The site-selective excitation and the dynamical electron–lattice interaction
on the luminescence of YBO
3
: Sb
3 þ
Lei Chen
a,f,n, An-Qi Luo
a, Yao Zhang
a, Xin-Hui Chen
a, Hao Liu
a, Yang Jiang
a,1, Shi-Fu Chen
b,2,
Kuo-Ju Chen
c, Hao-Chung Kuo
c, Ye Tao
d, Guo-Bin Zhang
ea
School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, China b
Department of Chemistry, Huaibei Normal University, Huaibei 235000, China c
Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, China d
Department of Photonic & Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan eNational Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230026, China
f
Semiconductor and Optoelectronic Technology Engineering Research Center of Anhui Province, Wuhu 241000, China
a r t i c l e
i n f o
Article history:
Received 19 December 2012 Received in revised form 19 February 2013 Accepted 24 February 2013 Available online 7 March 2013 Keywords: Phosphor Luminescence Site-selective excitation Electron–lattice interaction YBO3: Sb3 þ
a b s t r a c t
The phosphor of YBO3: Sb3 þwas synthesized by solid state reaction. Its crystal structure was examined
by using X-ray diffraction analysis. The morphology and particle size were characterized with a scanning electron microscopy. The properties of YBO3: Sb3 þluminescence was systematically studied
by exciting borate host, the1P
1and3P1levels of Sb3 þat the Ciand C1sites with 168, 203, 218, 252 and
262 nm, respectively, measured on the synchrotron radiation instruments. The different configurations of emission spectra excited with variant wavelengths show the character of wavelength-selective excitation, which was attributed to different occupations of the Sb3 þsite in the YBO
3crystal lattice.
Moreover, strong electron–lattice interaction (i.e., the Jahn–Teller effect) between Sb3 þand YBO
3host
was discriminated by comparing low temperature and room temperature spectra. The site-selective luminescence was confirmed by YBO3 activated with multiple Sb3 þ concentrations. The spectral
assignment was verified with fluorescence lifetime achieved by using the Time-Correlated Single-Photon Counting method.
&2013 Elsevier Inc. All rights reserved.
1. Introduction
The luminescence of Sb3 þ, which originates from ns2–nsnp
transition for its ns2 configuration, has a large cross section to
capture excitation energy and then transfer to other activators. Thus, Sb3 þ was promisingly served as either an activator or a
sensitizer in phosphors[1–5]. Typically, Sb3 þ was utilized as a
sensitizer for the activator of Mn2 þ in warm white phosphor of
3Ca3(PO4)2Ca(F,Cl)2:Sb3 þ,Mn2 þ for traditional fluorescent lamp
[4,5]. Accordingly, the spectroscopic properties of Sb3 þ ion have
attracted wide attention[6–13].
Although several theoretical and experimental researches had addressed the luminescent properties of Sb3 þ, there were still
some controversies over the assignment of Sb3 þ excited states
and the case of electron transitions[6–13]. For example, the three
excitation bands of Cs2NaScCl6which consists of a doublet around
275,000 cm1, a weaker band at about 31,600 cm1and a triplet around 34,700 cm1, were attributed to 1S
0-3P1,1S0-3P2 and
1S
0-1P1transitions, respectively; and the corresponding singlet
emission band was designed to 3P
1-1S0 transition [6]. Three
excitation bands with maxima at about 250 nm, 230 nm (weak) and 205 nm were attributed to 1S
0-3P1,1S0-3P2 and 1S0-1P1
transitions, while the emission bands peaked at 415 nm and 520 nm were attributed to the isolated Sb3 þ and the clusters of Sb3 þ in oxychloride phase of LaOCl, respectively [7]. As for
Ln(PO3)3:Sb3 þ (Ln ¼Sc, Lu, Gd), the doublet shape of the
excita-tion spectrum of Ln(PO3)3:Sb3 þ(Ln ¼Sc, Lu, Gd) was attributed to
the influence of different sites (not to the Jahn–Teller effect), but that was ascribed to the superposition of the Jahn–Teller effect and sites effect in Y(PO3)3: Sb3 þ [8].
Thanks to the recent development of 3-dimension television (3D TV) technology, the research on plasma display panel (PDP) catched the eye once again. A phosphor which could emit efficiently under vacuum ultraviolet (VUV, 100–200 nm) excita-tion was desired for improvement of PDPs efficiency. Moreover, a material that can convert 172 nm photons into ultraviolet or visible light was still desired for space particle detector for a long Contents lists available atSciVerse ScienceDirect
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Journal of Solid State Chemistry
0022-4596/$ - see front matter & 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jssc.2013.02.034
n
Corresponding author at: School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, China. Fax: þ 86 551 62901362.
