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Polymer-assisted hydrothermal synthesis of single crystal Pb1 − xLaxTiO3 nanorods
Materials Letters 63 (2009) 937–939
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m a t l e t
Polymer-assisted hydrothermal synthesis of single crystal Pb1 − xLaxTiO3 nanorods Y. Deng a,⁎, J.X. Zhou b, Y.L. Du c, K.R. Zhu d, Y. Hu a, D. Wu a, M.S. Zhang a, Y.W. Du a a
National Laboratory of Solid State Microstructures and Center for Materials Analysis, Nanjing University, Nanjing 210093, China Institute of Nano-Science, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, China c Department of Materials Science & Engineering, Nanjing University of Science and Technology, Nanjing 210094, China d School of Physics & Materials Science and Modern Experiment Technology Center, Anhui University, Hefei 230039, China b
a r t i c l e
i n f o
Article history: Received 14 November 2008 Accepted 17 January 2009 Available online 22 January 2009 PACS: 81.07.Bc 61.72.U 64.70.Nd 63.22.Gh 81.07-b
a b s t r a c t Single-crystal PbxLa1 − xTiO3 (PLT) nanorods of various La concentrations have been synthesized by polymerassisted hydrothermal method. The nanorods have diameters of 25–60 nm and average lengths of 3 μm. With tetragonal lattices structures, the PLT nanorods grow along the (001) direction. As La concentration increasing, the tetragonality c/a decreases and the Raman mode E(1TO) becomes softening. PLT nanorods with various lengths and diameters have been prepared by using different polymer additives. To fabricate well-crystallized nanorods, an annealing process after the hydrothermal treatment is proved to be necessary. © 2009 Elsevier B.V. All rights reserved.
Keywords: Nanorods Doping Phonon characteristics Fabrication and characterization
1. Introduction
2. Experimental
Lead titanate (PbTiO3, PT) and lead lanthanum titanate (Pb1 − xLaxTiO3, PLT) are important perovskite oxides which show excellent ferroelectric, piezoelectricity and nonlinear optical properties [1–3]. Due to the recent trend of miniaturization on microelectronic and optoelectronic devices with improved properties, nanosized PT and PLT are desirable materials for future application. Surprisingly large optical nonlinearity, pyroelectricity and giant dielectric permittivity have been investigated in PLT lowdimensional materials [3–5]. It's reasonable to believe that there should be more attractive properties in the one-dimensional nanostructures (1DNS) of PLT. However, there is little reported about the controllable growth and size-dependent properties of PLT 1DNS, though many efforts have been devoted to prepare low-dimensional materials of perovskite oxides in recent years [6–9]. In this work, we use a low-cost and convenient hydrothermal method to synthesize single crystalline PLT nanorods with high output-rate. By using different polymer-additives, nanorods with various lengths and diameters have been prepared. The structural characters and growth process of the PLT nanorods have also been investigated.
The PLT nanorods were fabricated by polymer-assisted hydrothermal technique. Firstly, two primary solutions were prepared as follows. Solution A: Pb(NO3)2 and La(NO3)3 in water; Solution B: Ti(OC4H9)4 in alcohol. Their molar ratios were determined according to the chemical formula PbxLa1 − xTiO3. Three kinds of PLT samples were prepared with La concentration of 0% (PLT0), 5% (PLT5) and 10% (PLT10), respectively. Subsequently, B was dropped into A, accompanying with constant-rate stirring for 15 min. NaOH was then slowly added until its concentration reaches 10 M/L. Under violently stirring, the additive polymer PVA (poly vinyl alcohol) (0.8 g/L) and PAA (poly acrylic acid) (0.1 g/L) were dropped into the mixture. The reaction mixture was then introduced into a stainless-steel Teflon-lined autoclave with 80% of its volume. Hydrothermal treatment was performed by placing the autoclave in an oven at 180 °C for 48 h, and then cooling to room temperature in air. The obtained powders were washed by distilled water and alcohol for several times. After drying, they were annealed in air at 450 °C for 3 h. Finally, the light yellow products were obtained. X-ray diffraction (XRD) was performed on a Rigaku D/MAX-RA diffractometer with CuKα as the incident radiation. The morphology and selected area electron diffraction (SAED) were detected on a transmission electron microscope (TEM) of JEM-200CX and a highresolution transmission electron microscope (HRTEM) of JEOL-2100. Scanning Electron microscope (SEM) was performed on a Hitachi-4800
⁎ Corresponding author. Tel.: +86 25 83592293; fax: +86 25 83595535. E-mail address: [email protected] (Y. Deng). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.01.043
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Y. Deng et al. / Materials Letters 63 (2009) 937–939
Fig. 3. Raman spectra of the PLT0, PLT5 and PLT10 nanorods at room temperature.
