Optical Materials xxx (2016) 1e6 Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat Tin-dioxide nanocrystals as Er3ỵ luminescence sensitizers: Formation of glass-ceramic thin films and their characterization Lidia Zur a, b, *, Lam Thi Ngoc Tran c, b, d, Marcello Meneghetti e, b, Van Thi Thanh Tran f, Anna Lukowiak g, Alessandro Chiasera b, Daniele Zonta c, b, h, Maurizio Ferrari b, a, Giancarlo C Righini a, i a Museo Storico della Fisica e Centro Studi e Ricerche “Enrico Fermi”, Piazza del Viminale 1, 00184, Roma, Italy IFN-CNR CSMFO Lab., and FBK Photonics Unit, via alla Cascata 56/C Povo, 38123, Trento, Italy Department of Civil, Environmental and Mechanical Engineering, Trento University Via Mesiano, 77, 38123, Trento, Italy d Ho Chi Minh City University of Technical Education, Vo Van Ngan Street, Linh Chieu Ward, Thu Duc District, Ho Chi Minh City, Viet Nam e di Trento, via Sommarive 14, Povo, 38123, Trento, Italy Dipartimento di Fisica, Universita f University of Science, Vietnam National University, Ho Chi Minh City, 227 Nguyen Van Cu Street, Ward 4, District 5, Ho Chi Minh City, Viet Nam g Institute of Low Temperature and Structure Research, PAS, Okolna St 2, 50-422, Wroclaw, Poland h Department of Civil and Environmental Engineering, University of Strathclyde, 75 Montrose Street, Glasgow, G11XJ, UK i IFAC e CNR, MiPLab., 50019, Sesto Fiorentino, Italy b c a r t i c l e i n f o a b s t r a c t Article history: Received 15 June 2016 Received in revised form 19 July 2016 Accepted 29 August 2016 Available online xxx Silica-tin dioxide thin lms doped with Er3ỵ ions were fabricated and investigated Different parameters such as heat-treatment temperatures, molar concentrations of SnO2 as well as Er3ỵ ions concentration were changed in order to obtain the best properties of presented thin films Using several techniques, thin films were characterized and proved to be crack-free, water-free and smooth after a heat-treatment at 1200 C Aiming to application in optics, the transparency of thin films was also evidenced by transmission spectra Based on the photoluminescence measurements, the mechanism of energy transfer from SnO2 nanocrystals to Er3ỵ ions was examined and discussed â 2016 Elsevier B.V All rights reserved Keywords: Glass-ceramics Thin films SnO2-SiO2 Rare-earths Luminescence Energy transfer Introduction Silica-based materials doped with rare earths (RE) are widely used in photonics They are promising materials that have found application in high brightness displays and laser emitters [1] Among others, Er3ỵ ion is one of the most studied RE dopants It can be used in optical amplifiers in the main optical telecommunication window at the 1.5 mm [2] Nowadays, fabrication of miniaturized, efficient integrated optics devices requires a high concentrations of rare earths in a small volume; one of the main problem arising is the concentration quenching effect due to the high content of active dopants This effect is observed for all RE after reaching a certain * Corresponding author Centro Fermi/CNR-IFN, Via alla Cascata 56/C, 38123, Povo, Trento, Italy E-mail address: zur@fbk.eu (L Zur) concentration, characteristic for each of these ions, due to clusters formation The ion-ion interactions in the clusters lead to reduction of luminescence efficiency [3] In order to overcome this problem, glass-ceramic materials containing RE embedded in nanocrystals can be prepared This reduces not only the clustering effect and energy transfer between ions but also allows energy transfers from crystals to RE ions [4,5], which enhances the luminescence quantum yield and compensates the small absorption cross section of the majority of rare earth ions Additionally, the glass-ceramic materials play a significant role because they merge the mechanical and optical properties of the glasses with a crystal-like environment for the RE ions [6] One of the promising materials that could be used to solve this problem is tin dioxide SnO2 is a wide-band gap semiconductor (Eg ¼ 3.6 eV at 300 K) with a maximum phonon energy below 630 cmÀ1 [7] Some papers about the RE doped to SnO2 