaporation) between others [19–22]. Polyethylene Terephthalate (PET) is a polymer, mainly used as container for beverages, with excellent characteristics as high transparency, flexibility and excellent chemical and thermal stabilities. However, PET water bottles are a serious pollution problem around the world, because they end up in landfills and take centuries to decompose. In order to reduce the presence of PET in the landfills, the search for alternative uses of PET are continually present [23,24]. In the present work, the use of PET as host lattice is reported. Cerium, terbium, dysprosium and europium were used as dopant materials, films doped with them, give rise to luminescent emissions at 450, 545, 573 and 612 nm respectively; films also have high light emissions at blue, green, yellow and red respectively. PET films were made by spray pyrolysis technique using corning glass, quartz and crystalline (1 1 1 and 1 1 0) silicon as substrates.

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Photoluminescence (PL) and cathodoluminescence (CL) emissions of these films are reported as well as optical and structural characteristics (by UV–Vis, IR spectroscopy AFM and SEM measurements). White light was also achieved using double dopant materials on PET films. This work aims to present a new way to deal with plastic waste, through a technological implementation. By using spray pyrolysis technique, the polymeric films deposition ensures low cost of a possible development of light emitting devices based on polymer films of PET and at the same time, at the best of our knowledge, this is the first time in which this deposition technique has been used to elaborate polymeric thin films. Similar studies, using other deposition techniques [25,26], report some drawbacks such as clusters formation and low homogeneity; the above is mainly due to the use of micrometer size powders or light emitting organic inks as active luminescent centers. In our case, rare earths were introduced as dopants into a polymer matrix, which ensures an even distribution of light emission sites and avoids the occurrence of clustering of light emission centers at localized points. This fact has been reported before in other host materials [27–29].

2. Experimental details PET films were prepared from recycled powdered water bottles, in powder shape; dissolved in N,N-Dimethylformamide (N,NDMF), supplied by J.T. Baker heated at 120 °C. The films were deposited at 240 °C on corning glass, quartz and crystalline silicon (1 1 1 and 1 0 0) by spray pyrolysis technique. This technique has been widely used to obtain films or coatings of different materials, mainly metallic oxides, since the technique is used under atmospheric pressure conditions. The spray pyrolysis technique is considered an inexpensive and scalable technique to obtain films and coatings with excellent properties. The technique consists in supplying an aerosol from a chemical solution which undergoes a pyrolytic decomposition on a hot substrate, leading to a solid film or coating on top of the surface used as substrate. However, the technique has not been attempted to obtain films thinner than 300 Å. Most of the films that are obtained with this technique usually range from 0.1 up to a few microns. For this work, PET films were prepared with a 0.0005 mol/l chemical solution formed with recycled powdered PET in N,NDMF, and using Rare Earths (RE3+) as dopants (RE chlorides) previously dissolved into diethyleneglycol in a 0.01 mol/l chemical solution; 6 ml were taken of these kinds of solutions and were added to PET chemical solutions; REs used at this work were cerium, terbium, dysprosium and europium in order to achieve blue, green, yellow and red light emissions. A molten tin bath was used as thermal energy source for substrates to get a pyrolytic reaction, furthermore, nitrogen (N2) at a flow rate of 5 l/min was used as carrier gas. It is important to mention that the reproducibility characteristics on parameters such as roughness, and IR measurements for these films are excellent. Actually spray pyrolysis is a technique known for the quality of the synthesized films and high reproducibility; including very thin films (around 30 nm) [30]. In addition, around 80 samples were synthesized and characterized, in order to make this report, to ensure the reproducibility of the different film characteristics including luminescence, transparency and roughness. Optical transmittance spectra were obtained with a Perkin Elmer Lambda 25 spectrophotometer in a wavelength range of 200–1100 nm, and a Thermo Scientific Nicolet 6700 FT-IR spectrophotometer in a wavelength range of 500–40,000 nm was used to achieve infrared analysis.