Fig. 1. XRD patterns of the PLT0, PLT5 and PLT10 nanorods.
and Energy dispersive spectroscopy (EDS) was detected on an Oxford EDX-250. The Raman spectrum were measured on a HR-800 Raman spectrometer under back-scattering geometry, using 488 nm exciting light at a power of 10 mW. 3. Results and discussion 3.1. The morphology and structural characterization of the PLT nanorods Fig. 1 represents the X-ray diffraction patterns of the PLT0, PLT5 and PLT10 samples. The sharp diffraction peaks imply that they are well crystallized. All the diffraction
Fig. 2. (a) TEM and (b) the corresponding EDS of the PLT5 nanorods, (c) the HRTEM of an individual PLT5 nanorod with its SAED in the upper inset and lattice image in the lower inset.
peaks can be well assigned as the tetragonal phase PT and there are no impurity lines. From the XRD data, the lattice constants a, b, c of the samples have been calculated [10] to be PLT0 (a = b = 3.91 Å, c = 4.12 Å), PLT5 (a = b = 3.92 Å, c = 4.01 Å) and PLT10 (a = b = 3.92 Å, c = 3.96 Å), respectively. It indicates that the La is effectively doped into the lattice, which can cause the decreasing of the tetragonality c/a [2,8]. From Fig. 1, we can also see that the (101) and (110) peaks in the PLT samples shift together as La concentration increasing, also revealing the decreasing of the tetragonality [2]. Fig. 2(a) shows the TEM image of the PLT5 nanorods, which exhibits almost the same morphology as the PLT0 and PLT10. The nanorods are straight and smooth, occupying a high proportion (N 80%) in the as-prepared products. Their diameters range from 25 nm to 60 nm and average lengths of 3 μm. Their corresponding EDS pattern is illustrated in Fig. 2(b), confirming that the PLT samples are in good stoichiometric proportion. An individual PLT5 nanorod is selected for HRTEM investigation, as shown in Fig. 2(c), the upper inset is its SAED pattern and the lower inset is its lattice image. From lower inset of Fig. 2(c), the inter-planar spacing along the growth-direction is measured to be 4.0 Å, which corresponds to (001) lattice constant, revealing that the nanorod grows along (001) direction [2,7]. The SAED result in upper inset of Fig. 2(c) further confirms the conclusion. Raman spectra of PLT0, PLT5 and PLT10 nanorods at room temperature are depicted in Fig. 3. All the Raman lines are assigned and compared with the results of Burns [1,2]. The lowest-frequency Raman line is the “soft mode” belonging to E(1TO) symmetry. As La concentration increasing from none to 10%, its intensity decreases and frequency downshifts from 80 cm− 1 to 66 cm− 1, accompanying with broadening of the line width.
Fig. 4. The SEM images of the products (a) with additive C without NaOH; (b) with additive A and NaOH; (c) with additive B and NaOH; (d) with additive C and NaOH.