http://dx.doi.org/10.1016/j.optmat.2016.08.041 0925-3467/© 2016 Elsevier B.V All rights reserved Please cite this article in press as: L Zur, et al., Tin-dioxide nanocrystals as Er3ỵ luminescence sensitizers: Formation of glass-ceramic thin films and their characterization, Optical Materials (2016), http://dx.doi.org/10.1016/j.optmat.2016.08.041 L Zur et al / Optical Materials xxx (2016) 1e6 nanocrystals have already been published [8,9], and investigations on increasing SnO2 concentration in silicate matrix were carried out [10,11] Particularly, strong enhancement of the Eu3ỵ emission was observed, thanks to energy transfer from the SnO2 nanocrystals to Eu3ỵ ions [12] In this paper, xSnO2-(100-x)SiO2 glass-ceramic thin lms doped with Er3ỵ ions are presented Inuence of the composition and heat-treatment process on the structural, optical, and spectroscopic properties of tin dioxide based glass-ceramics has been investigated by several characterization techniques Especially, we focused on the energy transfer between SnO2 and Er3ỵ ions as well as on the influence of rare earths on crystallization of SnO2 Experimental xSnO2e(100-x)SiO2 (x ¼ 10, 20, and 30 mol %) glass-ceramic thin lms doped with different concentrations of Er3ỵ (0, 1, 2, and mol %) were fabricated by sol-gel route using dip-coating technique Precursors, SnCl2$2H2O and Er(NO3)3$5H2O, were dissolved in ethanol and added to the starting solution that had been prepared by mixing tetraethyl orthosilicate (TEOS), ethanol, de-ionized water, and hydrochloric acid with the molar ratio of 1:21:2:0.01 The final mixture was stirred for 16 h Then, thin films activated by Er3ỵ were deposited on pure SiO2 and silicon substrates by dipcoating with the deeping rate cm/min After the first layer was dip-coated, there was a temporary annealing at 150 C during before next dip-coating step Final thin films having layers were heat-treated in air at various temperatures ranging from 500 C to 1200 C for h Structure, surface, transparency, and photoluminescence properties of the resulting thin films were characterized Using the EQUINOX 55 spectrometer, Fourier-transform infrared (FTIR) transmission measurements were performed to identify the structure, phases and especially existence of water and solvents in obtained thin films The crystallization of tin dioxide component was studied with X-ray diffraction patterns performed by D8eADVANCE The Atomic Force Microscopic images were taken by AFM-D3100 to supply the surface characterization of the thin films Then, the transparency of thin films was provided by the ultravioletevisible spectra Finally, using Horiba spectrometer, the photoluminescence of the Er3ỵ ions in the surrounding glassceramics environment was recorded upon an excitation at 300 nm, evidencing energy transfer from SnO2 crystals to Er3ỵ Fig Infrared spectra of 30SnO2e70SiO2 thin lm doped with 1% Er3ỵ as a function of annealing temperature published data [16,17] The majority of amorphous SiO2 is responsible for the broad absorption of asymmetric stretching vibration of ^SieOeSi^ linking bonds at 1062 cmÀ1 There are also other absorption bands concerning SieOeSi bindings: 811 cmÀ1 assigned to symmetric stretching and 457 cmÀ1 corresponding to the bending vibrational mode between adjoining SiO4 rings [18e20] The next weak band taken into account is the vibration absorption around 667 cmÀ1 of d(OeSneO) bonding [17] Finally, the weakest band appears at 551 cmÀ1 after heat treatment at 1200 C and is assigned to the vibration of SneOeSn bonding FT-IR spectra of thin films containing different SnO2 contents are presented in Fig Increasing concentration of SnO2 affects in broadening of the peak observed around 653 cmÀ1, connected with d(OeSneO) bonding The band at 561 cmÀ1 assigned to the vibration of SneOeSn bonding is broadening with increasing concentration of SnO2 One of the requirements of the xSnO2e(100-x)SiO2 glassceramic thin films is formation of SnO2 nanocrystals containing Er3ỵ ions, uniformly distributed in SiO2 matrix Thereby, the energy transfer from SnO2 nanocrystals to erbium is more efficient Results and discussion 3.1 Structural