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Characterization of crystallite morphology for doped strontium fluoride nanophosphors by TEM and XRD
Physica B ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Characterization of crystallite morphology for doped strontium fluoride nanophosphors by TEM and XRD J.H. O'Connell a, M.E. Lee a,n, M.Y.A. Yagoub b, H.C. Swart b, E. Coetsee b a b
Centre for HRTEM, Nelson Mandela Metropolitan University, PO Box 77000, Port Elizabeth ZA6031, South Africa Department of Physics, University of the Free State, PO Box 339, Bloemfontein ZA9300, South Africa
art ic l e i nf o
a b s t r a c t
Article history: Received 19 May 2015 Received in revised form 28 August 2015 Accepted 10 September 2015
Crystallite morphology for Eu-doped and undoped SrF2 nanophosphors have been determined by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The values for average crystallite size obtained by the application of the Scherrer equation and the full width at half maximum (FWHM) values for XRD peaks are compared to the results obtained using the hollow cone dark field (HCDF) TEM imaging technique. In the case of the TEM analysis, a bimodal crystallite size distribution was revealed with one of the distributions having a measured range of crystallite sizes which was in good agreement with the XRD data. HCDF in combination with FIB specimen preparation was found to be a promising technique for the determination of crystallite size distributions in nanophosphors which might facilitate a better understanding of their scintillation properties. & 2015 Elsevier B.V. All rights reserved.
Keywords: SrF2 Nanophosphors XRD DF TEM
1. Introduction Strontium fluoride (SrF2) is one of the most widely used optical materials due to its optical properties (wide bandgap and low phonon energy) as well as physical properties (low refraction index, high radiation resistance, mechanical strength and low hygroscopicity) [1,2]. Europium doped (optimally 2%) strontium fluoride (SrF2:Eu) nanophosphors have been shown to possess improved scintillation properties [3]. The reduction of crystallite size in a crystalline system can possibly result in modifications of their bulk properties because of the high surface area-to-volume (SA/Vol) ratio and there is a possibility that the nanocrystal size could have an effect on the luminescence lifetime, intensity and emission color [4]. It is therefore extremely important to accurately characterize the crystallite morphologies, size and distribution. Previous results reported for the characterization of the material by X-ray diffraction (XRD), based on the full width at half maxima (FWHM) for the X-ray peaks, produced values for the average dimensions of the nanocrystallites [5]. The line width broadening is sensitive to a number of factors such as the instrumental peak profile, crystallite size distribution, microstrain, solid–solution inhomogeneity and temperature factors [6]. This technique is also limited to a unimodal crystallite size distribution n
Corresponding author. E-mail address: [email protected] (M.E. Lee).
as well as average crystallite sizes below 80 nm. Therefore, due to the limitations of the XRD technique, analysis of exact morphology and orientation for the nanocrystallites requires alternative techniques such as transmission electron microscopy (TEM). Hollow cone dark field (HCDF) imaging in the TEM is able to provide information on crystal sizes for multi-modal crystallite size distributions and has an advantage over standard dark field (DF) imaging in that it can image crystals of different orientation simultaneously [7]. In this paper, it is shown that HCDF imaging in combination with the FIB technique, is a promising complimentary technique to XRD in order to better characterize the crystallite size distribution in Eu-doped and undoped SrF2 nanophosphor particles. Knowledge of the crystallite size distribution will possibly assist with the interpretation of optical performance data from these nanophosphors.
2. Experimental 2.1. Sample preparation Eu doped (1.5–10.0 mol%) and undoped SrF2 phosphor samples were synthesized by the hydrothermal method. For the hydrothermal process, all chemical reagents were of analytical grade and were used without further purification. For a typical synthesis, 1 mmol of Sr(NO3)2 was firstly dissolved in 30 ml distilled water, followed by the addition of 5 mmol of C10H14N2O8.2H2O (Na2EDTA,
http://dx.doi.org/10.1016/j.physb.2015.09.021 0921-4526/& 2015 Elsevier B.V. All rights reserved.
