Influence of Crystal Phase and Excitation Wavelength on

J. Phys. Chem. C 2008, 112, 19283–19292

19283

Influence of Crystal Phase and Excitation Wavelength on Luminescence Properties of Eu3+-Doped Sodium Yttrium Fluoride Nanocrystals Pushpal Ghosh and Amitava Patra* Department of Materials Science and Centre for AdVanced Materials, Indian Association for the CultiVation of Science, Kolkata 700 032, India ReceiVed: August 23, 2008; ReVised Manuscript ReceiVed: October 17, 2008

Here we report the preparation of Eu3+-doped cubic NaYF4 and hexagonal Na(Y1.5Na0.5)F6 nanocrystals and the crystal phase is modified by temperature of heating and varying the Y3+/F- ratio. The volume fraction of the cubic phase decreases from 100 to 55% with increasing the temperature from 80 to 400 °C, and the lattice strain can be modified by changing the crystal phase. The PL intensity, decay time, and quantum efficiency are found to be sensitive to the crystal phase and excitation wavelength. The PL intensity of hexagonal phase sample is stronger than the cubic phase sample under direct excitation and reversal result is obtained under charge transfer (CT) excitation. Under 394 nm excitation, the decay time increases from 5.92 to 7.58 ms with changing the crystal phase from cubic to hexagonal. It is interesting to note that the decay time also varies with changing the excitation wavelength. The quantum efficiency value increases from 4.3 to 67.4% with changing from charge transfer excitation to direct excitation, and this value can be tuned with changing the crystal phase. Analysis suggests that the modification of the quantum efficiencies of the rare-earth doped sodium yttrium fluoride nanocrystals can be done by changing the crystal phase and excitation wavelength. Introduction An enormous interest in nanostructured materials for photonic applications has emerged in recent years. One class of such materials is represented by rare-earth doped nanocrystals that have been investigated as candidates to be used as phosphors in luminescent displays, cold lamps, lasers, amplifiers, upconversion, and imaging of biological systems.1-4 It is well established that in the luminescence of rare-earth ions, the highest phonon frequencies of the host lattice are responsible for nonradiative relaxations.5 In accordance with the energy law, the presence of a large gap between emitting and terminal levels reduces the probability of nonradiative decay. Lower host phonon energy has a greater number of phonons connecting the emitting level with the next lower level. The more phonons will decrease the nonradiative relaxation probability and increase the quantum yield of luminescence. To overcome the phonon decay problem it is necessary to choose a lattice that has lower phonon energy. The fluoride matrix seems to be an ideal medium for preparation of highly luminescent materials because it has low phonon energy compared to oxide hosts.6 Over the past few years, a great deal of attention has been paid on rare-earth doped sodium yttrium fluoride nanocrystals and several groups have already demonstrated on efficient up-conversion luminescence properties of rare-earth doped sodium yttrium fluoride nanocrystals.7-15 From the fundamental point of view, the physical understanding of the luminescence properties of rareearth ions in nanocrystals with changing crystal phase and local structure are very important. As these potential applications are still very much in the design-phase; further fundamental research in this field remains a challenge. It is already reported that the crystal phase, size and concentration play important roles on emission properties of rare-earth doped nanocrystals.16,17 Liu et al.18 have demonstrated the high efficiency of hexagonal phase * To whom correspondence should be addressed. Phone: (91)-33-24734971. Fax: (91)-33-2473-2805. E-mail: [email protected].

than cubic phase in transparent glass ceramics due to multisite character of hexagonal phase. At ambient temperature and pressure, the NaYF4 exists in two polymorphs; the hexagonal structure and the cubic, depending on the synthesis condition. Zachariasen19 proposed the trigonal structure for NaLaF4 crystals. The crystal structure of NaYF4 has been elucidated by Burns.20 The space group is P6 symmetry for hexagonal NaYF4, where Y3+ ions are distributed over two crystallographic sites, namely, 1a and 1f, both sites having C3h point symmetry. Therefore, it is very important to study the effect of the crystal structure on the photoluminescence properties of RE doped sodium yttrium fluoride nanocrystals. As we move toward nanotechnology, it is worthwhile to investigate the role of the wavelength of excitation on the efficiency of rare-earth doped nanocrystals which plays key roles in controlling the efficiency. Rare-earth ion doped nanocrystal can be activated through either by directly exciting the 4f n energy levels or by exciting the charge transfer (CT) band. The opposite-parity excited-state of the RE ion (charge transfer of f n-1d) and same parity excited-state of the RE ion (direct transition of 4f n) appeared to be important factors for tuning the efficiency of the materials. During charge-transfer excitation of Eu3+-ions, a fast radiationless process, that is, the transition occurs from the charge-transfer state to the excited levels of the 5D0, then the 5D0 level can decay radiatively to the 7FJ levels. However, in direct excitation, the emission occurs from the excited levels of the 5D0 level to 7FJ levels. In oxide nanocrystal, RE ions in the vicinity of the oxygen impurities display the RE3+-O2- charge transfer (CT) transition resulting in a broad absorption band in the UV region. In comparison with RE3+-O2- charge transfer (CT) transition in oxide system, the corresponding RE3+-F- charge transfer (CT) transition in fluoride crystals is located at a much higher energy, that is, at 150 nm. It is already reported that F- ions are substituted by OH- or O2- in fluoride crystals.17

