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Effect of chemical etching on the surface morphology of laser-patterned lines with Er3+-doped CaF2 nanocrystals in oxyfluoride glass
Materials Research Bulletin 44 (2009) 2143–2146
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Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Effect of chemical etching on the surface morphology of laser-patterned lines with Er3+-doped CaF2 nanocrystals in oxyfluoride glass M. Kanno, T. Honma, T. Komatsu * Department of Materials Science and Technology, Nagaoka University of Technology, 1603-1 Kamitomioka-cho, Nagaoka 940-2188, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 29 May 2009 Received in revised form 27 June 2009 Accepted 30 June 2009 Available online 9 July 2009
The chemical etching behavior for the lines consisting of Er3+-doped CaF2 nanocrystals patterned on the surface of an oxyfluoride glass by using a laser-induced crystallization technique (laser: Yb-doped YVO4; wavelength: 1080 nm; power: 1.7 W; a scanning speed: 2 mm/s) in a nitric acid solution (1N HNO3) is examined, and the morphology change in the lines due to the etching is characterized from confocal laser microscope observations and photoluminescence (PL) spectrum measurements of Er3+ ions. The higher and wider bumps compared with the bump of the original line (no etching) are observed in etched lines, and in particular, the surrounding of lines is etching away preferentially, forming the groove in both sides of line. PL spectra of Er3+ ions with strong intensities are observed from etched lines, suggesting that Er3+doped CaF2 nanocrystals are largely present just at the surface of etched lines. It is found that the chemical etching rate (1.2 102 mm/min) of the crystallized bulk sample is smaller than that (5.4 102 mm/min) of the precursor bulk glass, suggesting that CaF2 nanocrystals formed have a high resistance against the chemical attack. ß 2009 Elsevier Ltd. All rights reserved.
Keywords: A. Glasses A. Nanostructures A. Optical materials D. Luminescence D. Surface properties
1. Introduction Transparent oxyfluoride-based crystallized glasses (glass-ceramics) consisting of fluoride nanocrystals have received much attention [1–6]. In such materials, for instance, rare-earth (RE3+) ions are incorporated into fluoride nanocrystals embedded in SiO2based oxide glass matrices with high thermal/chemical stabilities, and photoluminescence (PL) spectra with strong intensities are observed from RE3+-doped fluoride nanocrystals with low phonon energies. Usually, oxyfluoride-based crystallized glasses are fabricated through well-controlled heat treatments in an electric furnace. Recently, a laser-induced crystallization technique has been applied to oxyfluoride glasses, and lines consisting of LaF3 or CaF2 nanocrystals have been patterned [7–9]. Spatially selected crystallization of fluoride crystals in oxyfluoride glasses would give a high potential for optical device applications such as waveguidetype amplifications, and it is of interest to characterize morphologies and properties of fluoride crystal lines patterned by lasers. Very recently, Hirokawa et al. [10] and Honma et al. [11] applied a combination technique of laser irradiation and chemical etching (e.g., 1N HNO3 solution) to nonlinear optical Ba2TiGe2O8 crystal dots and lines on the glass surface and proposed that its technique is effective in constructing the micro-architecture of crystal dot array and patterned line. The key-point in their studies is that
* Corresponding author. Tel.: +81 258 47 9313; fax: +81 258 47 9300. E-mail address: [email protected] (T. Komatsu). 0025-5408/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2009.06.018
chemical etching rates of the precursor glass and crystallized part in an acid solution are different largely [10,11]. It is of interest and importance to clarify the chemical etching behavior in oxyfluoride glasses and crystallized glasses with fluoride nanocrystals for their device applications. There have been, however, no reports on chemical etching of oxyfluoride glasses and their ceramics so far. The purpose of this study is to examine the chemical etching behavior for laser-induced crystal lines consisting of Er3+-doped CaF2 nanocrystals in oxyfluoride glasses and to clarify the change in the surface morphology and PL spectra after chemical etching. In this study, it has been clarified that the chemical etching rate for the patterned lines in a nitric acid solution (1N HNO3) is smaller than that for the precursor oxyfluoride glass (non-irradiated part). A combination technique of laser-induced crystallization and simple chemical etching is proposed to be effective in designing the micro-architecture of lines with CaF2 nanocrystals in oxyfluoride glasses. Lines consisting of Er3+-doped CaF2 nanocrystals in oxyfluoride glasses have been already successfully patterned using a laser-induced crystallization technique by our research group [7,9]. 2. Experimental In this study, an oxyfluoride glass with the composition of 41.5SiO2–21.3Al2O3–4.8CaO–12.6NaF–16.4CaF2–2.9NiO–0.5ErF3 (mol.%) (designated here as Glass A) was examined [9]. The glasses were prepared using a conventional melt quenching method. Commercial powders of raw materials of SiO2, Al2O3, CaCO3, NaF,