and morphological characterization FT-IR transmission spectra of the thin films xSnO2e(100-x)SiO2 doped with Er3ỵ dip-coated on the silicon substrate have been analyzed FT-IR analysis is used to study rare earth doped materials, giving important information about the structure, and allows tracking the process of removing organic groups as well as hydroxyl groups Hydroxyl groups are well known as luminescence quenching agents [13e15], so the heat treatment process is essential to remove these groups in order to achieve efcient luminescence of Er3ỵ Fig presents the infrared transmission spectra of 30SnO2e70SiO2 thin films doped with 1% of Er3ỵ heattreated at different temperatures After annealing at 600 C, peaks at about 1630 and 3404 cmÀ1 were observed that are typical for bending and stretching vibration of OH group coming from residual water in thin films When films were heat-treated at higher temperatures (800÷1200 C), disappearance of these two bands implies that hydroxyls groups were eliminated by the annealing These results are in accordance with previously Fig Infrared spectra of xSnO2e(100-x)SiO2 thin lms doped with 1% Er3ỵ as a function of SnO2 content Please cite this article in press as: L Zur, et al., Tin-dioxide nanocrystals as Er3ỵ luminescence sensitizers: Formation of glass-ceramic thin films and their characterization, Optical Materials (2016), http://dx.doi.org/10.1016/j.optmat.2016.08.041 L Zur et al / Optical Materials xxx (2016) 1e6 Primarily, the influence of heat-treatment process on formation of SnO2 nanocrystals was carefully examined Fig shows the diffraction patterns of the 1%Er3ỵ:70SiO2e30SnO2 thin lms annealed at 800 C, 1000 C and 1200 C The diffraction peaks at 2q ¼ 26.9 , 34.1, 38.2 , and 52.0 , which can be assigned to the (110), (101), (200), (211), and (112) planes of SnO2 rutile crystal phase (JCPDS 41-1445), are present only after the heat-treatment at 1000 C or higher temperature In addition, several diffraction peaks with low intensity appear (marked with asterisk) According to previous results [21], it is possible to conclude that a new crystalline phase is formed; however, no known crystalline structure associated with Er/O, Er/Si, or Er/O/Si compounds matches with the data of XRD on Fig SnO2 nanocrystal size calculated from Scherrer formula for films annealed at 1000 C and 1200 C shows that the heat-treatment temperature affects slightly the crystals size Previously published results shown that the sintering process influences the agglomeration and the particle size growth The sintering temperature promotes enlargement of grain boundaries and consequently particle size increases as a function of annealing temperature [22,23] The structure of the xSnO2e(100-x)SiO2 thin films was also investigated as a function of the SnO2 content, and presented in Fig 4(a) Based on diffraction patterns, using Scherrer formula, the size of the tin dioxide nanocrystals was calculated and displayed in Table We observed that the size of nanocrystals increases with SnO2 content Moreover, one can see that increasing concentration of SnO2 not cause formation of new phase in investigated films Obtained results are in accordance to the FT-IR results discussed above In order to provide complete examination of xSnO2e(100-x) SiO2 system, the structure of thin films as a function of Er3ỵ concentration has been investigated (Fig 4(b)) Due to formation of SnO2 nanocrystals, part of the dopant is in amorphous SiO2 and part is embedded in SnO2 nanocrystals Incorporation of Er3ỵ into SnO2 will cause defects in the crystal lattice, causing the loss of symmetry in the crystal Thus, as a result, slight change in the size of SnO2 nanocrystals is observed with increasing concentration of Er3ỵ ions Similar behavior has been observed before in SnO2 doped with Er3ỵ [24] Additionally, no other phase was formed with increasing concentration of erbium The microstructure of thin films is also a relevant factor for application of planar waveguides AFM images of thin films with 1% Fig X-Ray diffraction patterns of xSnO2e(100-x)SiO2 thin films as a function of (a) SnO2 content