Roughness and morphology were measured with an Atomic Force Microscope Veeco CP Research, that is capable to measure both arithmetic average and root mean square roughness (RMS), at this work RMS measurements are reported. RMS parameter represents the standard deviation of the distribution of surface heights, so it is an important parameter to describe the surface roughness by statistical methods. This parameter is more sensitive than arithmetic average height to large deviation from the mean line. The mathematical definition and the digital implementation of this parameter are as follows:

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Z 1 1 Rq ¼ fyðxÞg2 dx l 0 vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u n u1 X Rq ¼ t y2 n i¼1 i where Rq is the root mean square roughness, n is the number of intersections of the profile at the mean line and y is the relative height. The RMS mean line is the line that divides the profile so that, the sum of the squares of the deviations of the profile height from it is equal to zero [31]. SEM images were obtained in a Scanning Electron Microscope JEOL using an acceleration voltage of 1 kV and 500 as magnification. Photoluminescence (PL) measurements were carried out using a SPEX Fluoro-Max-P spectrophotometer. Finally CL measurements were performed in a stainless steel vacuum chamber with a cold cathode electron gun (Luminoscope, model ELM-2 MCA, RELION Co.). The thin films were placed inside the vacuum chamber and evacuated to 102 Torr. The electron beam was deflected through a 90° angle to focus onto the luminescent film normal to the surface; the diameter of the electron beam on the film was 3 mm approximately. The emitted light was collected by an optical fiber and fed into a SPEX Fluoro-Max-P spectrofluorimeter. The applied current of electron beam was 0.05 mA with an accelerating voltage in the range from 1 kV to 2 kV for all kinds of thin films. All measurements were carried out at room temperature. 3. Results and discussion Fig. 1(a) shows the IR spectrum of an undoped PET film, which is similar to the one previously reported by Marck [32]. The rest of images Fig. 1(b to f) show the IR spectra characteristics of doped PET films, it is possible to observe that those spectra are similar to only PET film spectrum, save for two bands (at 1540 and 1410 cm1) that are associated to the presence of (ACH2A) bonds

Fig. 1. IR spectra from undoped PET films (a). Rare earths doped PET films (b–f).


due to diethyleneglycol used as RE chlorides dissolvent during film synthesis [33]. Fig. 2 shows the UV–Vis-near IR characteristics of PET films. From this plot it is observed that undoped films, are highly transparent (close to 95% T) in the whole visible range, and comparable to the best quality bulk crystal PET [34]. While doped films are also transparent in the whole visible range but less than undoped films (between 80 and 90% T); films do not present any coloration in any case, regardless of which dopant is used. In Fig. 3, Microscopy (SEM and AFM) images are presented. Fig. 3(A) shows the typical SEM belonging to a Tb doped PET film. It is observed that PET film is composed by overlapping drops that coat the substrate surface to form a solid layer; these kind of films are transparent, with low roughness, thin and solid. Fig. 3(B) corresponds also to an AFM Tb doped PET film; it is observed that the surface is pretty smooth (RMS roughness is 630 ± 76 Å) and homogeneous. This kind of samples have the same characteristics of bulk PET, they are transparent and its roughness is like common PET bottles [23,34]. It is remarkable that RE doped and non doped PET films have similar morphological characteristics as surface planarity, roughness and homogeneity. So, dopant materials do not change these kinds of characteristics, but just the luminescent ones. Fig. 3(C) corresponds to an histogram that shows statistically, the roughness variation; their values are in a range from 200 to 1000 Å, the average value is close to 650 Å; and it gives a general idea about the topography of PET films. PL measurements are also shown. Fig. 4(A) shows a non-doped PET sample spectrum (a 300 nm excitation wavelength was used to achieve this spectrum), where two bands can be observed, the first one is centered at 370 nm and the second one at 470 nm (very weak); it is well known that these bands have its origin in the C@O bonds; this group usually gives rise to a very likely localized p⁄ n transition, and the subsequent fluorescence around 400 nm is an emission from the S1(n ? p⁄) state [35–37]. It should be noticed that these excitation and emission bands are weak as compared to those obtained from RE3+ doped films. Nevertheless, the PET light emission, covers the absorption of every rare earth, this represents an energy transfer between PET matrix and rare earths. The PET emission is due mainly to the presence of organic bonds like: p, n and ⁄p (which absorb in the region from 200 to 500 nm about) in the polymer chains of PET. Fig. 4(B) shows the photoluminescence spectra of Ce3+ PET films, the excitation wavelength used was 300 nm. The observed luminescence spectra peaks observed are broad and they are located at approximately 370 and 450 nm, they are associated with inter-level transitions of the electronic energy states of Ce3+ (5d ? 4f emission of Ce3+ ions). The highest emission intensity was obtained at 450 nm. The excitation spectrum for the 450 nm