Please cite this article as: J.H. O'Connell, et al., Physica B (2015), http://dx.doi.org/10.1016/j.physb.2015.09.021i
J.H. O'Connell et al. / Physica B ∎ (∎∎∎∎) ∎∎∎–∎∎∎
2
Fig. 1. Schematic diagram showing technique for producing (a) conventional TEM DF imaging and (b) hollow cone TEM DF imaging.
Fig. 2. SEM SE images showing two distinct particle growth morphologies for the SrF2 particles (a) dense particles with flat crystal faces parallel to the surface and (b) large particles consisting of plate like crystals perpendicular to the particle surface with a high concentration of void space between crystals.
2.2. XRD The nanophosphors were characterized by X-ray diffraction (XRD) (Bruker Advance D8 diffractometer using Cu Kα radiation (λ ¼0.154 nm)) to identify the crystal structure and average grain size of the nanocrystallites. The estimated particle size S was calculated using the diffraction peaks and Scherrer's equation [6].
S=
Fig. 3. XRD pattern for (a) SrF2 and (b) SrF2:Eu 3.0 mol% samples. The vertical lines are the standard X-ray lines for SrF2 from the 00-086-2418 card.
Table 1 Calculated particle size of the undoped and Eu doped SrF2 using the Scherrer equation. Eu concentration (mol%)
Crystallite size (nm)
Undoped 1.5 3.0 10.0
7.6 7.3 6.8 4.5
ethylenediamine tetraacetic acid disodium salt) and 2 mmol of NaBF4 under constant stirring conditions for 10 minutes. The solution was transferred into a 125 ml autoclave lined with Teflon and heated at 160 °C for one hour and naturally cooled down to room temperature [8]. The product was collected by centrifugal separation and washed with water and ethanol respectively. Finally, the product was dried for 10 h in an oven at 60 °C. Eu doped SrF2 samples were prepared by the same hydrothermal technique by the addition of Eu(NO3)3.5H2O to the reaction solution described above.
0.9λ β cos θ
where S is the average size of SrF2 particles, λ is the wavelength of the X-rays (0.154 nm) and β is the full-width at half maximum of the X-ray peak at the Bragg angle θ. 2.3. Electron microscopy Scanning electron microscope (SEM) secondary electron (SE) imaging was performed in a JEOL JSM-7001F at 2 kV. Particles were thinned using an FEI Helios 650 Nanolab Focused Ion Beam Scanning Electron Microscope (FIBSEM) to obtain electron transparent sections for TEM analysis. HAADF STEM and HCDF imaging as well as selected area diffraction (SAD) was performed in a JEOL JEM 2100 at 200 kV. Elemental analysis of the doped material was performed in the TEM using an Oxford Instruments™ SDD EDS detector. In centered DF imaging, as shown in Fig. 1(a), the electron beam is tilted in order to center the diffraction spot of interest on the optical axis. The image is formed by excluding all diffracted beams apart from the spot of interest with the objective aperture. In this case, only crystals that diffract the incident beam through the objective aperture appear bright and thus all bright crystals in the image contain planes of similar periodicity and similar orientation (defined by the size of the aperture). In the case of hollow cone DF imaging, the electron beam is precessed to sweep all the spots (or
Please cite this article as: J.H. O'Connell, et al., Physica B (2015), http://dx.doi.org/10.1016/j.physb.2015.09.021i
J.H. O'Connell et al. / Physica B ∎ (∎∎∎∎) ∎∎∎–∎∎∎
3
Fig. 4. (a) HAADF STEM micrograph of SrF2 particles containing nanocrystallites (b) SAD pattern of the particle shown in (a) with a rotationally averaged plot (insert) of the rings showing the major reflections (c) EDS spectrum showing the elemental composition of the Eu-doped SrF2 crystallites.
Fig. 5. (a)–(d) HCDF micrographs for the 111, 200, 220 and 311 diffraction rings respectively.