Y. Deng et al. / Materials Letters 63 (2009) 937–939
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cubic structural transition of PbTiO3 lattice, the A1(2TO) mode shifts towards the “silent mode” B1 + E and get completely overlapped in the cubic phase [1]. 3.2. The effect of the polymer-additives and the annealing process Without NaOH, the hydrothermal reaction cannot produce PLT nanorods by the polymer assisting. Fig. 4(a) shows the products prepared without adding NaOH, where all the other synthesis conditions are the same. We can see that there are nanoparticles with average size of 70 nm instead of 1DNS. Moreover, we find that the prepared PLT nanorods will exhibit different lengths and diameters when different kinds of polymeradditives are used. As we know, when introduced into the hydrothermal reaction, the PVA will attach to the surfaces of the 1DNS. Notably, there will be much more PVA attached to the sides of the 1DNS than to their tips, which will benefit the longitudinal growth of nanorods. Moreover, the highly water-soluble PAA is considered to be effective on limiting the radial growth of 1DNS during the hydrothermal process. And some polymer additives can selectively limit the growth of some crystalline planes by lowering the surface tension in aqueous solution [9]. For comparison, three polymeradditives of A (PVA of 0.2 g/L), B (PVA of 0.8 g/L) and C (PVA of 0.8 g/L and PAA of 0.1 g/L) were used to synthesize PLT5 nanorods, respectively. The SEM images of the corresponding products were shown in Fig. 4(b)–(d). We can clearly see that the additive A produce the short nanorods (average length of 500 nm and diameter of 60 nm), B produce the longer nanorods (average length of 1 μm and diameter of 40 nm), and C produce the long nanorods (average length of 3 μm and diameter of 30 nm). It is worth pointing out that the choosing of additives can effectively control the growth of PLT nanorods [9]. Further researches on fabricating series PLT nanorods with designed length and diameter are now carrying on. To synthesize well-crystallized PLT nanorods, the annealing process after the hydrothermal reaction is considered to be necessary. Fig. 5(a) represents the SEM image of the intermediate products right after the hydrothermal treatment. They are curving nanorods with diameter of 20–60 nm and length up to 8 μm. However, the HRTEM in Fig. 4(b) reveals that the nanorod has only partly crystallized, with an armorphase “shell” of 5 nm thickness in average. The Raman spectrum of the PLT5 nanorods without annealing is shown in Fig. 5(e). We can see that most Raman lines are weak and broad, and the peaks of E(1TO), A1(2TO) are so weak that they are difficult to recognize, indicating that the samples need more crystallization [1,2,7]. With increasing annealing temperatures, the Raman spectra of the PLT5 nanorods are also depicted in Fig. 5(e). We find that all the Raman lines become well intensive and sharp when annealing temperature reaches 450 °C [1,2]. By further SEM and HRTEM observations, as shown in Fig. 5(c) and (d), we can see that the samples have completely crystallized and become straight nanorods.
4. Conclusion In summary, single-crystal PLT nanorods have been synthesized by polymer-assisted hydrothermal method with high out-put rate (N80%). The PLT nanorods are in tetragonal phase and grow along the (001) direction. The Raman modes of the PLT nanorods are assigned and mode softening behaviors of E(1TO) caused by La doping are recorded. By using different kinds of polymer additives, nanorods of various sizes have been prepared. Acknowledgements This work has been supported by the National Natural Science Foundation of China, Grants Nos. 50802039 and 10174034, and the National key Project for Basic Research No. 2005CB623605. Fig. 5. (a) SEM and (b) HRTEM images of the PLT5 nanorods without annealing treatment. (c) SEM and (d) HRTEM of PLT5 nanorods with annealing treatment at 450 °C for 3 h. (e) Raman spectra of the PLT5 nanorods without annealing and annealing at 300 °C, 400 °C and 450 °C for 3 h, respectively.
It is a typical mode softening behavior, implying that the increasing La weakens the ferroelectricity of the PLT nanorods [2,11]. However, as La concentration increasing, the “silent mode” B1 + E maintains its frequency without downshifting like other modes. It can be attributed to the invariability of the B1 + E symmetry during the tetragonal-tocubic transition [1]. As La concentration increasing, A1(2TO) mode shifts to lower wave numbers, almost merges into B1 + E mode for PLT10. We attribute the phenomenon to the tetragonality decreasing of the PLT nanorods. As known, during the tetragonal-to-
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