Fig. 2. UV–Vis spectra from undoped PET films (a). Rare earths doped PET films (b– f).

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emission peak is also shown (dashed line). There are two excitation peaks, one located at around 265 nm and the other one at 300 nm. These results are similar to those previously reported for the luminescent properties of cerium doped films synthesized by spray pyrolysis using cerium chlorides as precursors. [38]. It was found before significant changes in luminescence due to different states of coordination, depending on the method of preparation used with Ce3+ [39]. In this case, with the synthesis method used, no variations associated with different states of coordination seem to be present. As mentioned before, these experiments were performed on multiple occasions always with similar results, confirming a single coordination state so it can also be noticed a uniform impurity distribution. As far as we know this is the first report of luminescent Ce doped polymeric films synthesized by the ultrasonic spray pyrolysis technique. The photoluminescence spectra for Tb3+-doped PET films are shown in Fig. 4(C), the luminescence peaks observed are associated with inter-level transitions within the electronic energy states of Tb3+ions, particularly those corresponding to transitions among levels 5D4 to 7F6, 7F5, 7F4 and 7F3, located at 490, 545, 590, and 622 nm respectively. The dominant peak for these spectra is the one associated with the transition 5D4 to 7F5 at 545 nm, which gives the characteristic green light emission identified with the presence of Tb3+ ions. In order to achieve these spectra a 300 nm excitation wavelength was used. The general behavior of these results is similar to that reported previously for other kinds of matrices [40]. The excitation spectrum is also given (dashed line) and as it can be observed excitation wavelengths from 250 to 300 nm are capable to excite the samples, being 300 nm the best one. This is due to an energy transfer process (as it was explained above) from the PET matrix to the luminescent center. Fig. 4(D) illustrates the room temperature photoluminescence excitation and emission spectral characteristic of the synthesized polymeric films, the spectra shown correspond to Dy3+ doped PET films, the excitation spectrum (dashed line) was measured in the 200–400 nm range for the 573 nm emission peak and shows just a peak (that starts at 250 nm and have a maximum at 300 nm) this kind of behavior is repeatable for every RE dopant. The PL emission spectrum was obtained at an excitation wavelength of 300 nm, it exhibits three distinct emission peaks [41]; located at 573 nm, associated with the hypersensitive transition 4 F9/2 to 6H13/2 which gives the characteristic yellow light emission identified with the presence of Dy3+ ions; at 485 nm corresponding to the less sensitive transition 4F9/2 to 6H15/2; and finally at 667 nm corresponds to the 4F9/2 to 6H11/2 transition. Fig. 4(E) shows the photoluminescent spectra of Eu3+-doped polymeric films. The luminescence spectra peaks observed are associated with inter level transitions within the electronic energy states of Eu3+ ions, particularly those associated with transitions levels 5D0 to 7F1, 7F2, 7F3 and 7F4, and they are located at 590, 612, 650, and 697 nm, respectively. PL spectra show the characteristics emission displaying a dominant peak associated with the transition 5D0 to 7F2 at 612 nm, so that the light emission is predominantly red. These spectra were generated with a 300 nm excitation wavelength corresponding to the excitation peak presented in the same Fig. 4(E) (dashed line). These results are similar to those previously reported for other kinds of matrices doped with europium [42] and once more the energy transfer is observed. Fig. 4(F) shows PL spectra of a double doped Ce and Dy PET films. This kind of sample was designed to achieve white light emission, due to necessary proper mix of blue and yellow light emissions. The spectra in this figure show the characteristic emission peaks for cerium and dysprosium at 400 to 450 nm and 573 nm respectively, the presence of both RE3+ are observed, Dy3+ as was mentioned above, has an inter-level transition (4F9/2 to 6H15/2) located at 483 nm (blue light emission); this transition

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Fig. 3. (a) SEM image, (b) AFM image.