E-mail addresses: shanggan2009@qq.com (L. Chen), apjiang@hfut.edu.cn (Y. Jiang), chshifu@chnu.edu.cn (S.-F. Chen).
1
Fax: þ86 551 62904358. 2Fax: þ86 561 3806611.
time, because the vacuum ultraviolet photomultiplier tube that was used to detect the 172 nm photons radiated by xenon discharge was very costly [14–16]. YBO3 was a promising host
for these applications due to its high UV (200–400 nm) transpar-ency, strong absorption in the VUV wavelength range, exceptional optical damage threshold, easy synthesis, good maintenance, and well chemical inert properties[17–19].
Earlier in 1986, the luminescence of YBO3: Sb3 þ had been
investigated by Oomen et al. [20]. In his research, one broad emission band which consisted of two nearly overlapping bands and each with its own excitation spectrum was observed, but the short-limit wavelength of excitation spectrum was measured only to 250 nm[20]. The experiments implemented under UV excita-tion were unable to reveal the full-scale properties of Sb3 þ,
because the host absorption band of YBO3 and the 1S0-1P1
transition of Sb3 þ may lie beyond the spectral range of UV spectrometer. Correspondingly, some interesting phenomena or some important mechanisms were ignored. For example, the strongly dynamical electron–lattice interaction between host and activator which had been observed in YPO4 [12]and Ca(S,
Se)[21]and Cs2NaMCl6(M¼ Sc, Y, La)[9], but it was not observed
in YBO3previously reported by Oomen et al.[20]. To obtain highly
efficient luminescence, nevertheless, the effect of instantaneous polarization field which was caused by the thermal dynamical vibration of cations and polyanion groups must be suppressed. For which, the mechanism must be identified first of all. So, it is necessary to investigate the properties of Sb3 þ luminescence
under VUV excitation. For these reasons, the photoluminescence of YBO3: Sb3 þ was further investigated in this work. The
phos-phor of YBO3: Sb3 þ was synthesized with solid-state reaction
method and its micro-structure was examined by using X-ray diffraction analysis and scanning electron microscopy. The lumi-nescence properties were characterized by using a fluorescence spectrometer and the synchrotron radiation instruments. Besides the site-selective luminescence of Sb3 þ in YBO
3, the dynamical
electron–lattice interaction between the activator of Sb3 þand the
host of YBO3was confirmed.
2. Experimental
Samples were synthesized from Y2O3 (99.9%), Sb2O3 (99.9%)
and H3BO3(99.5%) by a solid state reaction as following processes.
Firstly, a stoichiometric mixture of the raw materials with a 5% excess H3BO3 was thoroughly grounded and pre-fired at 800 1C
for 120 min. Then, the power was grounded for a next time and sintered at 1150 1C for 240 min in the air. After cooled down to room temperature with finance, the fired product was grounded and washed with 80–100 1C de-ionized water for several times to remove excess B2O3. Finally, it was dried at 120 1C.
Crystal structure of sample was identified by using an X-ray diffraction (XRD, Rigaku, D/Max-rB) with CuK
a
(l
¼1.5408 ˚A) radiation. Particles size and morphology were characterized with a scanning electron microscope (SEM, JEOL, JSM-6490LV). The UV excitation and emission spectra were collected with a Hitachi F-4500 fluorescence spectrometer at room temperature. The room-temperature VUV/UV excitation and emission spectra were measured at the National Synchrotron Radiation Laboratory (NSRL) VUV spectroscopy workstation on the U24 beam line. The workstation is equipped with a Seya–Namioka excitation monochromator (1200 g/mm, 100–400 nm), an ACS-257 emission monochromator (1200 g/mm, 200–700 nm), and a Hamamatsu H8259-01 photomultiplier detector. The cryogenic VUV/UV exci-tation and emission spectra were recorded at the VUV spectrum experimental station on the beam line 4B8 of Beijing Synchrotron Radiation Facilities (BSRF). A cryostat head with sample holderwas installed on a manipulator (MB1504; McAllister), used to adjust, optimize and restore the position of the sample. The fluorescence was collected and focused by two lenses and projected onto the entrance slit of a fluorescence monochromator (SP308, Acton), which holds three gratings covering the range from 190 to 1700 nm with spectral resolution of 0.2 nm. The fluorescence signals were detected by the photon counting heads (H6241, Hamamatsu), and the output pulses were input into the counter module (974, ORTEC). More parameters for this work-station please refer to Ref.[22]. During measurement, the electron energy in storage ring was kept at 800 MeV for NSRL and 2.5 GeV for BSRF. The beam current was in the range of 150–250 mA for NSRL and 235–265 mA for BSRF, respectively. The pressure in sample chamber was kept about 103Pa. All excitation spectra
were calibrated with the sodium salicylate. The lifetime of luminescence was measured with the single photon account method by using the FLUOROLOG-3-TAU Time-Correlated Sin-gle-Photon Counting Spectrofluorometer.