Burns G, Scott BA. Phys Rev B 1973;7:3088. Deng Y, Yin Z, Chen Q, Zhang MS, Zhang WF. Mater Sci Eng B 2001;84:248. Zhao QC, Liu Y, Shi WS, Ren W, Zhang LY, Yao X. Appl Phys Lett 1996;69:458. Kim BG, Cho SM, Kim TY, Jang HM. Phys Rev Lett 2001;86:3404. Tseng YK, Liu KS, Jiang JD, Lin IN. Appl Phys Lett 1998;72:3285. Lee S, Son T, Yun JD, Kwon H, Messing GL, Jun B. Mater Lett 2004;58:2932. Gu HS, Hu YM, You J, Hu ZL, Yuan Y, Zhang TJ. J Appl Phys 2007;101:024319. Wei X, Xu G, Ren ZH, Shen G, Han GR. Mater Lett 2008;62:3719. Xu G, Ren ZH, Du PY. Adv Mater 2005;17:907. Birks LS, Friedman H. J Appl Phys 1946;17:687. Ishikawa K, Yoshikawa K, Okada N. Phys Rev B 1988;37:5852.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m a t l e t
Polymer-assisted hydrothermal synthesis of single crystal Pb1 − xLaxTiO3 nanorods Y. Deng a,⁎, J.X. Zhou b, Y.L. Du c, K.R. Zhu d, Y. Hu a, D. Wu a, M.S. Zhang a, Y.W. Du a a
National Laboratory of Solid State Microstructures and Center for Materials Analysis, Nanjing University, Nanjing 210093, China Institute of Nano-Science, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, China c Department of Materials Science & Engineering, Nanjing University of Science and Technology, Nanjing 210094, China d School of Physics & Materials Science and Modern Experiment Technology Center, Anhui University, Hefei 230039, China b
a r t i c l e
i n f o
Article history: Received 14 November 2008 Accepted 17 January 2009 Available online 22 January 2009 PACS: 81.07.Bc 61.72.U 64.70.Nd 63.22.Gh 81.07-b
a b s t r a c t Single-crystal PbxLa1 − xTiO3 (PLT) nanorods of various La concentrations have been synthesized by polymerassisted hydrothermal method. The nanorods have diameters of 25–60 nm and average lengths of 3 μm. With tetragonal lattices structures, the PLT nanorods grow along the (001) direction. As La concentration increasing, the tetragonality c/a decreases and the Raman mode E(1TO) becomes softening. PLT nanorods with various lengths and diameters have been prepared by using different polymer additives. To fabricate well-crystallized nanorods, an annealing process after the hydrothermal treatment is proved to be necessary. © 2009 Elsevier B.V. All rights reserved.
Keywords: Nanorods Doping Phonon characteristics Fabrication and characterization
1. Introduction
2. Experimental
Lead titanate (PbTiO3, PT) and lead lanthanum titanate (Pb1 − xLaxTiO3, PLT) are important perovskite oxides which show excellent ferroelectric, piezoelectricity and nonlinear optical properties [1–3]. Due to the recent trend of miniaturization on microelectronic and optoelectronic devices with improved properties, nanosized PT and PLT are desirable materials for future application. Surprisingly large optical nonlinearity, pyroelectricity and giant dielectric permittivity have been investigated in PLT lowdimensional materials [3–5]. It's reasonable to believe that there should be more attractive properties in the one-dimensional nanostructures (1DNS) of PLT. However, there is little reported about the controllable growth and size-dependent properties of PLT 1DNS, though many efforts have been devoted to prepare low-dimensional materials of perovskite oxides in recent years [6–9]. In this work, we use a low-cost and convenient hydrothermal method to synthesize single crystalline PLT nanorods with high output-rate. By using different polymer-additives, nanorods with various lengths and diameters have been prepared. The structural characters and growth process of the PLT nanorods have also been investigated.
The PLT nanorods were fabricated by polymer-assisted hydrothermal technique. Firstly, two primary solutions were prepared as follows. Solution A: Pb(NO3)2 and La(NO3)3 in water; Solution B: Ti(OC4H9)4 in alcohol. Their molar ratios were determined according to the chemical formula PbxLa1 − xTiO3. Three kinds of PLT samples were prepared with La concentration of 0% (PLT0), 5% (PLT5) and 10% (PLT10), respectively. Subsequently, B was dropped into A, accompanying with constant-rate stirring for 15 min. NaOH was then slowly added until its concentration reaches 10 M/L. Under violently stirring, the additive polymer PVA (poly vinyl alcohol) (0.8 g/L) and PAA (poly acrylic acid) (0.1 g/L) were dropped into the mixture. The reaction mixture was then introduced into a stainless-steel Teflon-lined autoclave with 80% of its volume. Hydrothermal treatment was performed by placing the autoclave in an oven at 180 °C for 48 h, and then cooling to room temperature in air. The obtained powders were washed by distilled water and alcohol for several times. After drying, they were annealed in air at 450 °C for 3 h. Finally, the light yellow products were obtained. X-ray diffraction (XRD) was performed on a Rigaku D/MAX-RA diffractometer with CuKα as the incident radiation. The morphology and selected area electron diffraction (SAED) were detected on a transmission electron microscope (TEM) of JEM-200CX and a highresolution transmission electron microscope (HRTEM) of JEOL-2100. Scanning Electron microscope (SEM) was performed on a Hitachi-4800
⁎ Corresponding author. Tel.: +86 25 83592293; fax: +86 25 83595535. E-mail address: [email protected] (Y. Deng). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.01.043
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Y. Deng et al. / Materials Letters 63 (2009) 937–939
Fig. 3. Raman spectra of the PLT0, PLT5 and PLT10 nanorods at room temperature.