10.1021/jp807539r CCC: $40.75  2008 American Chemical Society Published on Web 11/14/2008

19284 J. Phys. Chem. C, Vol. 112, No. 49, 2008 Considering the best of our knowledge, the influence of crystal phase and excitation wavelength on the luminescence properties of Eu3+-doped sodium yttrium fluoride nanocrystals is not clear. A very broad range of unprecedented optical properties can be observed by changing the crystal phase and excitation wavelength. In the present study, we are addressing the following issues: how the quantum efficiency depends on crystal phase and the wavelength of excitation; and how they influence the quantum efficiency, emission intensity, and decay time of Eu3+ ions in sodium yttrium fluoride nanocrystals. Of particular interest to our research program is how the luminescence properties vary with crystal phase and excitation wavelength with the hope that such knowledge will enable us to construct efficient nanomaterials for photonic and biophotonic applications.

Ghosh and Patra

Figure 1. X-ray powder diffraction patterns of 1.0 mol % Eu3+-doped sodium yttrium fluoride nanocrystals at different temperatures prepared by microemulsion.

Experimental Section Several researchers have used various techniques such as cothermolysis,8 thermal decomposition, liquid-solid two-phase approaches,14 and solid state synthesis route12b for preparing the sodium yttrium fluoride nanocrystal. In this present study, we used a microemulsion-based wet chemical method and a hydrothermal method. Microemulsion Method. Two sets of nanoreactors were prepared through water-in-oil (w/o) type emulsion with sodium bis-(2-ethylhexyl) sulfosuccinate (AOT, Loba Chemie) and isooctane as the anionic surfactant and organic liquid phase, respectively. AOT (400 mM, 8.891 g), 44.6 mL of isooctane (Merck), and 1.9 cc of water were mixed carefully to prepare the microemulsion. A 0.425 M solution of Y(NO3)3 · 6H2O (Indian Rare Earth Ltd.), 0.425 M NaCl (Merck), and the required amount of europium nitrate (for 1 mol % Eu2O3, Aldrich) were taken in 3.5 mL of water and stirred for a few minutes. This solution mixture was then added to one set of microemulsion and stirred for 30 min. The amount of water was adjusted to keep the water to surfactant ratio (w/o) at 15. A 1.7 M NH4F solution (Merck) was added to the other set of microemulsions and stirred for 30 min. The fluoride containing microemulsion was then added to the previous emulsion and stirred for 1 h. In this preparation technique, Y3+/F- ratio and pH of the solution were maintained at 1:4 and 6:7, respectively. Finally, a white precipitate was obtained. Particles were then collected by centrifugation (6000 rpm), then the particles were washed twice with acetone and methanol and dried at 80 °C for 12 h in a vacuum oven and the crystalline particles were collected. Finally, samples are heated at 400, 600, and 720 °C for 1 h, with the rate of heating at 12 °C/ minute. Hydrothermal Method. For complex assisted hydrothermal method, Span80 or Sorbitan monooleate (Fluka) was used as template.21a A total of 30.0 mL of 1-butanol (Merck), 20 mL of water, and 4.5 cc of Span 80 were mixed and stirred for 10 min. This solution was then divided into two parts. In one part, a solution mixture of 0.42 M of Y(NO3)3 · 6H2O (Indian Rare Earth Ltd.), 0.42 M of NaCl (Merck), and the required amount of europium nitrate (for 1 mol % Eu2O3, Aldrich) were added and stirred for 10 min. A 1.7 M solution of NH4F was added to another part of the solution and stirred for 5 min. Finally, the two solutions were mixed and this mixture was put into a Teflonlined stainless steel autoclave where the temperature was maintained at 100 °C for 2 h. In this preparation technique, Y3+/ F- ratio and pH were maintained at 1:4 and 6:7, respectively. Finally a white precipitate was obtained. Particles were then washed, collected, and heated at 600 °C to get the desired crystal phase. Another sample was prepared by a similar

method at fixed Y3+/F- ratio (1:8) and fluoride concentration 3.4 (M). The particles were then washed, centrifuged, and heated at 600 °C to get the desired crystal phase. Transmission electron microscopy (TEM, JEOL Model 200) was used to study the morphology and particle size of the resulting powders. The crystalline phases of annealed powders were identified by X-ray diffraction (XRD) using a Siefert XRD 3000 P. The crystallite sizes of the nanocrystals were calculated following the Scherrer’s equation

D ) Kλ/β cos θ

(1)

where K ) 0.9, D represents crystallite size (Å), λ is the wavelength of Cu KR radiation, and β is the corrected halfwidth of the diffraction peak. The volume fraction of the cubic and hexagonal (χh) and (χc) phases were estimated from the integrated peak intensity of the (100)h, (110)h, and (101)h planes of the hexagonal phase and the (111)c plane of the cubic phase using the following equation