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CaF2, NiO, and ErF3 were mixed and melted in a platinum crucible with a lid at 1400 8C for 1.5 h in air in an electric furnace. The melts were poured onto an iron plate and pressed to a thickness of 2 mm with another iron plate. The glass transition, Tg, crystallization onset, Tx, and crystallization peak, Tp, temperatures for the glasses (bulk samples) were determined using differential thermal analyses (DTA) at a heating rate of 20 K/min. The formation of CaF2 crystals in the crystallized samples was examined from X-ray diffraction (XRD) analyses (Cu Ka radiation) at room temperature. The incorporation of Er3+ ions into CaF2 crystals was confirmed from PL spectra of Er3+ ions by using a micro-PL spectrum apparatus (Tokyo Instruments Co., Nanofinder; Ar+ laser with a wavelength of l = 488 nm). The glasses were mechanically polished to a mirror finish with CeO2 powders. Continuous-wave (cw) Yb:YVO4 fiber lasers with l = 1080 nm were irradiated onto the surface of the glasses using an objective lens (magnification: 50; numerical aperture: NA = 0.8). The sample was put on the stage and mechanically moved during laser irradiations to construct crystal lines. After laser irradiation, wet chemical etchings with a nitric acid (HNO3) solution (concentration: one normality, 1N) and different etching times at room temperature were carried out. The surface morphology of etched crystal lines was observed with a confocal scanning laser (CSL) microscope (Olympus-OLS 3000). The PL spectra of Er3+ ions in the crystal lines were measured by using a micro-PL spectrum apparatus. 3. Results and discussion 3.1. Chemical etching rates of bulk precursor glass and crystallized samples The DTA pattern showed that the glass prepared in this study gives the values of Tg = 575 8C, Tx = 620 8C, and Tp = 635 8C [9]. It was confirmed from the XRD patterns and PL spectra that Er3+doped CaF2 nanocrystals are formed in the samples obtained by heat treatments at Tx and Tp for 2 h, as already reported in previous paper [9]. That is, the average particle size, d, of CaF2 crystals was estimated to be d = 17 nm for the sample heat-treated at Tx = 620 8C and d = 20 nm for the sample heat-treated at Tp = 635 8C from the peak width of the (2 2 0) plane in the XRD patterns by using Scherrer’s equation [9]. Prior to the study of the chemical etching behavior for laserinduced crystal lines consisting of Er3+-doped CaF2 nanocrystals, the chemical etching rate of the bulk precursor oxyfluoride glass and the crystallized (Tp = 635 8C, 1 h) glass was examined. Some parts of the surfaces of glass and crystallized samples were masked by using an epoxy resin and were immersed into a solution of 1N HNO3 for 30, 60, 90, and 120 min. After removing the epoxy resin, the etching depth was measured from CSL microscope observations. The results are shown in Fig. 1. It is found that the etching thickness increases almost linearly with increasing etching time. Furthermore, it is seen that the chemical etching rate of the crystallized sample, i.e., 1.2 102 mm/min, is smaller than that (5.4 102 mm/min) of the precursor glass. This new information would be very important for practical applications of aluminosilicate-based oxyfluoride glass-ceramics with RE3+-doped nanocrystals. To the best of our knowledge, there have been no reports on mechanical properties and chemical durability for oxyfluoride glass-ceramics, although numerous studies on optical properties of RE3+ ions in oxyfluoride glass-ceramics have been reported. Kiczenski and Stebbins [12] examined the fluorine sites in calcium aluminosilicate oxyfluoride glasses by using a 19F magicangle spinning nuclear magnetic resonance (MAS NMR) technique and proposed that most of the F bonding is to Al, i.e., the formation of Al–F–Ca bonds, with roughly 0–30% Si–F–Ca bonds. Therefore,
Fig. 1. Chemical etching thickness of the crystallized (635 8C, 2 h) bulk sample and the precursor bulk glass in a solution of 1N HNO3 as a function of etching time.