Fig. 4. PL spectra from undoped PET films (a). Rare earths doped PET films (b–f).

is enhanced by the presence of cerium ions, and get a leveling of yellow and blue emissions, resulting in a white light emission. The excitation wavelength to generate these spectra was also 300 nm. It is notorious that even using two different kinds of REs a single wavelength (300 nm) is capable to excite both of them, when ordinarily Ce ions are excited using 250 nm, and Dy ions using 360 nm [38,41], due to polymeric matrix energy transfer. This is a simple and cheap way to generate white light polymeric films. It is important to notice, that in all cases, PET polymeric thin films are homogeneous, with similar characteristics as transparency and luminescent emission intensities. The light emitted by the films is completely homogeneous as observed in all the inset photographs in Fig. 4. This is due mainly, that in our case we use rare earths as dopants (no oxides, sulfides or ternary oxides) which are distributed homogeneously over the flat surface of the films. Fig. 5(A) shows the diagram CIE (Commission international de l’ éclairage) [43], which precisely defines the three primary colors of additive color synthesis, from which all others can be created, depending on its particular coordinates (x, y). The global emission generated from different kinds of doped and non doped PET films

deposited glass substrates was characterized by its chromaticity coordinates in a CIE diagram, the results are given in Fig. 5(B). Their corresponding coordinates are shown in Table 1 and according to the CIE diagram non doped films fall in the ‘‘violet’’ area. While single doped PET films, fall in blue, green, yellow and red areas in agreement with the type of dopant used. Just double doped films (Ce – Dy) fall into ‘‘White’’ area, due to combined blue and yellow emissions from dopants. In order make a complete luminescent study, cathodoluminescence measurements were practiced in PET and PET:RE3+ thin films; the measurements were performed in these films using an electron acceleration voltage in the 1–2 kV range. Fig. 6(A) shows a non-doped PET sample CL spectrum, where two bands can be observed, the first one is centered at 428 nm and the second one at 471 nm, these luminescence spectra are interpreted as being the result due to a cascade transition from the excited state of molecules localized in the PET polymer [44]. These bands are present in every spectra from PET:RE3+ samples, with an intensity level similar to RE3+ characteristic emissions. Fig. 6(B) shows the CL spectrum of Ce3+ PET films, this spectrum exhibits a broad band centered at approximately 410 nm which is


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Fig. 5. (a) CIE 1931 color space chromaticity diagram, (b) global emission generated from doped and undoped PET films.

Table 1 Experimental CIE diagram corresponding coordinates are presented. Kind of sample

x

y

PET PET PET PET PET PET

0.2216 0.1801 0.2721 0.4229 0.5331 0.2801

0.2082 0.1607 0.4516 0.4178 0.2717 0.2692

only – Ce – Tb – Dy – Eu Ce – Dy

characteristic of the electronic transitions in the Ce3+ ion (4f1). The observed emission bands are due to the 2T2g (5d) to 2F7/2, 2F5/2 (4f) transitions. These results are similar to those previously reported for the cathodoluminescent properties of cerium doped films, synthesized by spray pyrolysis using cerium chlorides as precursors

hosted in an inorganic matrix [45]. It is important to mention that in this case, two bands (from PET transitions) at 420 y 471 nm are mounted on the broad band of cerium and are pretty intense, even more than Ce3+ ion characteristical emission. Fig. 6(C) shows peaks that are characteristic of transitions between electronic energy levels of Tb3+ ions. The major peak centered at 546 nm corresponds to the transition 5D4 to 7F5, while the transitions 5D4 to 7F6, 5D4 to 7F4, and 5D4 to 7F3 of the Tb ion are related to the emission peaks at 489, 587, and 623 nm, respectively. These emissions are similar to those observed in cathodoluminescent studies in inorganic matrixes such as Ba2B5O9Cl and ZrO2 into thin films shape [46,47]. Again, two bands (from PET transitions) at 420 y 471 nm are observable and are as intense as Tb3+ characteristic peaks. The CL spectrum for Dy3+ doped PET films are shown in Fig. 6(D), the emission peaks observed are associated to the

Fig. 6. CL spectra from undoped PET films (a). Rare earths doped PET films (b–f).