χc)Ic(111)/[Ih(100) + Ih(110) + Ic(111) + Ih(101)] (2) The excitation and emission spectra and decay times were recorded in Fluro MaX-P (Horiba Jobin Yvon) spectrometer, using solid sample holder at room temperature. The experimental photoluminescence quantum yield of the samples was measured using Integrating Sphere F-3018 (Horiba Jobin Yvon). For this purpose, desirable amount of Eu3+-doped NaYF4 nanoparticles were dispersed in a small amount of ethanol under ultrasonic agitation. Then this clear solution was added to ethanolic solution of PVP (Polyvinyl pyrrolidone), maintaining NaYF4/ PVP as 1:1, and stirred for 2 h. A homogeneous solution was used for quantum yield measurement. Thermogravimetric and differential thermal analysis (TG and DTA) curves were obtained from SDT (Quanta chrome), with the heating rate at 12 °C/min, using N2 gas passing. Results and Discussion Structural Investigations. Figure 1 depicts the XRD pattern of 1 mol % Eu3+-doped sodium yttrium fluoride nanocrystals heated at 80, 400, 600, and 720 °C. It is clearly seen from Figure 1 that the crystal phase can be modified by changing the heating temperature. The phase composition, lattice parameters, and crystallite size as a function of heating temperature are summarized in Table 1. Figure 1a shows the XRD pattern of 80 °C dried 1 mol % Eu3+-doped NaYF4 nanocrystals. The strong peaks at 28.100° (111), 32.533° (200), and 46.782° (220) are due to cubic NaYF4 nanocrystals (JCPDS Card No- 6-342, SG Fm3m, Z ) 2), indicating the formation of 100% cubic phase.

Eu3+-Doped Sodium Yttrium Fluoride Nanocrystals

J. Phys. Chem. C, Vol. 112, No. 49, 2008 19285

TABLE 1: Phase Composition, Crystallite Sizes, and Cell Parameters of Na(Y1.5Na0.5)F6 and NaYF4 Nanocrystals

temperature (°C)

condition micro-emulsion Y3+/F- ) 1:4

80 400 600 720 600 600

hydrothermal Y3+/F- ) 1:4 hydrothermal Y3+/F- ) 1:8

crystal phase

crystallite size (nm) (Scherrer’s equation)

cubic cubic (55%) hexagonal (45%) hexagonal (90%); cubic (10%) cubic (80%); hexagonal (20%) cubic hexagonal

10.2 19.6 26.0 27.7 31.2 24.0 23.7

Using Scherrer’s equation, the calculated crystallite size is 10.2 nm, considering 100 intensity (111) plane. Figure 1b shows the XRD pattern of 400 °C heated 1.0 mol % Eu3+-doped sodium yttrium fluoride nanocrystals. The peaks at 17.045° (100), 29.815° (110), 30.687° (101), and 43.350° (201) are due to hexagonal phase (JCPDS Card No. 16-334, (SG P63/m, Z ) 1). Therefore, a mixture of hexagonal and cubic phases is observed at 400 °C heated sample. The volume fraction of the cubic phase (χc) decreases from 100 to 55% with increasing the temperature from 80 to 400 °C. Figure 1c depicts the XRD pattern of 600 °C heated sample. All the peaks are well indexed with hexagonal Na(Y1.5Na0.5)F6 except the peaks at 28.552° (111) and 47.519° (220), which are for cubic NaYF4. Here, we obtained 90% hexagonal phase and 10% cubic phase. It is interesting to note that the volume fraction of cubic phase increases to 80% when the sample heated at 720 °C (Figure 1d). Considering the 100 intensity peak, the calculated crystallite sizes are 19.6, 27.7, and 31.2 nm for 400, 600, and 720 °C heated samples, respectively. Particle size increases with increasing the temperature of heating, which is consistent with previous work.7 In the present study, we found the phase transformation with changing the temperature of heating, which is given below:

) ) ) ) ) ) )

cell volume (Å3)

5.4913 5.5054 5.9860; c ) 3.5146 5.9805; a ) 3.5170 5.4224 5.5157 5.9828c ) 3.5160