Ca2+ ions with the high field strength can more effectively compete with Al3+ for bonding with F in aluminosilicate oxyfluoride glasses [12]. It is known that the viscosity of Al-free silicate melts decreases with the partial substitution of 2F for O2 [13], where the mobile fluoride anion may serve to catalyze network bond breaking [12]. On the other hand, as demonstrated in the dental science and technology, calcium fluoride (CaF2) at the surface of teeth can act as protecting layers, reducing acid dissolution of teeth [14]. It is, therefore, considered that the formation of CaF2 crystals in oxyfluoride glasses would decrease the amount of F ions acting as anions for the breaking of Si–O–Si bonds in the glassy phase and would increase the chemical resistance against the attack of acids such as HNO3. From these reasons, it is expected that the chemical etching rate of the crystallized sample with CaF2 nanocrystals is much smaller than that of the precursor oxyfluoride glass, as obtained in the present study (Fig. 1). 3.2. Chemical etching of crystal lines with Er3+-doped nanocrystals As reported in previous papers [7–9], lines consisting of fluoride crystals such as LaF3 or CaF2 are patterned in aluminosilicate oxyfluoride glasses by laser irradiations. The CSL micrograph for the sample obtained by irradiations of Yb:YVO4 fiber lasers (l = 1080 nm) with a power of P = 1.7 W and a scanning speed of S = 2 mm/s in Glass A is shown in Fig. 2(a). The bump is observed in the laser-irradiated parts, and the values of height (h) and width (w) are h 1 mm and w 3 mm. The formation of bump at the surface is a typical feature in the laser-induced crystallization in oxide and oxyfluoride glasses, indicating that at least the temperature in the laser-irradiated region is going up over the glass transition temperature [15,16]. The CSL micrographs for the samples obtained by chemical etchings in a nitric acid solution (1N HNO3) for the etching times of t = 30 and 60 min at room temperature are shown in Fig. 2(b) and (c). The higher and wider bumps compared with the bump of the original line (no etching) are observed. Furthermore, in particular, the surrounding of the lines is etching away preferentially, forming the groove in both sides of the lines. The height and width of the bumps evaluated from CSL measurements for the etched lines are summarized in Fig. 3. The results shown in Figs. 2 and 3 clearly indicate that the chemical etching rate in the lines changes largely depending on the position of line. Very recently, Kanno et al. [9]
M. Kanno et al. / Materials Research Bulletin 44 (2009) 2143–2146
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proposed the structure (morphology) model for the cross-section of laser-patterned lines consisting of Er3+-doped CaF2 nanocrystals in oxyfluoride glasses. In their model [9], two regions, i.e., the center part having Er3+-doped CaF2 nanocrystals and the surrounding part having a different structure compared with the structure of the original glass (no laser radiated part), are formed in the laser-patterned lines. Considering the temperature distribution and crystal growth process (i.e., nucleation and crystal growth) in the laser-irradiated region, their model giving the formation of two regions would be reasonable. In other words, the results shown in Fig. 2 might indicate the spatial distribution of structural changes in the laser-patterned line. Because the chemical etching rate of the crystallized sample is smaller than that of the precursor glass (Fig. 1), the increase in the height and width of bumps with increasing etching time would be expected. The laser irradiation processing (scanning) is regarded as a rapid heating and rapid cooling in a local part, and consequently stresses would be induced in the laser-irradiated region. It is expected that stresses induced by laser irradiations would reduce by annealing at around the glass transition temperature. We annealed the laser-patterned (P = 1.7 W and S = 2 mm/s) line at 575 8C (Tg of Glass A) for 30 min before etching and then applied a chemical etching (1N HNO3, t = 60 min). The CSL micrograph for this sample is shown in Fig. 4. It is seen that no grooves are formed at the surrounding of the line. A similar behavior has been observed in oxide glasses, where nonlinear optical Ba2TiGe2O8 crystal dots and lines are patterned by laser irradiations [10,11]. Fig. 2. Confocal scanning laser micrographs for the sample obtained by irradiations of Yb:YVO4 fiber lasers (l = 1080 nm) with a power of P = 1.7 W and a scanning speed of S = 2 mm/s. (a) is for the original (no etching) line; (b) and (c) are for the lines obtained by chemical etchings in a nitric acid solution (1N HNO3) for the etching times of t = 30 and 60 min at room temperature, respectively.
Fig. 3. Morphologies of the original (no etching) and chemically etched (1N HNO3, time: t = 10, 30, and 60 min) lines for the laser-patterned line evaluated from confocal scanning laser micrographs.
3.3. PL spectra of Er3+ in chemical etched lines The PL spectrum at room temperature for the original (not etching) line patterned by a laser irradiation (P = 1.7 W, S = 2 mm/s) (Fig. 2(a)) in Glass A is shown in Fig. 5. The PL spectrum for the glass part (not laser-irradiated part) is also included in Fig. 5. In this experiment, the excitation light with l = 488 nm was irradiated perpendicularly against the sample surface. The clear emissions corresponding to the f–f transitions of 2H11/2 ! 4I15/2, 4S3/2 ! 4I15/2, and 4F9/2 ! 4I15/2 for Er3+ ions are observed from the patterned lines. Compared with the glass part, the lines give clearly the so-called Stark splitting in the f–f transition of 4S3/2 ! 4I15/2. Furthermore, although the PL intensity due to the 4F9/2 ! 4I15/2 transition in the glass part is extremely small, the clear PL peaks are observed in the lines. Qiao et al. [3] reported a similar PL spectrum for Er3+-doped CaF2 nanocrystals formed in oxyfluoride glass-ceramics. Bensalah et al. [4] synthesized Er3+-doped CaF2 nanoparticles (20 nm) using a reverse micelle method and reported the PL spectra (the excitation
Fig. 4. Confocal scanning laser micrograph for the laser-patterned (P = 1.7 W and S = 2 mm/s) line. The patterned line was first annealed at 575 8C for 30 min and then etched in 1N HNO3 for 60 min.