and (b) Er3ỵ concentration Table The average SnO2 crystallite sizes in xSnO2e(100-x)SiO2 thin lms doped with Er3ỵ in function of composition Er3ỵ [mol%] Fig X-Ray diffraction patterns of 30SnO2e70SiO2 thin film doped with 1% Er3ỵ as a function of heat-treatment temperature SnO2 [mol%] 10 20 30 48 nm 62 nm 64 nm 61 nm 61 nm of Er3ỵ and various concentrations of SnO2 annealed at 1200 C are presented in Fig 5(a) Annealing at this temperature promoted formation of the SnO2 nanocrystals, with a dense and homogenous distribution on the thin film surface It should be noted that when the content of SnO2 is low, the size of formed nanocrystals is small (what was confirmed by XRD analysis), which consequently leads to lower average value of surface roughness Increasing concentration of tin dioxide caused increase of the surface roughness The values of root mean square roughness (Rrms) of the thin films are 6.30 nm, 8.53 nm and 10.11 nm for SnO2 content of 10%, 20% and 20%, respectively Please cite this article in press as: L Zur, et al., Tin-dioxide nanocrystals as Er3ỵ luminescence sensitizers: Formation of glass-ceramic thin films and their characterization, Optical Materials (2016), http://dx.doi.org/10.1016/j.optmat.2016.08.041 L Zur et al / Optical Materials xxx (2016) 1e6 Fig 3D AFM images of the surface of xSnO2e(100-x)SiO2 thin films as a function of (a) SnO2 and (b) Er3ỵ concentration The inuence of rare earths concentration on the surface roughness was also examined, as shown in Fig 5(b) Thin films containing 30% of SnO2 with the concentration of Er3ỵ increased up to 3% have been chosen for doing this analysis Obtained results show the influence of erbium on formation of SnO2 nanocrystals It can be seen that the average roughness of the films decreases due to increasing concentration of Er3ỵ The root mean square roughness (Rrms) of the thin films decreases with increasing content of Er3ỵ and is equal to 10.11 nm (1% of Er3ỵ), 9.97 nm (2% of Er3ỵ) and 5.40 nm (3% of Er3ỵ) We can conclude that the incorporation of Er3ỵ ions inhibits the growth of SnO2 crystals Taking into account the results obtained from AFM microstructure analysis as a function of SnO2 and Er3ỵ concentration, the proper composition can be chosen as a compromise between the SnO2 and Er3ỵ content in the thin films 3.2 Optical characterization SnO2eSiO2 thin lms doped with Er3ỵ ions dip-coated on pure silica substrate were heat-treated at 500 C for h in order to collect UV-VIS transmittance spectra The spectra were recorded for samples with different SnO2 and Er3ỵ concentration, as shown in Fig It is clearly seen that, irrespective of the thin films composition, the transmittance always remains close to 90% Moreover, the transmittance spectra recorded for 30SnO2e70SiO2 thin films with different Er3ỵ ions concentration (Fig 6(b)) show that the increasing concentration of Er3ỵ has no effect on the transparency of investigated thin films 3.3 Spectroscopic characterization Spectroscopic properties of Er3ỵ ions doped into xSnO2e(100-x) SiO2 thin lms dip-coated on the silicon substrates and heat treated at 1200 C for h have been analyzed as a function of the film composition Deep and entire analysis of SiO2eSnO2 system was possible by changing the concentration of SnO2 in the range from 10 to 30%, while the Er3ỵ content was set at 1% Infrared photoluminescence spectra of discussed glass-ceramic thin films were recorded upon an excitation at 300 nm and are presented in Fig 7(a) This excitation wavelength corresponds to the interband electronic transition of the SnO2 nanocrystals The narrowing of the emission peaks, associated to the Stark multiplets, evidences that the rare earth ions were embedded in the SnO2 nanocrystals The emission intensity increases with the SnO2 content This behavior reects an increase in the number of the Er3ỵ ions embedded in the SnO2 nanocrystals with the SnO2 content When the concentration of SnO2 increased from 10% up to 30%, the luminescence intensity of 1550 nm region was enhanced about 10 times With increasing concentration of SnO2, the density as well as the size of nanocrystals