3. Results
Although numerous studies have been carried out about the crystal structure of YBO3, there were a larger amount of
con-troversies over them[23–33]. One recent revision was carried out by Chadeyron by adopting single-crystal techniques combining with11B nuclear magnetic resonance (NMR) and infrared
spectro-scopy (IR) studies[23]. Chadeyron attributed the crystal structure of the YBO3to the space group P63/m[23]. As for the method of
X-ray diffraction, however, it was not easy to identify the space group. Recently, Lin’s study, using neutron diffraction, shows the space group C2/c[24]. Nevertheless, either Chadeyron’s research or Lin’s work, they both approbated that there are two kinds of Y3 þ site and Y3 þ is 8-coordinated with oxygen atoms. Later,
Tanner used the low-temperature high-resolution emission spec-troscopy to probe the local site symmetries of Eu3 þions accom-modated on the Y3 þsites in YBO
3: Eu3 þ(0.5 at%)[25]. The use of
point group selection rules enabled a consistent spectral inter-pretation by envisaging distinct Ciand C1symmetry Eu3 þsites, in
accordance with the neutron diffraction study of Lin’s[24,25].
Fig. 1presents the XRD pattern of the as-prepared sample of Y0.92BO3: Sb3 þ0.08. By comparing the XRD patterns with JCPDS:
88-0356 (the P63/m space group) revised by Chadeyron et al.[23]in 1997 by using the x-ray diffraction of the YBO3single crystal and
10 20 30 40 50 60 70 * * * * * + + + + + + + + + + +
Diffraction intensity, a.u.
2θ, degree +: JCPDS 88-0356 *: JCPDS 01-073-7388 + * * * * * * * * * *
JCPDS: 01-073-7388 (the C2/c space group) resolved by Lin et al.[24]
in 2004 using neutron diffraction method, the diffraction peaks shown inFig. 1 is more consistent with the later one. Lin’s work indicated that the low-temperature YBO3crystallizes in a C-centered
monoclinic cell with C2/c space symmetry, the unit cell parameters being a¼11.3138(3) ˚A, b¼6.5403(2) ˚A, c¼9.5499(2) ˚A, and
b
¼112.902(1)1 [24]. Fig. 2 shows that phosphor particles have a potato-like profile with average size about 2–5m
m. Figs. 1 and 2demonstrated that the sample of YBO3: Sb3 þ was well
synthesized here.
Fig. 3 presents the emission spectra of (Y0.92Sb0.08)BO3 by
exciting with 230, 258 and 270 nm at room temperature mea-sured with a Hitachi F-4500 spectrophotometer, in which one broad emission band with peak around 410 nm is observed. Typically, the asymmetric configuration of the emission spectra indicates that the broadband consists of more than one peak. This conclusion is demonstrated with cryogenic spectra inFig. 4.Fig. 4
plots the emission spectra by exciting with 168, 203, 218, 225, 252, and 262 nm test on the beam line of 4B8 of BSRF synchrotron instrument at 14 K. The asymmetric configuration is observed in all spectra, but the emission spectrum excited with 262 nm
clearly shows that the emission band comprises of two peaks: one at about 410 nm and the other at approximate 470 nm.
Fig. 5presents the emission spectra of (Y0.92Sb0.08)BO3under
168, 204, 215, 250, and 260 nm excitation measured on the U24 beam line of the NSRL synchrotron instrument at room tempera-ture. Although the emission spectra inFig. 5 differ a little from those in Fig. 3, the asymmetric configuration is the common character of them and they are all measured at room temperature. One possible explanation for this difference is caused by the resolution or sensibility of spectrometers. However, the config-urations of the emission spectra measured at low temperature in
Fig. 4 are completely different from those collected at room temperature shown in Fig. 5. The significant difference of the emission spectra between measured at room temperature and collected at low temperature shows the temperature-dependent luminescence of YBO3: Sb3 þ.