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study, although it is experimentally difficult to measure emission spectra for the localized part. Mekhlouf et al. [17] have reported the local crystallization of LaF3 nanocrystals in an oxyfluoride glass using a combination technique of ultraviolet (UV) laser (l = 244 nm) irradiations and heat treatments at temperatures below the glass transition temperature. Gonzalez-Perez et al. [18] reported that cw argon laser irradiations can achieve successfully a local crystallization in Tm3+-doped oxyfluoride glasses. Spatially selected crystallization of fluoride crystals in oxyfluoride glasses would give high potentials for optical device applications such as waveguide-type amplifications. As demonstrated in this study, the combination technique of laser patterning and simple chemical etching gives a new method for design and control of the morphology of optical functional fluoride crystals in oxyfluoride glasses. It should be again emphasized that the chemical etching rate of crystallized samples with CaF2 nanocrystals is smaller than that of the precursor oxyfluoride glass. Further studies on chemical etching behavior for other oxyfluoride glass-ceramics consisting of RE3+doped fluoride nanocrystals are desired. 4. Conclusions
Fig. 5. Photoluminescence spectra at room temperature for the lines obtained by chemical etchings in a nitric acid solution (1N HNO3) for the etching times of t = 10 and 60 min at room temperature. Two-dimensional PL intensities (mapping) for the band at 539 nm measured for the line surfaces are also shown. The bright and black colors indicate the strong and weak PL intensities of 539 nm light, respectively.
is 355 nm) of Er3+ ions similar to that in Fig. 5. The spectrum shown in Fig. 5 clearly indicates that Er3+ ions are incorporated into CaF2 crystals formed in the lines. Because the PL peak at 539 nm is typical for Er3+ ions incorporated into CaF2 crystals, it is considered that the intensity of the PL peak at 539 nm would be used as a probe to examine the coordination environments of Er3+ ions in the laserirradiated part. The two-dimensional PL intensities for the band at 539 nm are measured for the original line surface, and the results (mapping) are shown in Fig. 5. It is seen that the PL intensity of the band at 539 nm from the center (top) of the patterned line is small compared with the off-centered part. The PL spectra at room temperature for the chemically etched (t = 30 and 60 min) lines are also shown in Fig. 5, together with the two-dimensional PL intensity mappings for the band at 539 nm. The bright and black colors indicate the strong and weak PL intensities of 539 nm light, respectively. It is seen that the intensity of the f–f transitions of 2H11/2 ! 4I15/2, 4S3/2 ! 4I15/2, and 4F9/ 4 3+ ions increases gradually with increasing etching 2 ! I15/2 for Er time. Furthermore, in the two-dimensional PL intensity mapping for the sample obtained by the etching for t = 60 min, strong emissions are clearly observed from the center of the line. As clarified in this study, the chemical etching rate of the crystallized sample is smaller than that of the precursor glass, and the main reason for such a phenomenon would be that CaF2 crystals have a high resistance against the chemical etching in a nitric acid solution. That is, in the process of chemical etching, CaF2 nanocrystals tend to remain in the laser-patterned line part and might appear at the surface of the etched lines. It is of interest to compare emission spectra monitored at 539 nm between the chemically etched laser-irradiated part and the glassy part (not laser-irradiated part). We are now under consideration for such a
The effect of the chemical etching (1N HNO3) on the morphology of the lines consisting of Er3+-doped CaF2 nanocrystals patterned on the surface of an aluminosilicate-based oxyfluoride glass by using a laser-induced crystallization technique was examined from confocal scanning laser microscope observations and photoluminescence spectrum measurements of Er3+ ions. The higher and wider bumps compared with the bump of the original line (no etching) were observed in etched lines. The surrounding of lines was etching away preferentially, forming the groove in both sides of lines, and it was found that the annealing before etching is effective to eliminate the groove. It was suggested from photoluminescence spectra that Er3+-doped CaF2 nanocrystals are largely present just at the surface of etched lines. Acknowledgment This work was supported from the Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports, Culture and Technology, Japan. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
Y. Wang, J. Ohwaki, Appl. Phys. Lett. 63 (1993) 3268. M.J. Dejneka, J. Non-Cryst. Solids 239 (1998) 149. X. Qiao, X. Fan, J. Wang, M. Wang, J. Non-Cryst. Solids 351 (2005) 357. A. Bensalah, M. Mortier, G. Patriarche, P. Gredin, D. Vivien, J. Solid State Chem. 179 (2006) 2636. Y. Kishi, S. Tanabe, J. Alloys Compd. 408–412 (2006) 842. D. Chen, Y. Wang, Y. Yu, E. Ma, F. Bao, Z. Hu, Y. Cheng, Mater. Chem. Phys. 95 (2006) 264. M. Kusatsugu, M. Kanno, T. Honma, T. Komatsu, J. Solid State Chem. 181 (2008) 1176. T. Honma, M. Kusatsugu, T. Komatsu, Mater. Chem. Phys. 113 (2009) 124. M. Kanno, T. Honma, T. Komatsu, J. Am. Ceram. Soc. 92 (2009) 825. N. Hirokawa, T. Honma, T. Komatsu, J. Am. Ceram. Soc. 91 (2008) 2170. T. Honma, N. Hirokawa, T. Komatsu, Appl. Surf. Sci. 255 (2008) 3126. T.J. Kiczenski, J.F. Stebbins, J. Non-Cryst. Solids 306 (2002) 160. D.B. Dingwell, Am. Mineral. 74 (1989) 333. H.B. Pan, B.W. Darvell, Arch. Oral Biol. 52 (2007) 861. T. Komatsu, R. Ihara, T. Honma, Y. Benino, R. Sato, H.G. Kim, T. Fujiwara, J. Am. Ceram. Soc. 90 (2007) 699. T. Komatsu, T. Honma, IEEE.J. Select, Top. Quant. Electron. 14 (2008) 1289. S.E. Mekhlouf, A. Boukenter, M. Ferrari, F. Goutaland, N. Ollier, Y. Ouerdane, J. NonCryst. Solids 353 (2007) 506. S. Gonzalez-Perez, I.R. Martin, F. Lahoz, D. Jaque, P. Haro-Gonzalez, N. Capuj, J. Lumin. 128 (2008) 905.