increases, which makes easier the process of incorporation of the rare earth ions in the nanocrystals Therefore, more Er3ỵ ions are excited due to exciton mediated energy transfer from SnO2 nanocrystals After analysis of obtained results, 30SnO2e70SiO2 composition has been selected to examine the spectroscopic properties as a function of Er3ỵ concentration Fig 7(b) shows the luminescence spectra for 30SnO2e70SiO2 thin films doped with 1, and 3% of Er3ỵ It is well known that with increasing concentration of rare earth ions the quenching effect is observed, due to clustering Please cite this article in press as: L Zur, et al., Tin-dioxide nanocrystals as Er3ỵ luminescence sensitizers: Formation of glass-ceramic thin lms and their characterization, Optical Materials (2016), http://dx.doi.org/10.1016/j.optmat.2016.08.041 L Zur et al / Optical Materials xxx (2016) 1e6 Fig Ultravioletevisible spectra of xSnO2-(100-x)SiO2 thin films as a function of (a) SnO2 and (b) Er3ỵ content and ions-ions interaction [25,26] Different investigation are carried out in order to allow higher doping of rare earth ions in different materials which is important from application point of view One of the solutions of this problem is the existence of nanocrystals in amorphous matrix, making the RE distribution easier In xSnO2e(100-x)SiO2 glass-ceramic thin films, SnO2 nanocrystals would appear to ease the dispersion of Er3ỵ ions in silica matrix Nevertheless, the quenching effect remains The emission intensity decreases very slightly when the concentration of Er3ỵ reaches 2%, then some reduction of intensity is observed for thin lm containing 3% of Er3ỵ Fig shows the PL emission spectra for the 1Er3ỵ: 30SnO2e70SiO2 thin lm as a function of the excitation wavelengths The spectra exhibit similar shapes, but different intensities It appears that the most suitable pumping region is around 290e300 nm This is not surprising, as this region corresponds to the SnO2 band gap Conclusions The effects of SnO2 and Er3ỵ concentration on the structure and distribution of erbium ions in glass-ceramic thin films were investigated by FT-IR spectroscopy, X-ray diffraction and Atomic Force Microscopy Obtained films are crack-free and exhibit a Fig Photoluminescence spectra of Er3ỵ doped to xSnO2e(100-x)SiO2 as a function of (a) SnO2 and (b) Er3ỵ concentration in thin lms with 1%Er3ỵ (a) and 30%SnO2 (b) Fig Photoluminescence spectra of 30SnO2e70SiO2 thin lms doped with 1% of Er3ỵ as a function of different excitation wavelengths transmittance of around 90% over the 400 nme1.1 mm region Size of the nanocrystals increases with the SnO2 content and reaches about 61 nm for 2% Er3ỵ:30SnO2e70SiO2 thin lm The properties of erbium as well as the role of SnO2 nanocrystals as Er3ỵ luminescence sensitizers were experimentally confirmed based on emission spectra Thin films with composition 30SnO2e70SiO2 Please cite this article in press as: L Zur, et al., Tin-dioxide nanocrystals as Er3ỵ luminescence sensitizers: Formation of glass-ceramic thin films and their characterization, Optical Materials (2016), http://dx.doi.org/10.1016/j.optmat.2016.08.041 L Zur et al / Optical Materials xxx (2016) 1e6 doped with 2% of Er3ỵ appears to be the most attractive in terms of luminescence efficiency [11] Acknowledgements The 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glass-ceramic thin films and their characterization, Optical Materials (2016), http://dx.doi.org/10.1016/j.optmat.2016.08.041 ... clustering Please cite this article in press as: L Zur, et al., Tin-dioxide nanocrystals as Er3ỵ luminescence sensitizers: Formation of glass-ceramic thin films and their characterization, Optical Materials... 234e250 Please cite this article in press as: L Zur, et al., Tin-dioxide nanocrystals as Er3ỵ luminescence sensitizers: Formation of glass-ceramic thin lms and their characterization, Optical Materials... this article in press as: L Zur, et al., Tin-dioxide nanocrystals as Er3ỵ luminescence sensitizers: Formation of glass-ceramic thin films and their characterization, Optical Materials (2016), http://dx.doi.org/10.1016/j.optmat.2016.08.041