Moreover, the wavelength-selective excitation of YBO3: Sb3 þ
luminescence is highlighted by comparing the emission spectrum upon 252 nm with 262 nm excitation, as displayed in the right hand side of Fig. 4.Fig. 4demonstrates that the emission band peaked at about 410 nm is sensitive to the excitation of 168, 203, 218, 225 and 252 excitation while the emission band peaked at about 470 nm is sensitive to the excitation of 262 nm. The doublet structure could be discriminated clearly from the emission upon 168, 203, 218, 225, 252, and 262 nm excitation at 14 K. However,
Fig. 2. Particle size and morphology of the YBO3: Sb3 þphosphor.
320 400 480 560 640 0 3000 6000 9000 Ci
Emission intensity, a.u.
Wavelength, nm 230 258 270 3P 1 1S0 C1
Fig. 3. The emission spectra of (Y0.92Sb0.08)BO3by exciting with 230, 258 and 270 nm collected with the Hitachi F-4500 spectrophotometer at room temperature. 350 400 450 500 550 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 168nm ex 203nm ex 218nm ex 225nm ex
Emission intensity, a.u.
Wavelength nm 350 400 450 500 550 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 Ci Ci C1 252nm ex 262nm ex Wavelength nm C1
Fig. 4. The emission spectra of (Y0.92Sb0.08)BO3by exciting with 168, 203, 218, 225, 252 and 262 nm measured on the BSRF synchrotron instrument at 14 K.
300 350 400 450 500 550 600 0 3000 6000 9000 C1 Ci
Emission intensity, a.u.
Wavelength, nm λex=168 nm λex=204 nm λex=215 nm λex=250 nm λex=260 nm 3P 1 1S0
Fig. 5. The emission spectra of (Y0.92Sb0.08)BO3by exciting with 168, 204, 215, 250 and 260 nm test on the NSRL synchrotron instrument at room temperature.
it is difficult to determine the precise position of the doublet peaks inFig. 5, besides the dominant one at about 400 nm. This difference indicates that the wavelength-selective excitation is also temperature-dependent.
From the doublet emission peaks inFig. 4, we can conclude that all emission spectra presented in Figs. 3 and 5 should comprise two peaks as disclosed by their assymetric configura-tion. Thus, the emission spectra could be deconvoluted by fitting with two Gaussian Functions. Fig. 3 presents one of the fitted spectra upon the emission excited with 230 nm. The others are omitted.
Fig. 6 presents the excitation spectra of (Y0.92Sb0.08)BO3 by
motoring the emission at 407/460 nm collected with Hitachi F-4500 spectrometer at room temperature, in which two absorp-tion bands with peak at about 231/249 and 272/285 nm are observed, respectively. In contrast to the excitation bands upon 407 nm, the two excitation bands upon 460 nm red-shift about 18 nm and 13 nm, respectively.
The excitation spectra of (Y0.92Sb0.08)BO3by motoring 407 and
460 nm measured on the NSRL instrument at room temperature are shown inFig. 7. Quadruplet at 202, 211, 217 and 222 nm in
the band region of 190–240 nm and a doublet at 250 and 264 nm in the band region of 240–280 nm are observed in the excitation spectra upon 407 nm emission, while triplet peaks at 200, 214 and 226 nm in the band region of 190–240 nm and a singlet at 259 nm in the band region of 240–280 nm are presented by monitoring the emission of 460 nm. Besides configuration, the relative intensity of the excitation peaks differs from each other. Moreover, as far as the same temperature and the same monitor-ing wavelength conditions are considered, the excitation spectra inFig. 6differentiate significantly from those inFig. 7.
Fig. 8 presents the excitation spectra by monitoring the emission at 407 or 460 nm measured on the BSRF instrument at 14 K. By comparingFig. 7 withFig. 8at the same condition of monitoring wavelength, either 407 or 460 nm, the differences caused by temperature could be discriminated clearly. Firstly, the quadruple peaks at 202, 212, 218 and 225 nm in the band region of 190–240 nm upon 407 nm emission were clearly exhibited in
Fig. 8, but they are obscure inFig. 7, especially for the minor peak at 217 nm. Secondly, the splits of the triplet peaks at 200, 216 and 228 nm upon 460 nm emission at 14 K are much larger than those at room temperature. Thirdly, the doublet structure of the excitation band in the region of 240–280 nm at 14 K inFig. 8is more clearly displayed than inFig. 7. These characters demon-strate the strongly temperature-dependent luminescence of YBO3: Sb3 þ.