Please cite this article as: J.H. O'Connell, et al., Physica B (2015), http://dx.doi.org/10.1016/j.physb.2015.09.021i
J.H. O'Connell et al. / Physica B ∎ (∎∎∎∎) ∎∎∎–∎∎∎
4
Fig. 6. (a) Line profile across three grains to obtain and (b) the intensity profile for measuring the grain sizes.
an entire diffraction ring) corresponding to a certain inter-planar spacing (as defined by the precession amplitude) through the objective aperture which is positioned around the optical axis. The result is an image where all crystals possessing planes of similar periodicity near parallel to the optical axis but unrestricted in rotation about the optical axis appear bright. In this work, this process was repeated for all the high intensity diffraction rings of SrF2.
3. Results and discussion 3.1. SEM analysis The morphology of the as-grown SrF2 powder was found to consist of spherical particles having two distinct types as shown by the SEM SE images in Fig. 2(a) and (b). Firstly, dense particles exhibiting flat crystals faces parallel to the spherical surface (Fig. 2 (a)) having a size in the range of 0.8–1.0 μm. Secondly, particles having a surface consisting of flat, plate like crystals, almost perpendicular to the particle surface (Fig. 2(b)) having a size in the range 1.2–2.5 μm. The latter type was of much lower density due to void spaces between the constituent flat crystals as can be seen from the micrograph. The two morphologies could possibly be due to different nucleation conditions and/or inhomogeneous growth conditions in the reaction solution. 3.2. XRD-Scherrer analysis XRD spectra are shown for the doped and undoped material in Fig. 3(a) and (b). Both spectra are in good agreement with the reference lines from the 00-086-2418 card and confirming the face-centered cubic structure with space group: Fm3 m. The average crystallite sizes as determined from the FWHM of the peaks using the Scherrer equation, as a function of doping levels, are shown in Table 1. The crystallite size is clearly affected by high doping concentrations. However, XRD gives no information on the actual distribution of crystallite sizes in the particles. 3.3. TEM analysis In this study 1.5 mol% Eu-doped SrF2 sample was used, as shown in Fig. 2(a) and (b) the as-grown particles were too large to allow for electron beam transmission in the TEM. SAD on this material produced very weak diffraction rings due to excessive
absorption of the primary beam. It was therefore necessary to prepare electron transparent sections of the particles to reveal the crystallites for TEM analysis. Fig. 4(a) shows a high angle annular dark field (HAADF) STEM micrograph of a FIB section of one of the smaller, dense particles. The corresponding SAD pattern is shown in Fig. 4(b) with a rotationally averaged intensity profile showing the major reflections used for HCDF imaging. The EDS spectrum shown in Fig. 4 (c) confirms the elemental composition of the Eu-doped SrF2 crystallites. HCDF micrographs for the 111, 200, 220 and 311 rings, respectively, are shown in Fig. 5(a), (b), (c) and (d). A bimodal crystal size distribution is clearly visible with larger grains of the order of 50–150 nm and smaller grains of the order of 5–10 nm. The average crystallite size for the smaller grain distribution, as obtained by the hollow cone technique, is in agreement with those calculated from the Scherrer equation. The individual crystallite sizes were determined by plotting their intensity profiles as shown in Fig. 6(a) and (b) and using the FWHM of each crystallite as representing its size. The disadvantage of this technique is the low counting statistics and high time consumption due to the requirement of having to analyze a large number of grains.
4. Conclusion Two complementary techniques namely, the XRD-Scherrer method and HCDF TEM imaging were used to determine the crystallite size distribution in SrF2 nanophosphor particles. Since the XRD method has limitations which includes the ability to reveal the presence of a multimodal crystallite size distribution and only provides an estimate for the macroscopic average crystallite size of a unimodal population. HCDF TEM provides valuable information on the grain size distribution, irrespective of crystallite size distribution. The limitations of this technique being the time consuming requirement of a large number of measurements required to provide a meaningful statistical measurement for range of crystallite size as well as the average value. This paper has shown that, HCDF in combination with FIB, to be a promising technique for a better understanding of the crystallite size distribution in nanophosphor particles. This information might therefore facilitate a better understanding of the luminescence properties which could possibly be influenced by the crystallite size which in turn will be a function of the crystal growth parameters.