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transitions within the electronic energy levels of the Dy3+ ion, it exhibits three distinct emission peaks located at 573 nm, associated with the hypersensitive transition 4F9/2 to 6H13/2; at 485 nm corresponding to the less sensitive transition 4F9/2 to 6H15/2; and at 667 nm corresponding to the 4F9/2 to 6H11/2 transition. These results are similar to those previously reported for the cathodoluminescent properties in an inorganic matrix (yttria) in powder shape, doped with dysprosium and synthesized by the polyol technique [41]. Once again, two bands from PET transitions are observable and have relative intensities similar to Dy3+ transitions. Fig. 6(E) shows the CL spectrum of Eu3+ doped polymeric films. The spectrum consist of a series of well resolved features at 598, 614, 650 and 690 nm, which are assigned to 5D0 to 7F1, 7F2, 7F3 and 7F4 transitions respectively. The most intense emission occurs at 611 nm and is due to 5D0 to 7F2 transition. Similar results have been obtained earlier for Y2O3:Eu3+ polycrystalline films and nano-powders and Al2O3:Eu3+ powders [48–51]. Two peaks, from PET transitions are observable in this case also; at 420 and 470 nm and has relative intensities similar to Eu3+ transitions. Finally, Fig. 6(F) shows a CL spectrum of a double doped (Ce3+ and Dy3+) PET film, The spectrum in this figure show the characteristic emission peaks for cerium and dysprosium at 400 to 450 nm and 573 nm respectively, the presence of both RE3+ are observed. It is important to mention that, two bands (from PET transitions) at 420 y 471 nm are mounted on the broad band of cerium and are pretty intense, even more than Ce3+ ion characteristic emission as occours in PET:Ce3+ films. On the other hand, Dy3+ as was mentioned above, has an inter-level transition (4F9/2 to 6H15/2) located at 483 nm (blue light emission) and this is seriously enhanced by 470 nm band PET transition. PL spectrum mentioned above (Fig. 4(F)) shows a leveling of yellow and blue emissions resulting in a white light emission, at this case, yellow and blue emissions are enhanced simultaneously and the global emission is mainly yellow such as occur in PET:Dy3+ films, this could be due to blue contribution of cerium is very weak. 4. Conclusions Polymeric PET films were deposited by spray pyrolysis technique, to our best knowledge, is the first time that is possible to achieve polymeric PET films by this technique. These films were doped using RE in order to achieve light emissions at different zones of electromagnetic spectrum. Non doped films are very transparent (95% T) and smooth; doped films are as less transparent than non doped ones, however, they are semitransparent (80% T average) and smooth also. Using Ce, Tb, Dy, and Eu was possible to observe blue, green, yellow, and red light emissions respectively; by means of an appropriate mix of Ce and Dy as dopants, it was also possible to obtain films that have white light emission. All of this using recycled and very cheap materials, trying to help to decrease plastic trash and contribute to the environmental improvement.

[3] [4] [5] [6] [7] [8]

[9]

[10]

[11]

[12]

[13]

[14]

[15] [16]

[17] [18]

[19]

[20]

[21]

[22]

[23]

[24]

[25]

[26] [27]

[28]

Acknowledgments [29]

The authors wish to acknowledge the technical assistance of Z. Rivera, A. Pineda, R. Fragoso, M. Guerrero and A. Soto from physics department of CINVESTAV-IPN. The authors also thank DGAPAUNAM, and SECITI (before ICyTDF) project # PICSO 12-119 to financial support. References [1] R. Reisfeld, Optical Properties of Colorants or Luminescent Species in Sol–Gel Glasses Structure and Bonding, Springer, 1992. [2] H. Busta, Field Emission flat Displays Vacuum Microelectronics, Wiley, 2001.