4. Discussion
The ground state of Sb3 þ for with [Xe]5s2electron
configura-tion is1S
0, whereas the excited state originating from 5s5p orbit
gives rise to four levels, namely3P
0,3P1,3P2, and1P1. The Mulliken
notation for these levels become1A
1g(
G
1þ) ground state and thethree excited states, 3T
1u (
G
1,G
4,G
3 andG
5), 1T1u, and theperturbed exciton (
G
4,)0.4 13The1T1uand3T1uexcited states arewell described by molecular orbits (MOs), a1gand t1u[4–13]. The
transition1S
0-1P1 is allowed, whose corresponding absorption
band is denoted as the C band; the transition1S0-3P1is partially
allowed by spin–orbit coupling (A band); the transition1S
0-3P2
is spin-forbidden (B band), but can be induced by lattice vibra-tions[4–13]. Correspondingly, the intensity of the B band usually is very weak. The transition 1S
0-3P0 is strictly forbidden for
D
J¼0, in which the total angular moment does not change. In order to identify the appropriate electron transitions through the assignment of fine structures in excitation and emission spectra of Sb3 þ, the site of Sb3 þ in crystal lattice has 220 240 260 280 300 320 340 360 380 0 3000 6000 9000 C1 C1 Ci 285 272 249 λem= 407 nm λem= 460 nmExcitation intensity, a.u.
Wavelength, nm 231 1S 0 1P1 1S 0 3P1 Ci
Fig. 6. The excitation spectra of (Y0.92Sb0.08)BO3by motoring 407 and 460 nm emission collected with the Hitachi F-4500 spectrophotometer at room temperature. 160 200 240 280 0.0 0.2 0.4 0.6 0.8 Ci C1 Ci 250 264 214 200 226 222 217
Excitation intensity, a.u.
Wavelength, nm λem = 407 nm λem = 460 nm 1S 0 1P1 Borate Host 1S 0 3P1 202 211 C1
Fig. 7. The excitation spectra of (Y0.92Sb0.08)BO3by motoring 407 and 460 nm emission measured on the NSRL synchrotron instrument at room temperature.
140 160 180 200 220 240 260 280 300 0.00 0.05 0.10 0.15 0.20 0.25 0.30 Ci Ci C1 212 225 200 1S 0 1P1
Excitation intensity, a.u.
Wavelength, nm λem = 407 nm λem = 460 nm Borate Host 1S 0 3P1 216 228 218 202 251 264 C1
Fig. 8. The excitation spectra of (Y0.92Sb0.08)BO3by motoring 407 and 460 nm emission measured on the BSRF synchrotron instrument at 14 K.
to be considered. By virtue of the same valence and similar ionic radius (Sb3 þ: 0.76, and Y3 þ: 0.89), as discussed above, the doped
Sb3 þions should occupy the sites Y3 þions in the crystal lattice of YBO3.
The nearest neighbor coordination environments of Y3 þ in
YBO3at the Ciand C1sites please refer toFig. 6in Ref.[24]and
Fig. 1in Ref.[25]. The eightfold Y3 þin C
isite is surrounded with
O1(2), O2(2), O3(2) and O4(2), while the C1site is coordinated with
O1(2), O2(2), O3(2), O4(1) and O5(1). The distances of the
nearest-neighbor oxygen atoms for which the mean values at the C1and Ci
sites are 235.6 and 238 pm, respectively[24,25]. According to the relationship between crystal field strength and ionic bond length, the parameter Dqwas represented as follows[34]:
Dq¼
3ze2r4
5R5 ð1Þ
where Dqpresents the crystal field stabilization energy (CFSE), R
the bond length between a central ion and ligand ions, r the mean size of a central ion, and Z the charge of a central ion. A shorter bond length implies the stronger crystal field strength and the larger split of excited states. Correspondingly, the center of the gravity of the excited state is greatly reduced due to the nephelauxetic effect, and the long wavelength emission occurs for the larger Stoke shift. Therefore, a reasonable explanation for the emission spectra inFigs. 3–5are all attributed to the3P
1-1S0
transition of Sb3 þ, the band peaked at about 410 nm is attributed
to Sb3 þ at the C
isite, and the band peaked at about 470 nm is
attributed to Sb3 þat the C
1site. Correspondingly, the excitation
bands peaked at 231 and 272 nm upon 407 nm inFig. 6could be ascribed to the1S
0-1P1and1S0-3P1transitions of Sb3 þat the Ci
site; and the excitation bands peaked at 249 and 285 nm upon 460 nm inFig. 6could be assigned to the1S
0-1P1and1S0-3P1
transitions of Sb3 þ at the C1site, respectively.