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Effect of chemical etching on the surface morphology of laser-patterned lines with Er3+-doped CaF2 nanocrystals in oxyfluoride glass M. Kanno, T. Honma, T. Komatsu * Department of Materials Science and Technology, Nagaoka University of Technology, 1603-1 Kamitomioka-cho, Nagaoka 940-2188, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 29 May 2009 Received in revised form 27 June 2009 Accepted 30 June 2009 Available online 9 July 2009
The chemical etching behavior for the lines consisting of Er3+-doped CaF2 nanocrystals patterned on the surface of an oxyfluoride glass by using a laser-induced crystallization technique (laser: Yb-doped YVO4; wavelength: 1080 nm; power: 1.7 W; a scanning speed: 2 mm/s) in a nitric acid solution (1N HNO3) is examined, and the morphology change in the lines due to the etching is characterized from confocal laser microscope observations and photoluminescence (PL) spectrum measurements of Er3+ ions. The higher and wider bumps compared with the bump of the original line (no etching) are observed in etched lines, and in particular, the surrounding of lines is etching away preferentially, forming the groove in both sides of line. PL spectra of Er3+ ions with strong intensities are observed from etched lines, suggesting that Er3+doped CaF2 nanocrystals are largely present just at the surface of etched lines. It is found that the chemical etching rate (1.2 102 mm/min) of the crystallized bulk sample is smaller than that (5.4 102 mm/min) of the precursor bulk glass, suggesting that CaF2 nanocrystals formed have a high resistance against the chemical attack. ß 2009 Elsevier Ltd. All rights reserved.
Keywords: A. Glasses A. Nanostructures A. Optical materials D. Luminescence D. Surface properties
1. Introduction Transparent oxyfluoride-based crystallized glasses (glass-ceramics) consisting of fluoride nanocrystals have received much attention [1–6]. In such materials, for instance, rare-earth (RE3+) ions are incorporated into fluoride nanocrystals embedded in SiO2based oxide glass matrices with high thermal/chemical stabilities, and photoluminescence (PL) spectra with strong intensities are observed from RE3+-doped fluoride nanocrystals with low phonon energies. Usually, oxyfluoride-based crystallized glasses are fabricated through well-controlled heat treatments in an electric furnace. Recently, a laser-induced crystallization technique has been applied to oxyfluoride glasses, and lines consisting of LaF3 or CaF2 nanocrystals have been patterned [7–9]. Spatially selected crystallization of fluoride crystals in oxyfluoride glasses would give a high potential for optical device applications such as waveguidetype amplifications, and it is of interest to characterize morphologies and properties of fluoride crystal lines patterned by lasers. Very recently, Hirokawa et al. [10] and Honma et al. [11] applied a combination technique of laser irradiation and chemical etching (e.g., 1N HNO3 solution) to nonlinear optical Ba2TiGe2O8 crystal dots and lines on the glass surface and proposed that its technique is effective in constructing the micro-architecture of crystal dot array and patterned line. The key-point in their studies is that
* Corresponding author. Tel.: +81 258 47 9313; fax: +81 258 47 9300. E-mail address: [email protected] (T. Komatsu). 0025-5408/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2009.06.018
chemical etching rates of the precursor glass and crystallized part in an acid solution are different largely [10,11]. It is of interest and importance to clarify the chemical etching behavior in oxyfluoride glasses and crystallized glasses with fluoride nanocrystals for their device applications. There have been, however, no reports on chemical etching of oxyfluoride glasses and their ceramics so far. The purpose of this study is to examine the chemical etching behavior for laser-induced crystal lines consisting of Er3+-doped CaF2 nanocrystals in oxyfluoride glasses and to clarify the change in the surface morphology and PL spectra after chemical etching. In this study, it has been clarified that the chemical etching rate for the patterned lines in a nitric acid solution (1N HNO3) is smaller than that for the precursor oxyfluoride glass (non-irradiated part). A combination technique of laser-induced crystallization and simple chemical etching is proposed to be effective in designing the micro-architecture of lines with CaF2 nanocrystals in oxyfluoride glasses. Lines consisting of Er3+-doped CaF2 nanocrystals in oxyfluoride glasses have been already successfully patterned using a laser-induced crystallization technique by our research group [7,9]. 2. Experimental In this study, an oxyfluoride glass with the composition of 41.5SiO2–21.3Al2O3–4.8CaO–12.6NaF–16.4CaF2–2.9NiO–0.5ErF3 (mol.%) (designated here as Glass A) was examined [9]. The glasses were prepared using a conventional melt quenching method. Commercial powders of raw materials of SiO2, Al2O3, CaCO3, NaF,