Please cite this article as: J.H. O'Connell, et al., Physica B (2015), http://dx.doi.org/10.1016/j.physb.2015.09.021i
J.H. O'Connell et al. / Physica B ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Acknowledgments Part of this work is based on the research supported by the South African research Chairs Initiative of the Department of Science and Technology (84415).The financial assistance of the National Research Foundation (NRF), Nelson Mandela Metropolitan University and the Cluster program of the University of the Free State is gratefully acknowledged.
5
[5] M.Y.A. Yagoub, H.C. Swart, L.L. Noto, J.H. O'Connel, M.E. Lee, E. Coetsee., J. Lumin. 156 (2014) 150–156. [6] B.D. Cullity, S.R. Stock, Elements of X-ray Diffraction, 3rd Edition, Pearson, UK (2014), p. 200–209. [7] B. Yao, T. Sun, A. Warren, H. Heinrich, K. Barmak, K.R. Coffey, Micron 41 (2010) 177–182. [8] J.J. Peng, S. Hou, X. Liu, J. Feng, X. Yu, Y. Xing, Z. Su, Mater. Res. Bull. 47 (2012) 328–332.
References [1] K.V. Ivanovskikh, V.A. Pustovarov, M. Kirm, B.V. Shulgin., J. Appl. Spectrosc. 72 (2005) 564–568. [2] B.M. Van der Ende, L. Aarts, A. Meijerink., Adv. Mater. 21 (2009) 3073–3077. [3] S. Kurosawa, Y. Yokota, T. Yanagida, A. Yoshikawa, Phys. Status Solidi C 9 (2012) 2275–2278. [4] S.F. Lim, W.S. Ryu, R.H. Austin., Opt. Exp. 18 (2010) 2309–2316.
Please cite this article as: J.H. O'Connell, et al., Physica B (2015), http://dx.doi.org/10.1016/j.physb.2015.09.021i
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Characterization of crystallite morphology for doped strontium fluoride nanophosphors by TEM and XRD J.H. O'Connell a, M.E. Lee a,n, M.Y.A. Yagoub b, H.C. Swart b, E. Coetsee b a b
Centre for HRTEM, Nelson Mandela Metropolitan University, PO Box 77000, Port Elizabeth ZA6031, South Africa Department of Physics, University of the Free State, PO Box 339, Bloemfontein ZA9300, South Africa
art ic l e i nf o
a b s t r a c t
Article history: Received 19 May 2015 Received in revised form 28 August 2015 Accepted 10 September 2015
Crystallite morphology for Eu-doped and undoped SrF2 nanophosphors have been determined by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The values for average crystallite size obtained by the application of the Scherrer equation and the full width at half maximum (FWHM) values for XRD peaks are compared to the results obtained using the hollow cone dark field (HCDF) TEM imaging technique. In the case of the TEM analysis, a bimodal crystallite size distribution was revealed with one of the distributions having a measured range of crystallite sizes which was in good agreement with the XRD data. HCDF in combination with FIB specimen preparation was found to be a promising technique for the determination of crystallite size distributions in nanophosphors which might facilitate a better understanding of their scintillation properties. & 2015 Elsevier B.V. All rights reserved.
Keywords: SrF2 Nanophosphors XRD DF TEM
1. Introduction Strontium fluoride (SrF2) is one of the most widely used optical materials due to its optical properties (wide bandgap and low phonon energy) as well as physical properties (low refraction index, high radiation resistance, mechanical strength and low hygroscopicity) [1,2]. Europium doped (optimally 2%) strontium fluoride (SrF2:Eu) nanophosphors have been shown to possess improved scintillation properties [3]. The reduction of crystallite size in a crystalline system can possibly result in modifications of their bulk properties because of the high surface area-to-volume (SA/Vol) ratio and there is a possibility that the nanocrystal size could have an effect on the luminescence lifetime, intensity and emission color [4]. It is therefore extremely important to accurately characterize the crystallite morphologies, size and distribution. Previous results reported for the characterization of the material by X-ray diffraction (XRD), based on the full width at half maxima (FWHM) for the X-ray peaks, produced values for the average dimensions of the nanocrystallites [5]. The line width broadening is sensitive to a number of factors such as the instrumental peak profile, crystallite size distribution, microstrain, solid–solution inhomogeneity and temperature factors [6]. This technique is also limited to a unimodal crystallite size distribution n
Corresponding author. E-mail address: [email protected] (M.E. Lee).