The diffuse-reflection spectra of YBO3and (Y0.92Sb0.08)BO3are
presented in Fig. 9. Compared with pure YBO3 host, a broad
absorption band in the range of 260–400 nm with an edge about 299 nm is observed in the diffuse-reflection spectrum of (Y0.92Sb0.08)BO3. This absorption band must originate from the
Sb3 þ ions, and the position of this band corresponds to the
excitation band in the region of 240–340 nm in Fig. 6. So, the absorption is attributed to the transition of 1S
0-3P1 of Sb3 þ.
Restricted to the spectrometer, the absorption of high-energy levels which has been beyond the available range of the instru-ment is not observed fromFig. 9.
As revealed by the spectra of the Eu3 þ/Tb3 þ activated
(Y,Gd)BO3system, the absorption band of borate host is
indepen-dent of activators. Therefore, the excitation band in the region 150–190 nm with a peak at about 168 nm inFigs. 7 and 8can be attributed to the absorption of the host band (HB) [17–19,35]. However, it is difficult to assign the fine peaks, i.e., the doublets at about 251 and 264 nm in the region of 240–280 nm upon 407 nm, the singlet at about 259 nm in the region of 240–280 nm upon 460 nm, the quadruplets at about 202, 211, 217 and 222 nm in the region of 190–240 nm upon 407 nm, and the triplets at about 200, 216 and 228 nm upon 460 nm, in Figs. 7 and 8. More interestingly, what does the mechanism correspond for the formation of the fine structures in excitation and how to explain them?
One possible explanation for the fine peaks is ascribed to the effect of site-selective excitation. The excitation band peaked at about 202 (200) and 251 nm could be attributed to the1S
0-1P1
and1S
0-3P1transitions of the Cisite of Sb3 þ, respectively. The
others, including the triplets at 212, 218 and 225 nm upon 407 nm and the doublets at 216 and 228 nm upon 407 nm, in the region of 190–240 nm and the band peaked at about 262 nm in the region of 240–280 nm is attributed the 1S
0-1P1 and
1S
0-3P1transitions of Sb3 þat the C1site, respectively, as marked
in Figs. 7 and 8. Moreover, the triplets at 211, 217 and 222 nm could be interpreted as crystal field effect on the band splitting of C1 for its trigonal symmetry, besides considering as electron
vibration peaks. However, the presence of the1S
0-3P1absorption
of Sb3 þ at C
1 site (peaked at 264 nm) in the excitation by
monitoring 407 nm of the 3P1-1S0 emission of Sb3 þ at Cisite
cannot be interpreted by this mechanism, because electrons usually cannot transfer from a low energy state to a higher level automatically. If it happened, it must involve the phonon-coupling interaction. Moreover, the doublet splitting at 216 and 228 nm of C1site upon 460 nm emission could not be explained
by the site-selective luminescence for its C1symmetry.
The site-selective luminescence of Bi3 þ, which has the same
ns2 electronic configuration as Sb3 þ, in the YBO
3 host was
demonstrated by exciting/monitoring with different wavelengths at the low temperature in previous researches [36,37]. As for YBO3: Bi3 þ, the two emission bands peaked at 294 and 330 nm
occur simultaneously by exciting Cisite with 247 nm; however,
the emission band peaked at 294 nm does not show up by exciting C1 site with 265 nm. Meanwhile, the excitation band
peaked at 247 nm (Cisite) appears in the excitation spectra by
monitoring both 330 (C1 site) and 294 (Ci site) nm emissions,
while the band peaked at 265 nm (C1site) does not occur to the
excitation spectrum by monitoring 294 nm (Cisite) emission. The
energy transfer between the symmetric and asymmetric sites of Bi3 þions in YBO
3was demonstrated at 22 K[36,37]. The results
show that energy can be transferred from the1P
1and3P1level of
the Cisite to the1P1and3P1levels of the C1site, respectively, but
can hardly be transferred from the1P
1 (having less proportion)
and3P
1(almost not) levels of the C1site to the1P1and3P1levels
of the Cisite. For the same reason, if the doublets inFigs. 7 and 8
are caused by Sb3 þ occupying different sites, the doublet emis-sion peaks at 407 and 460 nm should occur simultaneously by exciting3P
1level of Cisite with 252 nm, and should only the band
peaked at about 460 nm appear by exciting 3P1 level of C1site
with 262 nm; likewise, the dominant excitation band peaked at about 264 nm and the subordinate band peaked at 251 nm should both occur by monitoring the emission of C1site at 460 nm, and
only the band peaked at 251 nm be observed by monitoring the emission of Cisite at 407 nm. Practically, the results shown in
Figs. 7 and 8are not consistent with these conclusions.