165.587 166.867 109.067 108.938 159.431 167.810 108.996

lattice strain -3.52% +0.14% -0.71% -0.45% +0.24% +0.02% -0.65%

720°C

hex(45%) 98 hex(90%) 98 cubic(80%) The transition from cubic to hexagonal phase [Fm3m f P63/ m] is a disorder to order phase transition with respect to cation.6,15 Yi et al.7 also reported an exothermic peak around 460 °C during the phase transformation from cubic to hexagonal phase. Sobolev et al.21b showed a transformation from hexagonal to cubic phase at ∼973 K. In the present study, the phase transformation process is confirmed by DTA and TGA studies (Supporting Information, S1). The strong exothermic peaks at 460 and 720 °C clearly confirm above phase transformation. Here, we present the control of the crystal phase as a function of Y3+/F- ratio. Figure 2 presents the XRD pattern of 600 °C heated 1 mol % Eu3+-doped cubic (Figure 2a) and hexagonal (Figure 2b) nanocrystal prepared by hydrothermal synthesis. It is found that 96% cubic NaYF4 and 94% hexagonal Na(Y1.5Na0.5)F6 nanocrystals were prepared at 1:4 and 1:8 Y3+/ F- ratio, respectively. The volume fraction of cubic and hexagonal crystal phase was calculated using eq 2. The peaks at 28.02° (111), 32.438° (200), 46.62° (220), and 55.21° (311) correspond to the cubic (NaYF4) phase (JCPDS File No. 6-342) and the peaks at 16.93° (100), 29.88° (110), 30.75° (101), 43.60° (201), and 53.53° (211) are due to the hexagonal Na(Y1.5Na0.5)F6 nanocrystal (JCPDS File No. 16-334). The cell parameters and

(compressive) (tensile) (compressive) (compressive) (tensile) (tensile) (compressive)

lattice volumes are listed in Table 1. Considering the (111) and (201) plane of cubic and hexagonal nanocrystals, the calculated crystallite sizes are 23.9 and 23.7 nm for cubic and hexagonal phases, respectively. It is well-known that the hexagonal phase is a thermodynamically stable and ordered phase due to the presence of different types (three types) of cationic sites with high coordination numbers. According to Burns,20 two-thirds of the Y3+ ions occupy one kind of site, one-third of the Na+ ions are randomly mixed in a second site with Y3+ ions and the remaining Na+ ions are distributed with vacancies on a third kind of site. For the cubic sodium yttrium fluoride, the fluorite type structure was established by Hund,22 where Ca2+ ionic sites are randomly occupied by Na+ and Y3+ ions. It is also established that the cationic sites are accommodated by a small portion of rare-earth ions. 23 With increasing the fluoride ion concentrations (1:8), the hexagonal phase is formed due to decrease the energy barrier because Y3+ or other cationic sites are conveniently coordinated by F- ions.16,24 Thus, it is expected that the crystal field symmetries of Eu3+ ions are different due to different crystal phases. Generally, the broadenings of the diffraction peaks depends upon strain and particle size. We calculate the strain using Williamson and Hall theorem25

β cos θ/λ ) 1/D + η sin θ/λ

400°C

cubic(100%) 98 cubic(55%) + 600°C

a a a a a a a

cell parameter (Å)

(3)

where β is the full width at half-maximum (fwhm), θ is the diffraction angle, λ is the X-ray wavelength, D is the effective particle size, and η is the effective strain. The strain is calculated from the slope and the crystallite size (D) is calculated from the intercept of a plot of β cos θ/λ against sin θ/λ. Figure 3a shows the plot of β cos θ/λ against sin θ/λ for 80 °C dried 1 mol % Eu3+-doped NaYF4 prepared by microemulsion method. The slope value is -0.035, indicating the presence of compres-

Figure 2. X-ray powder diffraction patterns of 1.0 mol % Eu3+ doped NaYF4 and Na(Y1.5Na0.5)F6 nanocrystals at 600 °C, prepared by hydrothermal method.

19286 J. Phys. Chem. C, Vol. 112, No. 49, 2008

Ghosh and Patra

Figure 3. Plot of β cos θ/λ against sin θ/λ for sodium yttrium fluoride sample prepared by microemulsion at 80 (a), 400 (b), 600 (c), 720 (d), and 600 °C heated hexagonal sample prepared by hydrothermal method (e) and 600 °C heated cubic sample prepared by hydrothermal method (f).

Figure 4. TEM micrograph (a), HRTEM (b), and FFT pattern (c) of 80 °C dried Eu-doped NaYF4 nanocrystal.

sive strain (-3.5%). The calculated crystallite size is 10.3 nm, which is in good agreement with the value obtained from Scherrer equation. Similarly, the compressive strain was obtained for small sized (7.6 nm) TiO2 nanoparticle.26 Figure 3b shows the plot for 400 °C heated sample prepared by microemulsion method where both cubic and hexagonal phases are present. The tensile strain (+0.14%) and compressive strain (-0.71%) are obtained on considering the cubic and hexagonal phases, respectively. The calculated crystallite sizes from the intercept for the cubic and hexagonal are 22 and 24 nm, respectively which matched well with the value obtained from Scherrer equation. Figure 3c presents the plot for the 600 °C heated sample prepared by the same method. XRD (Figure 1c) reveals the 90% hexagonal phase for this sample. Here, a negative slope (-0.0045) suggests the compressive strain (-0.45%) and the obtained crystallite size is 28 nm. However, the tensile strain (+0.24%) is obtained for 720 °C heated 1.0 mol %Eu3+-doped NaYF4 sample (Figure 3d), where 80% cubic phase was present. Again, we measured the lattice strain for 600 °C heated 96% cubic and 94% hexagonal sodium yttrium fluoride nanocrystals prepared by the hydrothermal method. Figure 3e shows the plot for 600 °C heated hexagonal Na(Y1.5