as well as average crystallite sizes below 80 nm. Therefore, due to the limitations of the XRD technique, analysis of exact morphology and orientation for the nanocrystallites requires alternative techniques such as transmission electron microscopy (TEM). Hollow cone dark field (HCDF) imaging in the TEM is able to provide information on crystal sizes for multi-modal crystallite size distributions and has an advantage over standard dark field (DF) imaging in that it can image crystals of different orientation simultaneously [7]. In this paper, it is shown that HCDF imaging in combination with the FIB technique, is a promising complimentary technique to XRD in order to better characterize the crystallite size distribution in Eu-doped and undoped SrF2 nanophosphor particles. Knowledge of the crystallite size distribution will possibly assist with the interpretation of optical performance data from these nanophosphors.
2. Experimental 2.1. Sample preparation Eu doped (1.5–10.0 mol%) and undoped SrF2 phosphor samples were synthesized by the hydrothermal method. For the hydrothermal process, all chemical reagents were of analytical grade and were used without further purification. For a typical synthesis, 1 mmol of Sr(NO3)2 was firstly dissolved in 30 ml distilled water, followed by the addition of 5 mmol of C10H14N2O8.2H2O (Na2EDTA,
http://dx.doi.org/10.1016/j.physb.2015.09.021 0921-4526/& 2015 Elsevier B.V. All rights reserved.
Please cite this article as: J.H. O'Connell, et al., Physica B (2015), http://dx.doi.org/10.1016/j.physb.2015.09.021i
J.H. O'Connell et al. / Physica B ∎ (∎∎∎∎) ∎∎∎–∎∎∎
2
Fig. 1. Schematic diagram showing technique for producing (a) conventional TEM DF imaging and (b) hollow cone TEM DF imaging.
Fig. 2. SEM SE images showing two distinct particle growth morphologies for the SrF2 particles (a) dense particles with flat crystal faces parallel to the surface and (b) large particles consisting of plate like crystals perpendicular to the particle surface with a high concentration of void space between crystals.
2.2. XRD The nanophosphors were characterized by X-ray diffraction (XRD) (Bruker Advance D8 diffractometer using Cu Kα radiation (λ ¼0.154 nm)) to identify the crystal structure and average grain size of the nanocrystallites. The estimated particle size S was calculated using the diffraction peaks and Scherrer's equation [6].
S=
Fig. 3. XRD pattern for (a) SrF2 and (b) SrF2:Eu 3.0 mol% samples. The vertical lines are the standard X-ray lines for SrF2 from the 00-086-2418 card.
Table 1 Calculated particle size of the undoped and Eu doped SrF2 using the Scherrer equation. Eu concentration (mol%)
Crystallite size (nm)
Undoped 1.5 3.0 10.0
7.6 7.3 6.8 4.5
ethylenediamine tetraacetic acid disodium salt) and 2 mmol of NaBF4 under constant stirring conditions for 10 minutes. The solution was transferred into a 125 ml autoclave lined with Teflon and heated at 160 °C for one hour and naturally cooled down to room temperature [8]. The product was collected by centrifugal separation and washed with water and ethanol respectively. Finally, the product was dried for 10 h in an oven at 60 °C. Eu doped SrF2 samples were prepared by the same hydrothermal technique by the addition of Eu(NO3)3.5H2O to the reaction solution described above.