Fig. 1. XRD patterns of the PLT0, PLT5 and PLT10 nanorods.
and Energy dispersive spectroscopy (EDS) was detected on an Oxford EDX-250. The Raman spectrum were measured on a HR-800 Raman spectrometer under back-scattering geometry, using 488 nm exciting light at a power of 10 mW. 3. Results and discussion 3.1. The morphology and structural characterization of the PLT nanorods Fig. 1 represents the X-ray diffraction patterns of the PLT0, PLT5 and PLT10 samples. The sharp diffraction peaks imply that they are well crystallized. All the diffraction
Fig. 2. (a) TEM and (b) the corresponding EDS of the PLT5 nanorods, (c) the HRTEM of an individual PLT5 nanorod with its SAED in the upper inset and lattice image in the lower inset.
peaks can be well assigned as the tetragonal phase PT and there are no impurity lines. From the XRD data, the lattice constants a, b, c of the samples have been calculated [10] to be PLT0 (a = b = 3.91 Å, c = 4.12 Å), PLT5 (a = b = 3.92 Å, c = 4.01 Å) and PLT10 (a = b = 3.92 Å, c = 3.96 Å), respectively. It indicates that the La is effectively doped into the lattice, which can cause the decreasing of the tetragonality c/a [2,8]. From Fig. 1, we can also see that the (101) and (110) peaks in the PLT samples shift together as La concentration increasing, also revealing the decreasing of the tetragonality [2]. Fig. 2(a) shows the TEM image of the PLT5 nanorods, which exhibits almost the same morphology as the PLT0 and PLT10. The nanorods are straight and smooth, occupying a high proportion (N 80%) in the as-prepared products. Their diameters range from 25 nm to 60 nm and average lengths of 3 μm. Their corresponding EDS pattern is illustrated in Fig. 2(b), confirming that the PLT samples are in good stoichiometric proportion. An individual PLT5 nanorod is selected for HRTEM investigation, as shown in Fig. 2(c), the upper inset is its SAED pattern and the lower inset is its lattice image. From lower inset of Fig. 2(c), the inter-planar spacing along the growth-direction is measured to be 4.0 Å, which corresponds to (001) lattice constant, revealing that the nanorod grows along (001) direction [2,7]. The SAED result in upper inset of Fig. 2(c) further confirms the conclusion. Raman spectra of PLT0, PLT5 and PLT10 nanorods at room temperature are depicted in Fig. 3. All the Raman lines are assigned and compared with the results of Burns [1,2]. The lowest-frequency Raman line is the “soft mode” belonging to E(1TO) symmetry. As La concentration increasing from none to 10%, its intensity decreases and frequency downshifts from 80 cm− 1 to 66 cm− 1, accompanying with broadening of the line width.
Fig. 4. The SEM images of the products (a) with additive C without NaOH; (b) with additive A and NaOH; (c) with additive B and NaOH; (d) with additive C and NaOH.