Na0.5)F6 sample prepared by hydrothermal method keeping Y3+/ F- ratio at 1:8. For the cubic sample, the Y3+/ F- ratio is 1:4 (Figure 3f). The compressive strain (-0.65%) and the tensile (+0.026%) strain are obtained for hexagonal and cubic sodium yttrium fluoride samples, respectively. Analysis reveals that the compressive strain is always obtained for the hexagonal phase and tensile strain is always obtained for the cubic phase, except at low crystallite size. Thus, the lattice strain can be modified with varying the crystal phase. Kang et al.27 also reported the compressive strain (-12.6%) for AlxGa1-x N thin film during transition from Wurtzite (W)-to-zinc blend (ZB). Results indicate that there is a strong relationship between lattice strain and crystal phase and the lattice strain may play an important role on modification of the optical properties of the rare-earth doped nanocrystals. Figure 4a shows the low magnification transmission electron microscope image of 80 °C dried Eu3+-doped NaYF4 nanocrystal prepared by microemulsion method. The average size of the particles is 9 nm, which is in good agreement with the results obtained from XRD and Williamson Hall plot. Figure 4b shows the high resolution image (HRTEM) of the nanocrystals and the measured lattice spacing is 2.72 Å, which corresponds to



Figure 5. TEM micrograph (a), HRTEM (b), and FFT pattern of region A of HRTEM (c) and FFT pattern of region B (d) of 400 °C heated Eu-doped NaYF4 nanocrystal.

Figure 6. TEM micrograph (a), HRTEM (b), and FFT pattern (c) of 600 °C heated Eu-doped Na(Y1.5 Na0.5)F6 nanocrystal.

the (200) plane of the cubic NaYF4. Figure 4c shows the fast Fourier transformation pattern (FFT) of a sample from a selected area of HRTEM and this diffraction pattern also confirms the presence of the cubic (200) plane. Figure 5a and b represent the low magnification TEM and HRTEM images of 400 °C heated sodium yttrium fluoride sample. The average particle size is 22 nm which is in good agreement with the value obtained from XRD. The measured lattice spacing of region A of the HRTEM image is 2.72 Å, which corresponds to the cubic (200) plane of sodium yttrium fluoride. The measured lattice spacing of region B is 2.89 Å, which corresponds to the (101) plane of the hexagonal phase. The fast Fourier transformation pattern (FFT) obtained from the selected area of the region A also confirms the cubic (200) plane (Figure 5c). The FFT pattern obtained from the selected area of region B confirms C (200),

H (100), and H (101) planes (Figure 5d). It is already seen from XRD analysis that a mixture of cubic and hexagonal phases is obtained in NaYF4 nanocrystals heated at 400 °C. It is seen from HRTEM image (Figure 5b) that cubic (200) plane of region A is slightly bending and converts to the H(101) plane, indicated by arrowhead. Similar results were reported for Wurtzite and Zinc Blende layers within CdSe nanocrystals.28,29 Penn et al.30 also showed the twin boundaries during anatase to rutile phase transformation of TiO2 nanoparticle. The energy difference between the cubic (200) plane and the hexagonal (101) plane is small, which may be the driving force for phase transformation from cubic to hexagonal NaYF4. Figure 6a and b represent the low magnification TEM and HRTEM images of 600 °C heated sodium yttrium fluoride nanocrystals. The measured lattice spacing is 5.05 Å, which corresponds to the (100) plane


Ghosh and Patra

Figure 7. TEM micrograph (a) and HRTEM (b) of 720 °C heated Eu-doped NaYF4 nanocrystal.

of hexagonal Na (Y1.5Na0.5) F6 nanocrystals (Figure 6b). The FFT pattern also confirms the presence of the (100) plane of hexagonal phase (Figure 6c), which matches with XRD results. Figure 7a represents the low magnification TEM images of 720 °C heated sodium yttrium fluoride and the average crystallite size is 32 nm which is in good agreement with the size obtained from XRD and Williamson plot. From the HRTEM image (Figure 7b), the spacing between lattice planes is 3.23 Å, which corresponds to the cubic (111) plane. The FFT pattern again confirms the presence of the cubic (111) plane, which matches with XRD data. Luminescence Studies. Figure 8 shows the excitation and emission spectra of different temperatures heated 1.0 mol % Eu3+-doped sodium yttrium fluoride nanocrystals. The peak in the range 225-300 nm is associated with charge transfer (CT) transition from 2p orbital of O2- to the incomplete 4f orbital of Eu3+ ions. This CT band is due to Eu3+-O2-, not for Eu3+-Fbecause Eu3+-F- CT band is located at higher energy, that is,

Figure 8. CT excitation (a) and emission spectra (b) of 400, 600, and 720 °C heated sodium yttrium fluoride nanocrystals prepared by microemulsion. CT excitation (c) and emission spectra (d) of 600 °C heated cubic and hexagonal sodium yttrium fluoride nanocrystals prepared by the hydrothermal method.