[30]

[31] [32] [33]

Y.A. Ono, Encyclopedia of Applied Physics, vol. 4, VCH, New York, 1993. R. Reisfeld, Lasers and Excited States of Rare Earths, Springer, 1977. G. Blasse, Luminescent Materials, Springer, 1994. F. Gan, Optical and Spectroscopic Properties of Glass, Springer, 1992. E. Lueder, Conference on Liquid Crystal Materials, Devices, and Applications, SPIE 3297, SPIE, 1998. R. Baeuerle, J. Baumbach, E. Lueder, J. Siegordner, A MIM driven display with colour filters on 200 diagonal plastic substrates, SID Symposium Digest of Technical Papers 30 (1999) 14–17. C.C. Wu, S.D. Theiss, G. Gu, M.H. Lu, J.C. Sturm, S. Wagner, S.R. Forrest, Integration of organic LED’s and amorphous Si TFT’s onto flexible and lightweight metal foil substrates, IEEE Electron Dev. Lett. 18 (1997). P.E. Burrows, G. Gu, V. Bulovic, Z. Shen, S.R. Forrest, M.E. Thompson, Achieving full-color organic light-emitting devices for lightweight, flat-panel displays, IEEE Trans. Electron Dev. 44 (1188) (1997) 1188–1203. Barrett Comiskey, J.D. Albert, Hidekazu Yoshizawa, Joseph Jacobson, An electrophoretic ink for all-printed reflective electronic displays, Nature 394 (253) (1998) 253–255. S.L. Jones, D. Kumar, Rajiv K. Singh, P.H. Holloway, Luminescence of pulsed laser deposited Eu doped yttrium oxide films, Appl. Phys. Lett. 71 (404) (1997) 404–406. R. Martínez-Martínez, M. García, A. Speghini, M. Bettinelli, C. Falcony, U. Caldino, Blue–green–red luminescence from CeCl3- and MnCl2-doped hafnium oxide layers prepared by ultrasonic spray pyrolysis, J. Phys.: Condens. Matter 20 (2008). 395205. G. Hirata, N. Perea, M. Tejeda, J.A. Gonzalez-Ortega, J. McKittrick, Luminescence study in Eu-doped aluminum oxide phosphors, Opt. Mater. 27 (2005) 1311–1315. X. Ouyang, A.H. Kitai, T. Xiao, Electroluminescence of the oxide thin film phosphors Zn2SiO4 and Y2SiO5, J. Appl. Phys. 79 (3229) (1996) 3229–3234. Xiaoxiang Han, Renxian Zhou, Xiaoming Zheng, Heng Jiang, Effect of rare earths on the hydrogenation properties of p-chloronitrobenzene over polymer-anchored platinum catalysts, J. Mol. Catal. A: Chem. 193 (2003) 103–108. L.H. Slooff, A. Van Blaaderen, A. Polman, G.A. Hebbink, S.I. Klink, F.C.J.M. Van Veggel, D.N. Reinhoudt, J.W. Hofstraat, J. Appl. Phys. 91 (2002) 3955–3980. Xiaodan Guo, Guangshan Zhu, Qianrong Fang, Ming Xue, Ge Tian, Jinyu Sun, Xiaotian Li, Shilun Qiu, Synthesis, structure and luminescent properties of rare earth coordination polymers constructed from paddle-wheel building blocks, Inorg. Chem. 44 (2005) 3850–3855. Wu Chung-Chih, James C. Sturm, Richard A. Register, Jing Tian, Elena P. Dana, Mark E. Thompson, Efficient organic electroluminescent devices using singlelayer doped polymer thin films with bipolar carrier transport abilities, IEEE Trans. Electron Dev. 44 (8) (1997) 1269–1281. Raquel Hernandez, Anne-Christine Franville, Payam Minoofar, Bruce Dunn, Jeffrey I. Zink, Controlled placement of luminescent molecules and polymers in mesostructured sol–gel thin films, J. Am. Chem. Soc. 123 (2001) 1248–1249. Christopher J. Buchkoa, Loui C. Chena, Yu Shena, David C. Martin, Processing and microstructural characterization of porous biocompatible protein polymer thin films, Polymer 40 (1999) 7397–7407. A. Piqué, R.A. McGill, D.B. Chrisey, D. Leonhardt, T.E. Mslna, B.J. Spargo, J.H. Callahan, R.W. Vachet, R. Chung, M.A. Bucaro, Thin Solid Films 355–356 (1999) 536–541. Bohwon Kim, Vladan Koncar, Eric Devaux, Claude Dufour, Pierre Viallier, Electrical and morphological properties of PP and PET conductive polymer fibers, Synth. Met. 146 (2004) 167–174. Yun-Wang Choi, Dae-Joong Moon, Jee-Seung Chung, Sun-Kyu Cho, Effects of waste PET bottles aggregate on the properties of concrete, Cem. Concr. Res. 35 (2005) 776–781. Christoph P. Hauser, Dominique T. Thielemann, Matthias Adlung, Claudia Wickleder, Peter W. Roesky, Clemens K. Weiss, Katharina Landfester, Luminescent polymeric dispersions and films based on oligonuclear lanthanide clusters, Macromol. Chem. Phys. 212 (2011) 286–296. Harald Hoppe, Niyazi Serdar Sariciftci, Morphology of polymer/fullerene bulk heterojunction solar cells, J. Mater. Chem. 16 (2006) 45–61. A.N. Meza-Rocha, E.F. Huerta, E. Zaleta-Alejandre, Z. Rivera-Álvarez, C. Falcony, Enhanced photoluminescence of Y2O3:Er3+ thin films by Li+ co-doping, J. Lumin. 141 (2013) 173–176. R. Martínez-Martínez, M. García, A. Speghini, M. Bettinelli, C. Falcony, U. Caldiño, Blue–green–red luminescence from CeCl3- and MnCl2-doped hafnium oxide layers prepared by ultrasonic spray pyrolysis, J. Phys.: Condens. Matter 20 (2008) 395205–395212. A. Ortiz, J.C. Alonso, V. Pankov, D. Albarran, Violet–blue photoluminescence in aluminium oxide Þlms prepared by ultrasonic spray pyrolysis, J. Lumin. 81 (1999) 45–51. S. Carmona-Tellez, J. Guzman-Mendoza, M. Aguilar-Frutis, G. Alarcon-Flores, M. Garcia-Hipolito, M.A. Canseco, C. Falcony, Electrical, optical, and structural characteristics of Al2O3 thin films prepared by pulsed ultrasonic sprayed pyrolysis, J. Appl. Phys. 103 (2008) 034105. E.S. Gadelmawla, M.M. Koura, T.M.A. Maksoud, I.M. Elewa, H.H. Soliman, Roughness parameters, J. Mater. Process. Technol. 123 (2002) 133–145. Herman F. Marck, Encyclopedia of Polymer Science and Engineering, second ed., vol. 16, John Wiley & Sons, 1989. Libin Bai, Gu Fang, Yi Feng, Yinghai Liu, Synthesis of microporous pH-sensitive polyacrylic acid/poly (ethylene glycol) hydrogels initiated by potassium diperiodatocuprate (III), Iranian Polym. J. 17 (2008) 325–332.

