J. Phys. Chem. C 2007, 111, 12137-12138
Reply to “Comment on ‘Judd-Ofelt Intensity Parameters and Spectral Properties of Gd2O3: Eu3+ Nanocrystals’”
12137
changes of fluorescence patterns or energy level positions”. The Ωλ intensity parameters for J-J′ transitions can be estimated theoretically from structural data or determined experimentally from absorption or emission spectra. In ref 1, we analyzed the spectral area data at room temperature using the 5D0-7F2 and 5D -7F transitions of the Eu3+ ion to determine experimentally 0 4 the Ωl (λ ) 2 and 4) intensity parameters by taking the 5D07F transition as the reference. 1 When the particles are much smaller than the light wavelength, the local electric field acting on the Eu3+ is determined by the combined effects of the Gd2O3 medium contained within the nanoparticle and that of the medium filling the voids. By ref 2, it is necessary to introduce an effective index of refraction for the medium, neff, which consists of the nanoparticles surrounded by the media with refractive index nmed
Chunxu Liu* and Junye Liu Key Laboratory of Excited State Processes, Chinese Academy of Sciences, Changchun Institute of Optics and Fine Mechanics and Physics, Chinese Academy of Sciences Changchun 130033
Kai Dou College of Medicine, UniVersity of Kentucky, Lexington, Kentucky 40536 ReceiVed: April 4, 2007; In Final Form: June 8, 2007
neff(x) ) x‚nGd2O3 + (1 - x)‚nmed
Through the use of selective excitation spectra, three nonequivalent Cs centers (A, B, and C) in monoclinic phase and C2 center in cubic phase were investigated in the nanocrystalline Gd2O3:Eu3+. The Judd-Ofelt intensity parameters Ωλ (λ ) 2, 4) for nanoparticles Gd2O3:Eu3+ were determined experimentally. It is indicated that the parameters Ωλ changed dramatically with the sizes of Gd2O3:Eu3+ from nanoparticles to bulk material. By decreasing the diameters of Gd2O3:Eu3+ from 135 to 15 nm, the quantum efficiencies of emitting level 5D0 reduced from 23.6 to 4.6% due to the increasing ratio of surface to volume.1 In using the single term Sellmeier equation (n2 - 1)-1 ) (-A)/λ2 + B in ref 1, we should choose A ) 75 × 10-16 m2 and B ) 0.3644 (for cubic Gd2O3), not A ) 62 × 10-16 m2 and B ) 0.3163 (for monoclinic Gd2O3), as pointed out in the comment. Then, the intensity parameters Ω2 and Ω4 for samples with different sizes have been calculated again, the original and new are listed in Table 1. The Ω2’s ratio is 10.41:6.51:9.83 ) 160%:1:151% and Ω4’s ratio is 6.46:4.70:7.29 ) 137%:1:155% for samples 4, 5, and 6, respectively. It is well known that the unfilled 4f subshell of rare earth ions is shielded by outer filled 5s2 and 5p6 subshells. Spectroscopic properties of lanthanide ions embedded in a host lattice are well described by the crystal field theory. The 4f-4f emission spectra of rare earth ions are the sensitive probes for the crystal field circumstances (CFC) surrounding them. The varying CFC of samples (sizes 15-135 nm) are responsible for the significant changes of emission peak numbers and widths, shown in Figure 5 in ref 1. “If the changes of JO parameters were ascribed to the size-induced modification of CF environment, then significant changes of fluorescence patterns or energy level positions would also be experimentally observed”. We do not know what is the standard of “significant
(1)
where x is the ‘‘filling factor” showing what fraction of space is occupied by the Gd2O3 nanoparticles. In our case, if we choose x ) 0.99 (usually the samples are compact), nmed ) 1 (in air), and neff ≈ nGd2O3, then the correction is not significant for us. The other point in the comment is that “Besides, also in Table 1 of ref 1, the measured fluorescence lifetimes were 0.066, 0.11, and 0.33 ms for nanocrystals of 15, 23, and 135 nm, respectively, which differ significantly from those values listed in Table 2 of ref 1. Liu et al. did not provide any reasonable explanation for this substantial discrepancy”. First, “the measured fluorescence lifetimes 0.066, 0.11, and 0.33 ms” cannot be found in Table 1 in ref 1, but 1.43, 2.04, and 1.41 ms are included in this Table 1 in ref 1. Second, these lifetime are not “the measured fluorescence lifetimes”, but are the 5D0 radiative lifetimes calculated by
S0-λ σλ S0-1 σ1
(4 in ref 1)
∑J A0-J
(9 in ref 1)
A0-λ ) A0-1 and AR )
AR is the radiative decay rate of 5D0 radiative rates obtained by summing over the radiative decay rates A0-J for each 5D0-7FJ (J ) 1, 2, 3, and 4) radiative transition. The measured fluorescence lifetimes of 5D0-7FJ (J ) 0, 1, 2, 3, and 4) were listed in Table 2. Therefore, the lifetime data listed in Table 1 and Table 2 are completely different things, and the substantial
TABLE 1. sample 4 (15 nm) 5D
mon cub a
0
-7F
5D
2 a
sample 5 (23 nm) 7 0- F4
5D
-7F
0
sample 6 (135 nm)
5D -7F 0 4
2
5D
-7F
0
5D
2
7 0- F4
n
Ω2
n
Ω4
n
Ω2
n
Ω4
n
Ω2
n
Ω4
2.165 1.950
8.18 10.41
2.147 1.933
5.00 6.46
2.166 1.949
5.13 6.51
2.146 1.932
3.63 4.70
2.166 1.950
7.74 9.83
2.147 1.933
5.66 7.29
Intensity parameters × 10-20.
* To whom correspondence should be addressed. Fax: +86-4316176337. E-mail:
[email protected].
discrepancy cannot be cancelled by any reasonable explanation. In conclusion, the Judd-Ofelt intensity parameters Ωλ (λ ) 2, 4) for nanoparticles Gd2O3:Eu3+ were determined experimentally
10.1021/jp072666m CCC: $37.00 © 2007 American Chemical Society Published on Web 07/25/2007
12138 J. Phys. Chem. C, Vol. 111, No. 32, 2007 using mainly the areas of J-J’ transitions but not data of position and splits of energy levels. Acknowledgment. The support of the Natural Science Foundation of China under project Grant 60308008 and 10274083 is gratefully acknowledged.
Comments References and Notes (1) Liu, C. X.; Liu, J. Y.; Dou, K. J. Phys. Chem. B 2006, 110, 20277. (2) Meltzer, R. S.; Feofilov, S. P.; Tissue, B.; Yuan, H. B. Phys. ReV. B 1999, 20, R14012.