around 150 nm. Haase et al.17 already mentioned that the CT band is due to Eu3+-O2- in NaGdF4/Eu3+ nanoparticles. The excitation maximum varies from 252 to 259 nm with changing the phase composition (Supporting Information, S2). However, the excitation bands are 252 and 256 nm for hexagonal (94%) and cubic (96%) phase samples, respectively (Figure 8c), where the crystal phase is different but the average particle size is same because both samples were prepared at 600 °C (Table 1). Therefore, the charge transfer band obviously depends on the crystal phase. According to Blasse and Grabmier,31 the shifting toward the lower energy is due to increasing covalency. The excitation intensity increases with changing the crystal phase, which may be due to the coupling of the Eu3+ ions with the lattice anions.32 The position of the charge transfer (CT) band in Eu3+-doped crystals can be predicted by calculating the environmental factor (he). According to Li and Zhang, 33,34 CT energy (Ect) decreases with the increase of the environmental factor (he). Taking the empirical formulas, Ect ) A + Be-khe (where A ) 2.804, B ) 6.924, and K ) 1.256 for the Eu3+ ion), the calculated environmental factors (he) are 0.95 and 0.99 for hexagonal and cubic crystal phase, respectively. Higher value of environmental factor suggests the increasing covalency in cubic phase again. Lattice strain may be another factor for shifting of band. Aumer et al.35 reported the red shifting of emission peak energy by 236 meV with changing the stress from -0. 86% (compressive) to +0.25% (tensile) in In0.08Ga0.92N quantum well (QW). In the present study, we have seen that the CT energy of the hexagonal sodium yttrium fluoride is blueshifted by 120 meV (6 nm) than the cubic phase with changing the lattice strain from +0.241% (tensile) to -0.446% (compressive). Figure 8b shows the emission spectra under excitation at their corresponding CT bands. The prominent emission bands are at 610, 628, and 592 nm. Generally, the emission spectra are attributed to the 5D0 f 7FJ (J ) 0-2) transition of the Eu3+ ions, that is, the band at 570 f 603 nm and the band at 603-640 nm are related to transition of 5D0 f 7F1 and 5D0 f 7F2, respectively. In europium, the 5D0 f 7F1 (592 nm) transition is mainly magnetically allowed (a magnetic-dipole transition), while 5D0 f 7F2 (610) is a hypersensitive forced electric-dipole transition being allowed only at low symmetries with no inversion center. The UV radiation is absorbed by the chargetransfer band of the Eu3+ ion. The ion then nonradiatively decays to lower 4fn levels, with the majority of the luminescence occurring from the 5D0 state of the Eu3+ ion, and then the 5D0 level can decay radiatively to the 7FJ levels. It is interesting to



Figure 9. (A) Excitation (a) and emission (b) spectra of 80, 400, 600, and 720 °C heated sodium yttrium fluoride nanocrystals prepared by microemulsion and (B) excitation (c) and emission (d) spectra of 600 °C heated cubic and hexagonal sodium yttrium fluoride nanocrystals prepared by hydrothermal method.

note that the emission intensity is higher in the cubic phase than hexagonal phase under charge transfer (CT) band excitation. Figure 8d shows the emission spectra of different phases under CT excitation. It is again seen that cubic nanocrystals show higher emission intensity (8×) than hexagonal nanocrystals under CT excitation. The possible explanation is the chargecompensating mechanism that influences the luminescence due to association of the charge compensator with the luminescence center. Structurally, there are three cationic sites in the hexagonal sodium yttrium fluoride where one site is occupied by Y3+, another site is occupied by 1/2Na+ and 1/2Y3+, and the third site is occupied by 1/2Na+ and vacancies. Dopant ions (RE) are usually accommodated into these sites and RE ions are always in close proximity of charge compensator vacancies or cation in hexagonal phase. However, Na+ and Ln3+ ions are randomly distributed in the cationic sublattice in the NaYF4 cubic phase, which lacks charge compensator. Blasse et al.36 already reported the better quantum efficiency of the Eu3+ ion without charge compensator in CaO system. It is worth mentioning that the photoluminescence properties are different during direct excitation. In direct excitation, the emission occurs from the excited levels of the 5D0 to 7FJ levels. Figure 9a and b depict the excitation and photoluminescence spectra of different temperatures heated 1 mol % Eu3+-doped sodium yttrium fluoride nanocrystals at 394 nm, that is, by direct excitation. The emission intensity increases with increasing the temperature from 80 (cubic) to 600 °C (hexagonal) and then sharply decreases when the temperature reaches to 720 °C (cubic). It reveals that the emission intensity is higher in the hexagonal phase than the cubic phase, which is opposite to CT excitation. Several researchers7,15,16 already demonstrated the higher efficiency in hexagonal sodium yttrium fluoride nanocrystals than the cubic phase due to the presence of three cationic sites inside the hexagonal phase and only one site for cubic crystal phase. Figure 9c and d depict the excitation and photoluminescence spectra of 600 °C heated 1 mol % Eu3+doped hexagonal Na(Y1.5 Na0.5)F6 and cubic NaYF4 samples prepared by hydrothermal method. Here, the PL intensity at the

Figure 10. Schematic energy level diagram and corresponding transition of Eu3+ ions.