0.9λ β cos θ
where S is the average size of SrF2 particles, λ is the wavelength of the X-rays (0.154 nm) and β is the full-width at half maximum of the X-ray peak at the Bragg angle θ. 2.3. Electron microscopy Scanning electron microscope (SEM) secondary electron (SE) imaging was performed in a JEOL JSM-7001F at 2 kV. Particles were thinned using an FEI Helios 650 Nanolab Focused Ion Beam Scanning Electron Microscope (FIBSEM) to obtain electron transparent sections for TEM analysis. HAADF STEM and HCDF imaging as well as selected area diffraction (SAD) was performed in a JEOL JEM 2100 at 200 kV. Elemental analysis of the doped material was performed in the TEM using an Oxford Instruments™ SDD EDS detector. In centered DF imaging, as shown in Fig. 1(a), the electron beam is tilted in order to center the diffraction spot of interest on the optical axis. The image is formed by excluding all diffracted beams apart from the spot of interest with the objective aperture. In this case, only crystals that diffract the incident beam through the objective aperture appear bright and thus all bright crystals in the image contain planes of similar periodicity and similar orientation (defined by the size of the aperture). In the case of hollow cone DF imaging, the electron beam is precessed to sweep all the spots (or
Please cite this article as: J.H. O'Connell, et al., Physica B (2015), http://dx.doi.org/10.1016/j.physb.2015.09.021i
J.H. O'Connell et al. / Physica B ∎ (∎∎∎∎) ∎∎∎–∎∎∎
3
Fig. 4. (a) HAADF STEM micrograph of SrF2 particles containing nanocrystallites (b) SAD pattern of the particle shown in (a) with a rotationally averaged plot (insert) of the rings showing the major reflections (c) EDS spectrum showing the elemental composition of the Eu-doped SrF2 crystallites.
Fig. 5. (a)–(d) HCDF micrographs for the 111, 200, 220 and 311 diffraction rings respectively.
Please cite this article as: J.H. O'Connell, et al., Physica B (2015), http://dx.doi.org/10.1016/j.physb.2015.09.021i
J.H. O'Connell et al. / Physica B ∎ (∎∎∎∎) ∎∎∎–∎∎∎
4
Fig. 6. (a) Line profile across three grains to obtain and (b) the intensity profile for measuring the grain sizes.
an entire diffraction ring) corresponding to a certain inter-planar spacing (as defined by the precession amplitude) through the objective aperture which is positioned around the optical axis. The result is an image where all crystals possessing planes of similar periodicity near parallel to the optical axis but unrestricted in rotation about the optical axis appear bright. In this work, this process was repeated for all the high intensity diffraction rings of SrF2.
3. Results and discussion 3.1. SEM analysis The morphology of the as-grown SrF2 powder was found to consist of spherical particles having two distinct types as shown by the SEM SE images in Fig. 2(a) and (b). Firstly, dense particles exhibiting flat crystals faces parallel to the spherical surface (Fig. 2 (a)) having a size in the range of 0.8–1.0 μm. Secondly, particles having a surface consisting of flat, plate like crystals, almost perpendicular to the particle surface (Fig. 2(b)) having a size in the range 1.2–2.5 μm. The latter type was of much lower density due to void spaces between the constituent flat crystals as can be seen from the micrograph. The two morphologies could possibly be due to different nucleation conditions and/or inhomogeneous growth conditions in the reaction solution. 3.2. XRD-Scherrer analysis XRD spectra are shown for the doped and undoped material in Fig. 3(a) and (b). Both spectra are in good agreement with the reference lines from the 00-086-2418 card and confirming the face-centered cubic structure with space group: Fm3 m. The average crystallite sizes as determined from the FWHM of the peaks using the Scherrer equation, as a function of doping levels, are shown in Table 1. The crystallite size is clearly affected by high doping concentrations. However, XRD gives no information on the actual distribution of crystallite sizes in the particles. 3.3. TEM analysis In this study 1.5 mol% Eu-doped SrF2 sample was used, as shown in Fig. 2(a) and (b) the as-grown particles were too large to allow for electron beam transmission in the TEM. SAD on this material produced very weak diffraction rings due to excessive