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CaF2, NiO, and ErF3 were mixed and melted in a platinum crucible with a lid at 1400 8C for 1.5 h in air in an electric furnace. The melts were poured onto an iron plate and pressed to a thickness of 2 mm with another iron plate. The glass transition, Tg, crystallization onset, Tx, and crystallization peak, Tp, temperatures for the glasses (bulk samples) were determined using differential thermal analyses (DTA) at a heating rate of 20 K/min. The formation of CaF2 crystals in the crystallized samples was examined from X-ray diffraction (XRD) analyses (Cu Ka radiation) at room temperature. The incorporation of Er3+ ions into CaF2 crystals was confirmed from PL spectra of Er3+ ions by using a micro-PL spectrum apparatus (Tokyo Instruments Co., Nanofinder; Ar+ laser with a wavelength of l = 488 nm). The glasses were mechanically polished to a mirror finish with CeO2 powders. Continuous-wave (cw) Yb:YVO4 fiber lasers with l = 1080 nm were irradiated onto the surface of the glasses using an objective lens (magnification: 50; numerical aperture: NA = 0.8). The sample was put on the stage and mechanically moved during laser irradiations to construct crystal lines. After laser irradiation, wet chemical etchings with a nitric acid (HNO3) solution (concentration: one normality, 1N) and different etching times at room temperature were carried out. The surface morphology of etched crystal lines was observed with a confocal scanning laser (CSL) microscope (Olympus-OLS 3000). The PL spectra of Er3+ ions in the crystal lines were measured by using a micro-PL spectrum apparatus. 3. Results and discussion 3.1. Chemical etching rates of bulk precursor glass and crystallized samples The DTA pattern showed that the glass prepared in this study gives the values of Tg = 575 8C, Tx = 620 8C, and Tp = 635 8C [9]. It was confirmed from the XRD patterns and PL spectra that Er3+doped CaF2 nanocrystals are formed in the samples obtained by heat treatments at Tx and Tp for 2 h, as already reported in previous paper [9]. That is, the average particle size, d, of CaF2 crystals was estimated to be d = 17 nm for the sample heat-treated at Tx = 620 8C and d = 20 nm for the sample heat-treated at Tp = 635 8C from the peak width of the (2 2 0) plane in the XRD patterns by using Scherrer’s equation [9]. Prior to the study of the chemical etching behavior for laserinduced crystal lines consisting of Er3+-doped CaF2 nanocrystals, the chemical etching rate of the bulk precursor oxyfluoride glass and the crystallized (Tp = 635 8C, 1 h) glass was examined. Some parts of the surfaces of glass and crystallized samples were masked by using an epoxy resin and were immersed into a solution of 1N HNO3 for 30, 60, 90, and 120 min. After removing the epoxy resin, the etching depth was measured from CSL microscope observations. The results are shown in Fig. 1. It is found that the etching thickness increases almost linearly with increasing etching time. Furthermore, it is seen that the chemical etching rate of the crystallized sample, i.e., 1.2 102 mm/min, is smaller than that (5.4 102 mm/min) of the precursor glass. This new information would be very important for practical applications of aluminosilicate-based oxyfluoride glass-ceramics with RE3+-doped nanocrystals. To the best of our knowledge, there have been no reports on mechanical properties and chemical durability for oxyfluoride glass-ceramics, although numerous studies on optical properties of RE3+ ions in oxyfluoride glass-ceramics have been reported. Kiczenski and Stebbins [12] examined the fluorine sites in calcium aluminosilicate oxyfluoride glasses by using a 19F magicangle spinning nuclear magnetic resonance (MAS NMR) technique and proposed that most of the F bonding is to Al, i.e., the formation of Al–F–Ca bonds, with roughly 0–30% Si–F–Ca bonds. Therefore,
Fig. 1. Chemical etching thickness of the crystallized (635 8C, 2 h) bulk sample and the precursor bulk glass in a solution of 1N HNO3 as a function of etching time.