hexagonal phase has an approximately six times stronger emission than the cubic phase sample. The multisite character and low energy phonon mode of hexagonal sample are the main reason for its better efficiency when excited by 394 nm light.15,16 Results suggest that the photoluminescence properties of these materials can be tuned by changing the excitation wavelength. Figure 10 shows the schematic energy level diagram of Eu3+ ions in sodium yttrium fluoride nanocrystals. During direct excitation (step 1), the lowest ground state (7F0) excited to 5L6 level and then nonradiative multiphonon relaxation (step 4) to


Ghosh and Patra observed which suggests the decay time arises from one source. In this case, the obtained decay times are 7.58 and 5.92 ms for hexagonal and cubic phase, respectively, which agrees well with emission spectra. The decay times for other samples are listed in Table 2. Analysis suggests that crystal phase and excitation wavelength control the emission intensity and decay time of Eu-doped sodium yttrium fluoride nanocrystals. Judd-Ofelt Parameters and Quantum Efficiency. Judd-Ofelt parameters are calculated to get more insight into the structural changes surrounding the Eu3+ ion due to change of crystal phases. The Judd-Ofelt parameter (Ω2) gives information on the intensities or nature of the hypersensitive transitions of the Eu3+ ion.37,38 The experimental intensity parameters (Ω2) were determined from the emission spectra for Eu3+ ion (Figures 8 and 9) based on the 5D0 f 7F2 electric dipole transition and the 5D0 f 7F1 magnetic dipole transitions as the reference and they are estimated according to eq 439

Figure 11. Normalized photoluminescence (PL) decays of 600 °C heated 1 mol % Eu3+-doped cubic and hexagonal sodium yttrium fluoride nanoparticles prepared by hydrothermal method and monitored at the (a) 5D0 f 7F2 transition (607 nm) and 250/256 nm excitation and (b) 5D0 f 7F2 transition (617 nm) and 394 nm excitation.

A)

the lowest excited state (5D0). Finally, it radiatively decays to 7 Fj levels (denoted as step 5). In case of CT excitation, the ground-state of Eu3+ is excited through Eu-O charge transfer which is represented as step 2. Then, the excited electrons can reach the ground states either by direct transition from the excited-state to the ground-state or they may go to the lower excited states of Eu3+ by nonradiative transitions (step3). The direct transition is forbidden because of very weak oscillator strength.4g Therefore, with the multiphonon relaxation (step 4), the excited photons reach the lower excited states (5D0). Finally, it gives the radiative transition, 5D0 f 7Fj (j ) 0-6) (step 5). Figure 11a shows the normalized PL decays of 600 °C heated cubic and hexagonal sodium yttrium fluoride nanocrystals, monitored at the 5D0 f 7F2 transition (607nm) and λex ) 250/256 nm. The average decay times for the cubic and hexagonal sodium yttrium fluoride nanocrystals are 4.40 and 3.34 ms, respectively which matches with the emission spectra. Biexponential decay is observed in both of these cases. The average decay time is calculated using ) a1τ1 + a2τ2, where a1 and a2 are the percentage of the decay components. For cubic nanocrystals, the decay components are τf ) 3.26 ms (81%) and τs ) 10.03 ms (19%), whereas the decay components are τf )2.42 ms (88%) and τs ) 10.02 ms (12%) for hexagonal. The fast component can be assigned to energy transfer from CT to Eu3+ ion and the slow component is due to 5D0 f 7F2 transition. Results suggest that energy transfer process in cubic phase is higher than hexagonal phase. Thus, the decay time increases in cubic phase under CT excitation. Figure 11b shows the normalized PL decays of 600 °C heated 1 mol % doped cubic and hexagonal sodium yttrium fluoride nanocrystals, monitored at the 5D0 f 7F2 transition (615 nm) under direct excitation at 394 nm. Here, the single component decay is

4e2ω3 1 χ 3pc3 2J + 1

∑ Ω2〈5D0|U(2)|7F2〉2

(4)

where A0λ is the coefficient of spontaneous emission, e is the electronic charge, ω is the angular frequency of the transition, p is Plank’s constant, c is the velocity of light, χ is the Lorentz local field correction and is expressed as χ ) η(η2 + 2)2/9, where η is the refractive index of the sample, which is experimentally determined, and 〈5D0|U(2)|7F 2〉2 is the squared reduced matrix elements whose value is independent of the chemical environment of the ion and is 0.0039 for J ) 2.40 Because the magnetic dipole 5D0 f 7F1 transition is relatively insensitive to the chemical environment around the Eu3+ ion, it can therefore be considered as a reference for the whole spectrum, and the coefficient of spontaneous emission is calculated according to the relation41

A0J ) A01(I0J /I01)(λ01/λ0J)

(5)

where γ01 and γ0J are the energy baricenters of the 5D0 f 7F1 and 5D0 f 7F2 transitions, respectively. A01 is the Einstein’s coefficient between 5D0 f 7F1 levels, and it is calculated using A01 ) η3(A0-1)vac; where η is the refractive index of the sample and (A0-1)vac ) 14.65 s-1. Radiative (Arad) and nonradiative (Anrad) transition and average decay time are related through the following equation