Y. Deng et al. / Materials Letters 63 (2009) 937–939
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cubic structural transition of PbTiO3 lattice, the A1(2TO) mode shifts towards the “silent mode” B1 + E and get completely overlapped in the cubic phase [1]. 3.2. The effect of the polymer-additives and the annealing process Without NaOH, the hydrothermal reaction cannot produce PLT nanorods by the polymer assisting. Fig. 4(a) shows the products prepared without adding NaOH, where all the other synthesis conditions are the same. We can see that there are nanoparticles with average size of 70 nm instead of 1DNS. Moreover, we find that the prepared PLT nanorods will exhibit different lengths and diameters when different kinds of polymeradditives are used. As we know, when introduced into the hydrothermal reaction, the PVA will attach to the surfaces of the 1DNS. Notably, there will be much more PVA attached to the sides of the 1DNS than to their tips, which will benefit the longitudinal growth of nanorods. Moreover, the highly water-soluble PAA is considered to be effective on limiting the radial growth of 1DNS during the hydrothermal process. And some polymer additives can selectively limit the growth of some crystalline planes by lowering the surface tension in aqueous solution [9]. For comparison, three polymeradditives of A (PVA of 0.2 g/L), B (PVA of 0.8 g/L) and C (PVA of 0.8 g/L and PAA of 0.1 g/L) were used to synthesize PLT5 nanorods, respectively. The SEM images of the corresponding products were shown in Fig. 4(b)–(d). We can clearly see that the additive A produce the short nanorods (average length of 500 nm and diameter of 60 nm), B produce the longer nanorods (average length of 1 μm and diameter of 40 nm), and C produce the long nanorods (average length of 3 μm and diameter of 30 nm). It is worth pointing out that the choosing of additives can effectively control the growth of PLT nanorods [9]. Further researches on fabricating series PLT nanorods with designed length and diameter are now carrying on. To synthesize well-crystallized PLT nanorods, the annealing process after the hydrothermal reaction is considered to be necessary. Fig. 5(a) represents the SEM image of the intermediate products right after the hydrothermal treatment. They are curving nanorods with diameter of 20–60 nm and length up to 8 μm. However, the HRTEM in Fig. 4(b) reveals that the nanorod has only partly crystallized, with an armorphase “shell” of 5 nm thickness in average. The Raman spectrum of the PLT5 nanorods without annealing is shown in Fig. 5(e). We can see that most Raman lines are weak and broad, and the peaks of E(1TO), A1(2TO) are so weak that they are difficult to recognize, indicating that the samples need more crystallization [1,2,7]. With increasing annealing temperatures, the Raman spectra of the PLT5 nanorods are also depicted in Fig. 5(e). We find that all the Raman lines become well intensive and sharp when annealing temperature reaches 450 °C [1,2]. By further SEM and HRTEM observations, as shown in Fig. 5(c) and (d), we can see that the samples have completely crystallized and become straight nanorods.
4. Conclusion In summary, single-crystal PLT nanorods have been synthesized by polymer-assisted hydrothermal method with high out-put rate (N80%). The PLT nanorods are in tetragonal phase and grow along the (001) direction. The Raman modes of the PLT nanorods are assigned and mode softening behaviors of E(1TO) caused by La doping are recorded. By using different kinds of polymer additives, nanorods of various sizes have been prepared. Acknowledgements This work has been supported by the National Natural Science Foundation of China, Grants Nos. 50802039 and 10174034, and the National key Project for Basic Research No. 2005CB623605. Fig. 5. (a) SEM and (b) HRTEM images of the PLT5 nanorods without annealing treatment. (c) SEM and (d) HRTEM of PLT5 nanorods with annealing treatment at 450 °C for 3 h. (e) Raman spectra of the PLT5 nanorods without annealing and annealing at 300 °C, 400 °C and 450 °C for 3 h, respectively.
It is a typical mode softening behavior, implying that the increasing La weakens the ferroelectricity of the PLT nanorods [2,11]. However, as La concentration increasing, the “silent mode” B1 + E maintains its frequency without downshifting like other modes. It can be attributed to the invariability of the B1 + E symmetry during the tetragonal-tocubic transition [1]. As La concentration increasing, A1(2TO) mode shifts to lower wave numbers, almost merges into B1 + E mode for PLT10. We attribute the phenomenon to the tetragonality decreasing of the PLT nanorods. As known, during the tetragonal-to-
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
Burns G, Scott BA. Phys Rev B 1973;7:3088. Deng Y, Yin Z, Chen Q, Zhang MS, Zhang WF. Mater Sci Eng B 2001;84:248. Zhao QC, Liu Y, Shi WS, Ren W, Zhang LY, Yao X. Appl Phys Lett 1996;69:458. Kim BG, Cho SM, Kim TY, Jang HM. Phys Rev Lett 2001;86:3404. Tseng YK, Liu KS, Jiang JD, Lin IN. Appl Phys Lett 1998;72:3285. Lee S, Son T, Yun JD, Kwon H, Messing GL, Jun B. Mater Lett 2004;58:2932. Gu HS, Hu YM, You J, Hu ZL, Yuan Y, Zhang TJ. J Appl Phys 2007;101:024319. Wei X, Xu G, Ren ZH, Shen G, Han GR. Mater Lett 2008;62:3719. Xu G, Ren ZH, Du PY. Adv Mater 2005;17:907. Birks LS, Friedman H. J Appl Phys 1946;17:687. Ishikawa K, Yoshikawa K, Okada N. Phys Rev B 1988;37:5852.