Another explanation is attributed to the superposition of the site-selective excitation and the dynamical electron–lattice interaction
300 400 500 600 700 75 90 105 Relative reflection, % Wavelength, nm (Y0.92Sb0.08)BO3 YBO3 1S 0 3P1
Fig. 9. The diffuse-reflection spectra of the pure host of YBO3and the phosphor of (Y0.92Sb0.08)BO3.
(i.e. the Jahn–Teller effect) [6–13]. The Jahn–Teller split strongly depends on temperature. The relationship of the Jahn–Teller split with temperature is expressed by Jacobs and Oyama[38]as following equation:
d
ðTÞ ¼d
ð0Þ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi coth_o
ef f 2KT r ð2Þ whered
(0) is the Jahn–Teller splitting at T¼0 K andoeff
is an average frequency over all modes involved. According to the formula (2), the Jahn–Teller split should increase with temperature. The intensity of luminescence usually decreases while the full width at half maximum (FWHM) increases with temperature[6,8,9]. The middle of the triplet peaks at 217 nm in the band region of 190–240 nm inFigs. 7 and 8clearly shows this character. Due to the linear and quadratic
interaction with the
Eg
and T2gwhere lattice vibrations, the A bandsplits into doublet while the C band splits into triplet [10]. The doublet peaks at 252 and 264 nm occurring to the excitation spectrum 407 nm inFigs. 7 and 8could be attributed to the adiabatic potential energy surface (APES) of the3P
1relaxed excited state which
has two minima (the high energy level denoted as ATand the low
energy level denoted as Ax). The triplets at 211, 217 and 222 nm in
the excitation spectra upon 407 nm and the doublets at 214 and 226 nm in the excitation spectra upon 460 nm inFigs. 7 and 8could be ascribed to the Jahn–Teller effect on the1S
0-1P1 transition of
Sb3 þat C
iand C1sites, respectively.
Herewith, the energy levels of Sb3 þ in YBO
3 including the
borate host absorption and the processes of excitation, emission, and nonradiative transition through electron relaxation could be described as the scheme inFig. 10, in which the dashed lines in energy level bands indicate the Jahn–Teller effect on the APES of
1P
1and3P1of Sb3 þ.
To seek for another evidence to support the above conclusion about the multiple sites involved in the observed transitions, the phosphors activated by multiple Sb3 þ concentrations were
synthesized.Fig. 11displays the emission and excitation spectra of (Y1 xSbx)BO3 (x¼0, 0.001, 0.005 and 0.01) collected by using
the F-4500 spectrometer at room temperature. With trace of Sb3 þ
doped into the YBO3 host, the Sb3 þ ions would preferentially
occupy the Cisite of Y3 þfor its high symmetry, but only a large
amount doped Sb3 þ will incorporate into the C
1 site for its low
symmetry. FromFig. 11(a)–(d), so, we observe that the configua-tion of the emission and excitation spectra of the
(Y0.999Sb0.001)BO3 are similar to those of (Y0.995Sb0.005)BO3 very
much, no matter by exciting/ monitoring the Cisite with 252/
400 nm or by exciting/monitoring the C1 site with 270/460 nm.
When the concentration of Sb3 þis increased to x¼0.01, however,
Fig. 10. The scheme of the energy levels of Sb3 þ
occupying at the sites of Ciand C1 in the crystal lattice of YBO3and the processes of excitation (Ex.), emission (Em.) and nonradiative transition through electron relaxation.