S. Carmona-Téllez et al. / Optical Materials 42 (2015) 99–105 [34] M. Wegelin, S. Canonica, A.C. Alder, D. Marazuela, M.J.-F. Suter, Th.D. Bucheli, O.P. Haefliger, R. Zenobi, K.G. McGuigan, M.T. Kelly, P. Ibrahim, M. Larroque, Does sunlight change the material and content of polyethylene terephthalate (PET) bottles?, J Water Supply: Res. Technol.-AQUA 50 (3) (2001) 125–133. [35] Liusheng Chen, Xigao Jin, Du Jinhuan, Renyuan Qian, On the origin of the fluorescence emission of poly(ethylene terephthalate) by excitation at 340 nm, Makromol. Chem. 192 (1991) 1309–1408. [36] M.R. Padhye, P.S. Tamhane, Luminescence studies in poly(ethylene terephthalate) films at 77 K. Investigation of emission centres in polymers, Die Angew. Makromol. Chem. 69 (1978) 33–45. [37] Rainer D. Beck, Pamela St. John, Marcos M. Alvarez, Fransois Diederich, Robert L. Whetten, Resilience of all-carbon molecules C60, C70, and C80: a surfacescattering time-of-light investigation, J. Phys. Chem. 95 (1991) 8402–8409. [38] Rong-Jun Xie, Naoto Hirosaki, Mamoru Mitomo, Yoshinobu Yamamoto, Takayuki Suehiro, Naoki Ohashi, Photoluminescence of cerium-doped aSiAlON materials, J. Am. Ceram. Soc. 87 (7) (2004) 1368–1370. [39] G. Lakshminarayana, Joseph A. Torres, Terri C. Lin, I.V. Kityk, Markus P. Hehlen, Sol–gel synthesis and characterization of fluoride-rich lanthanum-aluminosilicate gels doped with Ce3+ and Ti4+, J. Alloys Compd. 601 (2014) 67–74. [40] Jianhua Hao, Michael Cocivera, Optical and luminescent properties of undoped and rare-earth-doped Ga2O3 thin films deposited by spray pyrolysis, J. Phys. D: Appl. Phys. 35 (2002) 433–438. [41] R. Balderas-Xicohténcatl, R. Martínez-Martínez, Z. Rivera-Alvarez, J. SantoyoSalazar, C. Falcony, Photo and cathodoluminescence characteristics of dysprosium doped yttrium oxide nanoparticles prepared by polyol method, J. Lumin. 146 (2014) 497–501. [42] Gu Mu, Xin Xu, Xiaolin Liu, Longqing Qiu, Rui Zhang, Preparation and characterization of GdTaO4:Eu3+ sol–gel luminescence thin films, J. Sol–Gel Sci. Technol. 35 (2005) 193–196.