Ca2+ ions with the high field strength can more effectively compete with Al3+ for bonding with F in aluminosilicate oxyfluoride glasses [12]. It is known that the viscosity of Al-free silicate melts decreases with the partial substitution of 2F for O2 [13], where the mobile fluoride anion may serve to catalyze network bond breaking [12]. On the other hand, as demonstrated in the dental science and technology, calcium fluoride (CaF2) at the surface of teeth can act as protecting layers, reducing acid dissolution of teeth [14]. It is, therefore, considered that the formation of CaF2 crystals in oxyfluoride glasses would decrease the amount of F ions acting as anions for the breaking of Si–O–Si bonds in the glassy phase and would increase the chemical resistance against the attack of acids such as HNO3. From these reasons, it is expected that the chemical etching rate of the crystallized sample with CaF2 nanocrystals is much smaller than that of the precursor oxyfluoride glass, as obtained in the present study (Fig. 1). 3.2. Chemical etching of crystal lines with Er3+-doped nanocrystals As reported in previous papers [7–9], lines consisting of fluoride crystals such as LaF3 or CaF2 are patterned in aluminosilicate oxyfluoride glasses by laser irradiations. The CSL micrograph for the sample obtained by irradiations of Yb:YVO4 fiber lasers (l = 1080 nm) with a power of P = 1.7 W and a scanning speed of S = 2 mm/s in Glass A is shown in Fig. 2(a). The bump is observed in the laser-irradiated parts, and the values of height (h) and width (w) are h 1 mm and w 3 mm. The formation of bump at the surface is a typical feature in the laser-induced crystallization in oxide and oxyfluoride glasses, indicating that at least the temperature in the laser-irradiated region is going up over the glass transition temperature [15,16]. The CSL micrographs for the samples obtained by chemical etchings in a nitric acid solution (1N HNO3) for the etching times of t = 30 and 60 min at room temperature are shown in Fig. 2(b) and (c). The higher and wider bumps compared with the bump of the original line (no etching) are observed. Furthermore, in particular, the surrounding of the lines is etching away preferentially, forming the groove in both sides of the lines. The height and width of the bumps evaluated from CSL measurements for the etched lines are summarized in Fig. 3. The results shown in Figs. 2 and 3 clearly indicate that the chemical etching rate in the lines changes largely depending on the position of line. Very recently, Kanno et al. [9]
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proposed the structure (morphology) model for the cross-section of laser-patterned lines consisting of Er3+-doped CaF2 nanocrystals in oxyfluoride glasses. In their model [9], two regions, i.e., the center part having Er3+-doped CaF2 nanocrystals and the surrounding part having a different structure compared with the structure of the original glass (no laser radiated part), are formed in the laser-patterned lines. Considering the temperature distribution and crystal growth process (i.e., nucleation and crystal growth) in the laser-irradiated region, their model giving the formation of two regions would be reasonable. In other words, the results shown in Fig. 2 might indicate the spatial distribution of structural changes in the laser-patterned line. Because the chemical etching rate of the crystallized sample is smaller than that of the precursor glass (Fig. 1), the increase in the height and width of bumps with increasing etching time would be expected. The laser irradiation processing (scanning) is regarded as a rapid heating and rapid cooling in a local part, and consequently stresses would be induced in the laser-irradiated region. It is expected that stresses induced by laser irradiations would reduce by annealing at around the glass transition temperature. We annealed the laser-patterned (P = 1.7 W and S = 2 mm/s) line at 575 8C (Tg of Glass A) for 30 min before etching and then applied a chemical etching (1N HNO3, t = 60 min). The CSL micrograph for this sample is shown in Fig. 4. It is seen that no grooves are formed at the surrounding of the line. A similar behavior has been observed in oxide glasses, where nonlinear optical Ba2TiGe2O8 crystal dots and lines are patterned by laser irradiations [10,11]. Fig. 2. Confocal scanning laser micrographs for the sample obtained by irradiations of Yb:YVO4 fiber lasers (l = 1080 nm) with a power of P = 1.7 W and a scanning speed of S = 2 mm/s. (a) is for the original (no etching) line; (b) and (c) are for the lines obtained by chemical etchings in a nitric acid solution (1N HNO3) for the etching times of t = 30 and 60 min at room temperature, respectively.
Fig. 3. Morphologies of the original (no etching) and chemically etched (1N HNO3, time: t = 10, 30, and 60 min) lines for the laser-patterned line evaluated from confocal scanning laser micrographs.