Atot ) 1/τ ) Arad + Anrad

(6)

where Arad can be expressed as39

Arad ) A01

γ01 2 I0J ) I01 J-0 γ0J

∑

∑ A0J

(7)

J

As quantum efficiency is expressed as the ratio between the number of photons emitted by the Eu3+ ion and the number of photons absorbed by the Eu3+ ion and it is a balance between

TABLE 2: Decay Time, Judd-Ofelt Parameter (Ω2), and Luminescence Quantum Efficiency (η%) of Na(Y1.5Na0.5)F6 and NaYF4 Nanocrystals η% λex ) 394 nm sample

decay time (ms) λex ) 394 nm

calcd

expt

Ω2 10-20 cm2 λex ) 394 nm

microemulsion (80 °C) microemulsion (400 °C) microemulsion (600 °C) microemulsion (720 °C) hydrothermal (600 °C; H) hydrothermal (600 °C; C)

5.60 5.74 7.54 7.16 7.58 5.92

17.6 21.4 64.3 22.5 67.2 15.1

23.0 31.7 65.9 42 67.4 26

5.03 5.17 6.47 5.05 7.06 4.23

decay time (ms) λex ) 252/258 nm NP NP τ) τ) τ) τ)

2.17 2.35 3.34 4.40

(τf (τf (τf (τf

) ) ) )

1.92; τs ) 7.02) 2.2; τs ) 6.48) 2.42; τs ) 10.02) 3.26; τs ) 10.03)

η% λex ) 252/ Ω2 λex ) 252/ 258 nm, expt 258 nm NP 2 5.6 7.2 4.3 7.8

NP 11.85 23.00 25.00 10.74 13.70



radiative and nonradiative process, quantum efficiency can be expressed as

η)

Arad Arad + Anrad

(8)

J-O parameters (Ω2) for the sodium yttrium fluoride samples are calculated by the above explained method. The values of the J-O parameter (Ω2) are 7.06 × 10-20 cm2 and 4.23 × 10-20 cm2 for the hexagonal and cubic samples under excitation at 394 nm. The larger Ω2 value for the hexagonal sample suggests that the Eu3+ ion resides at a more asymmetric environment in the hexagonal phase than the cubic phase, which is consistent with emission spectra and lifetime data. The calculated quantum efficiencies are 67.2 and 15.1% for hexagonal and cubic samples under excitation at 394 nm, and the measured quantum efficiencies are 67.4 and 26% for hexagonal and cubic phases, respectively. Haase et al.42 reported 70% photoluminescence quantum yield for green emitting CePO4/Tb/LaPO4 core-shell nanoparticle. Veggel et al.4b also reported maximum 54% quantum yield by surface modification. The measured J-O parameters for samples prepared by the microemulsion method are given in Table 2. Ω2 values are 5.026 × 10-20, 5.17 × 10-20, 6.47 × 10-20, and 5.049 × 10-20 cm2 for 80, 400, 600, and 720 °C heated samples, respectively. Here, the maximum Ω2 value is obtained for the hexagonal phase (600 °C), which matches with emission spectra and decay time results. The J-O value for CT excitation is much higher than direct excitation (Table 2). The higher intensity ratio between electric dipole and magnetic dipole is the main reason for the higher J-O value for the CT excitation. The quantum efficiencies for all these samples are listed in Table 2. The quantum efficiencies of these samples are much lower at CT excitation than direct excitation. It is to be mentioned that we use only experimental quantum yield for CT excitation because we found an error in calculating the efficiency using eq 4 as the refractive index value of hexagonal (1.37) is always higher than the cubic phase (1.02). To avoid the error, we compare the experimental efficiency values for this purpose. Analysis suggests that the crystal phase and excitation wavelength play an important role on tuning the quantum efficiency, emission intensity, and decay time of Eu3+doped sodium yttrium fluoride nanocrystals. Conclusions In conclusion, solution based techniques are promising routes for the synthesis of Eu-doped sodium yttrium fluoride nanocrystals and the crystal phase can be tuned by changing the heating temperature and Y3+/F- ratio. It is also found that photoluminescence properties of Eu3+ ions are sensitive to the crystal structure and wavelength of excitation. The emission intensity of the peak at 614 nm (5D0 f 7F2) for Eu3+ ions for hexagonal nanocrystals is higher than the cubic sample under direct excitation, and reversal results are obtained under charge transfer excitation. The decay time changes from 7.58 to 5.92 ms with changing the crystal phase from hexagonal to cubic under direct excitation. However, the decay time changes from 7.58 to 3.34 ms with changing the excitation wavelength from direct to charge transfer at hexagonal crystal phase. The larger Ω2 value for hexagonal sample suggests that Eu3+ ion resides at more asymmetric environment in hexagonal phase than cubic phase which is consistent with emission spectra and lifetime data. The quantum efficiency value can be tuned with changing the crystal phase and wavelength of excitation. The crystal phase and wavelength of excitation are found to introduce significant

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Influence of Crystal Phase and Excitation Wavelength on

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