105

[43] Deane B. Judd, David L. Macadam, Spectral distribution of typical daylight as a function of correlated color temperature, J. Opt. Soc. Am. 54 (1964) 1032– 1040. [44] Yutaka Ito, Dependence of the doppler-broadening of the positron annihilation radiation in C60 fullerenes on the temperature, Radiat. Phys. Chem. 64 (2002) 273–275. [45] M. García-Hipólito, J. Guzmán-Mendoza, E. Martínez, O. Alvarez-Fregoso, C. Falcony, Growth and cathodoluminescent characteristics of blue emitting cerium-doped zinc aluminate layers synthesized by spray pyrolysis technique, Phys. Stat. Sol. A 201 (2004) 1510–1517. [46] Jianhua Hao, J. Gao, Michael Cocivera, Tuning of the blue emission from europium-doped alkaline earth chloroborate thin films activated in air, Appl. Phys. Lett. 82 (2003) 2778–2780. [47] M. García-Hipólito, R. Martínez, O. Alvarez-Fregoso, E. Martínez, C. Falcony, Cathodoluminescent and photoluminescent properties of terbium doped ZrO2 films prepared by pneumatic spray pyrolysis technique, J. Lumin. 93 (2001) 9– 15. [48] Gary A. West, K.W. Beeson, Low-pressure metalorganic chemical vapor deposition of photoluminescent Eu-doped Y2O3 films, J. Mater. Res. 5 (7) (1990) 1573–1580. [49] D. Kumar, K.G. Cho, Zhan Chen, V. Craciun, P.H. Holloway, Rajiv K. Singh, Cathodoluminescent properties of pulsed-laser-deposited Eu-activated Y2O3 epitaxial films, Phys. Rev. B 60 (1991) 13331–13334. [50] G. Hirata, N. Perea, M. Tejeda, J.A. Gonzalez-Ortega, J. McKittrick, Luminescence study in Eu-doped aluminum oxide phosphors, Opt. Mater. 27 (2005). [51] Nguyen Vu, Tran Kim Anh, Gyu-Chul Yi, W. Strek, Photoluminescence and cathodoluminescence properties of Y2O3:Eu nanophosphors prepared by combustion synthesis, J. Lumin. 122 (2007) 776–779.