absorption of the primary beam. It was therefore necessary to prepare electron transparent sections of the particles to reveal the crystallites for TEM analysis. Fig. 4(a) shows a high angle annular dark field (HAADF) STEM micrograph of a FIB section of one of the smaller, dense particles. The corresponding SAD pattern is shown in Fig. 4(b) with a rotationally averaged intensity profile showing the major reflections used for HCDF imaging. The EDS spectrum shown in Fig. 4 (c) confirms the elemental composition of the Eu-doped SrF2 crystallites. HCDF micrographs for the 111, 200, 220 and 311 rings, respectively, are shown in Fig. 5(a), (b), (c) and (d). A bimodal crystal size distribution is clearly visible with larger grains of the order of 50–150 nm and smaller grains of the order of 5–10 nm. The average crystallite size for the smaller grain distribution, as obtained by the hollow cone technique, is in agreement with those calculated from the Scherrer equation. The individual crystallite sizes were determined by plotting their intensity profiles as shown in Fig. 6(a) and (b) and using the FWHM of each crystallite as representing its size. The disadvantage of this technique is the low counting statistics and high time consumption due to the requirement of having to analyze a large number of grains.
4. Conclusion Two complementary techniques namely, the XRD-Scherrer method and HCDF TEM imaging were used to determine the crystallite size distribution in SrF2 nanophosphor particles. Since the XRD method has limitations which includes the ability to reveal the presence of a multimodal crystallite size distribution and only provides an estimate for the macroscopic average crystallite size of a unimodal population. HCDF TEM provides valuable information on the grain size distribution, irrespective of crystallite size distribution. The limitations of this technique being the time consuming requirement of a large number of measurements required to provide a meaningful statistical measurement for range of crystallite size as well as the average value. This paper has shown that, HCDF in combination with FIB, to be a promising technique for a better understanding of the crystallite size distribution in nanophosphor particles. This information might therefore facilitate a better understanding of the luminescence properties which could possibly be influenced by the crystallite size which in turn will be a function of the crystal growth parameters.
Please cite this article as: J.H. O'Connell, et al., Physica B (2015), http://dx.doi.org/10.1016/j.physb.2015.09.021i
J.H. O'Connell et al. / Physica B ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Acknowledgments Part of this work is based on the research supported by the South African research Chairs Initiative of the Department of Science and Technology (84415).The financial assistance of the National Research Foundation (NRF), Nelson Mandela Metropolitan University and the Cluster program of the University of the Free State is gratefully acknowledged.
5
[5] M.Y.A. Yagoub, H.C. Swart, L.L. Noto, J.H. O'Connel, M.E. Lee, E. Coetsee., J. Lumin. 156 (2014) 150–156. [6] B.D. Cullity, S.R. Stock, Elements of X-ray Diffraction, 3rd Edition, Pearson, UK (2014), p. 200–209. [7] B. Yao, T. Sun, A. Warren, H. Heinrich, K. Barmak, K.R. Coffey, Micron 41 (2010) 177–182. [8] J.J. Peng, S. Hou, X. Liu, J. Feng, X. Yu, Y. Xing, Z. Su, Mater. Res. Bull. 47 (2012) 328–332.
References [1] K.V. Ivanovskikh, V.A. Pustovarov, M. Kirm, B.V. Shulgin., J. Appl. Spectrosc. 72 (2005) 564–568. [2] B.M. Van der Ende, L. Aarts, A. Meijerink., Adv. Mater. 21 (2009) 3073–3077. [3] S. Kurosawa, Y. Yokota, T. Yanagida, A. Yoshikawa, Phys. Status Solidi C 9 (2012) 2275–2278. [4] S.F. Lim, W.S. Ryu, R.H. Austin., Opt. Exp. 18 (2010) 2309–2316.
Please cite this article as: J.H. O'Connell, et al., Physica B (2015), http://dx.doi.org/10.1016/j.physb.2015.09.021i