3.3. PL spectra of Er3+ in chemical etched lines The PL spectrum at room temperature for the original (not etching) line patterned by a laser irradiation (P = 1.7 W, S = 2 mm/s) (Fig. 2(a)) in Glass A is shown in Fig. 5. The PL spectrum for the glass part (not laser-irradiated part) is also included in Fig. 5. In this experiment, the excitation light with l = 488 nm was irradiated perpendicularly against the sample surface. The clear emissions corresponding to the f–f transitions of 2H11/2 ! 4I15/2, 4S3/2 ! 4I15/2, and 4F9/2 ! 4I15/2 for Er3+ ions are observed from the patterned lines. Compared with the glass part, the lines give clearly the so-called Stark splitting in the f–f transition of 4S3/2 ! 4I15/2. Furthermore, although the PL intensity due to the 4F9/2 ! 4I15/2 transition in the glass part is extremely small, the clear PL peaks are observed in the lines. Qiao et al. [3] reported a similar PL spectrum for Er3+-doped CaF2 nanocrystals formed in oxyfluoride glass-ceramics. Bensalah et al. [4] synthesized Er3+-doped CaF2 nanoparticles (20 nm) using a reverse micelle method and reported the PL spectra (the excitation
Fig. 4. Confocal scanning laser micrograph for the laser-patterned (P = 1.7 W and S = 2 mm/s) line. The patterned line was first annealed at 575 8C for 30 min and then etched in 1N HNO3 for 60 min.
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study, although it is experimentally difficult to measure emission spectra for the localized part. Mekhlouf et al. [17] have reported the local crystallization of LaF3 nanocrystals in an oxyfluoride glass using a combination technique of ultraviolet (UV) laser (l = 244 nm) irradiations and heat treatments at temperatures below the glass transition temperature. Gonzalez-Perez et al. [18] reported that cw argon laser irradiations can achieve successfully a local crystallization in Tm3+-doped oxyfluoride glasses. Spatially selected crystallization of fluoride crystals in oxyfluoride glasses would give high potentials for optical device applications such as waveguide-type amplifications. As demonstrated in this study, the combination technique of laser patterning and simple chemical etching gives a new method for design and control of the morphology of optical functional fluoride crystals in oxyfluoride glasses. It should be again emphasized that the chemical etching rate of crystallized samples with CaF2 nanocrystals is smaller than that of the precursor oxyfluoride glass. Further studies on chemical etching behavior for other oxyfluoride glass-ceramics consisting of RE3+doped fluoride nanocrystals are desired. 4. Conclusions
Fig. 5. Photoluminescence spectra at room temperature for the lines obtained by chemical etchings in a nitric acid solution (1N HNO3) for the etching times of t = 10 and 60 min at room temperature. Two-dimensional PL intensities (mapping) for the band at 539 nm measured for the line surfaces are also shown. The bright and black colors indicate the strong and weak PL intensities of 539 nm light, respectively.
is 355 nm) of Er3+ ions similar to that in Fig. 5. The spectrum shown in Fig. 5 clearly indicates that Er3+ ions are incorporated into CaF2 crystals formed in the lines. Because the PL peak at 539 nm is typical for Er3+ ions incorporated into CaF2 crystals, it is considered that the intensity of the PL peak at 539 nm would be used as a probe to examine the coordination environments of Er3+ ions in the laserirradiated part. The two-dimensional PL intensities for the band at 539 nm are measured for the original line surface, and the results (mapping) are shown in Fig. 5. It is seen that the PL intensity of the band at 539 nm from the center (top) of the patterned line is small compared with the off-centered part. The PL spectra at room temperature for the chemically etched (t = 30 and 60 min) lines are also shown in Fig. 5, together with the two-dimensional PL intensity mappings for the band at 539 nm. The bright and black colors indicate the strong and weak PL intensities of 539 nm light, respectively. It is seen that the intensity of the f–f transitions of 2H11/2 ! 4I15/2, 4S3/2 ! 4I15/2, and 4F9/ 4 3+ ions increases gradually with increasing etching 2 ! I15/2 for Er time. Furthermore, in the two-dimensional PL intensity mapping for the sample obtained by the etching for t = 60 min, strong emissions are clearly observed from the center of the line. As clarified in this study, the chemical etching rate of the crystallized sample is smaller than that of the precursor glass, and the main reason for such a phenomenon would be that CaF2 crystals have a high resistance against the chemical etching in a nitric acid solution. That is, in the process of chemical etching, CaF2 nanocrystals tend to remain in the laser-patterned line part and might appear at the surface of the etched lines. It is of interest to compare emission spectra monitored at 539 nm between the chemically etched laser-irradiated part and the glassy part (not laser-irradiated part). We are now under consideration for such a
The effect of the chemical etching (1N HNO3) on the morphology of the lines consisting of Er3+-doped CaF2 nanocrystals patterned on the surface of an aluminosilicate-based oxyfluoride glass by using a laser-induced crystallization technique was examined from confocal scanning laser microscope observations and photoluminescence spectrum measurements of Er3+ ions. The higher and wider bumps compared with the bump of the original line (no etching) were observed in etched lines. The surrounding of lines was etching away preferentially, forming the groove in both sides of lines, and it was found that the annealing before etching is effective to eliminate the groove. It was suggested from photoluminescence spectra that Er3+-doped CaF2 nanocrystals are largely present just at the surface of etched lines. Acknowledgment This work was supported from the Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports, Culture and Technology, Japan. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
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