350 400 450 500 550 600 0 20 40 60 80 λex= 252 nm x = 0 x = 0.001 x = 0.005 x = 0.01 Emission, a.u. Wavelength, nm 0 20 40 60 80 Emission, a.u. Wavelength, nm x = 0 x = 0.001 x = 0.005 x = 0.01 λex= 270 nm 0 20 40 60 80 100 Excitation, a.u. Wavelength, nm x = 0 x = 0.001 x = 0.005 x = 0.01 λem= 400 nm (Y1-xSbx)BO3 (Y1-xSbx)BO3 (Y1-xSbx)BO3 (Y1-xSbx)BO3 210 240 270 300 0 20 40 60 80 x = 0 x = 0.001 x = 0.005 x = 0.01 Excitation, a.u. Wavelength, nm λem= 460 nm x = 0 350 400 450 500 550 600 x = 0 x = 0.001 x = 0.005 x = 0.01 x = 0 210 240 270 300
Fig. 11. The emission spectra of (Y1 xSbx)BO3(x¼ 0, 0.001, 0.005 and 0.01) excited with 252 and 270 nm for (a) and (b), respectively; and the excitation spectra by monitoring the emission at 400 and 460 nm for (c) and (d) respectively.
an extra emission and absorption of Sb3 þ at C
1site is observed
obviously by comparing with the emission and absorption of Sb3 þat Cisite, especially fromFig. 11(b)–(d). The phenomena are
consistent with expection.
By comparing with Bi3 þ which has the same ns2 electronic
configuration with Sb3 þ, we found that strong luminescence
could be obtained by doping with very little Bi3 þ (even trace of
0.02% M) into YBO3in our experiments. However, a larger amount
of Sb3 þ ( the nominal optimal concentration is 8% M) should be
doped so as to obtain efficient luminescence. This phenomenon suggests that electrons could transfer in a long range within different Bi3 þsites; however, it only happens in a short range for
Sb3 þ, which may be restrained by the strong electron–lattice coupling interaction between Sb3 þ and YBO
3host. A reasonable
explanation for this difference is that the spin–orbit interaction of the heavy Bi3 þ ion is much stronger than the electron–lattice interaction of Sb3 þion[9].
To verify the availability of the above analyses, the fluores-cence lifetime of Sb3 þemission was measured at room tempera-ture.Fig. 12presents the fluorescence decay curves by monitoring the emission at 400 and 460 nm, respectively, by exciting each with either 252 or 262 nm. The fluorescence decay spectra were fitted with exponential decay function as
I ¼ A exp t
t
þC: ð3Þ
The fitted lifetime of
t
for (a) (l
ex¼252 nm,l
em¼400 nm), (b)(
l
ex¼252 nm,l
em¼460 nm), (c) (l
ex¼262 nm,l
em¼400 nm) and(d) (
l
ex¼262 nm,l
em¼460 nm) are 0.00991, 0.01028, 0.01068and 0.01102 ms, respectively. The lifetime of a forbidden transi-tion usually is millisecond. So, the microsecond lifetime indicates that the emission must come from an allowed transition, which supports the above attribution of the emission band to the
3P
1-1S0of Sb3 þ. Nevertheless, the lifetime of an allowed
transi-tion from the p to the s orbit through electric dipole interactransi-tion usually is nano second. A reasonable explanation for the
microsecond lifetime may be caused by energy transfer among different sites of Sb3 þand phonon–electron coupling, as
consist-ing with the above analysis of the electron–lattice interaction on the luminescence of YBO3: Sb3 þ. Therefore, the attribution of the
doublet emission inFigs. 3–5to the3P
1-1S0transition and the
assignment of the doublet excitation in the band region of 240– 280 nm to the1S
0-3P1transition inFigs. 7 and 8are reasonable.
Restricted to experimental instruments, the lifetime of lumines-cence as corresponding to the excitation at quadruple peaks about 202, 211, 217 and 222 nm in the region of 190–240 nm was not carried out.
Finally, the parameters about the Jahn–Teller effect were calculated by using the molecular-orbital approximation. The formulae about the sum of the Coulomb energy and the energy difference between the ground state and excited state F0, the
spin–orbit coupling constant
z
, the exchange energy G, and the dipole strength ratio of the C band to the A band Rth, are given byFukuda and Sugano as follows[39,40]:
F0¼14
z
þW0, where W0¼12ðWAþWCÞ ð4Þz
¼23ðWBW0Þ þ13½6ðWBW0Þ2ðWBW0Þ21=2 ð5Þ G ¼3 4z
ðWBW0Þ ð6Þ Rth¼ ðWCWAÞ þW ðWCWAÞW , where W ¼ 2z
2ðWBW0Þ ð7ÞWA, WBand WCcorrespond to the peak wavelength of A, B and C
band, respectively.
The strongest absorption at about 251 and 212 nm were considered as the mean value of C1and Cisite on the position of
A and B bands, respectively. However, the B band which origi-nates from a spin-forbidden transition of 1S
0-3P2 induced by
lattice vibrations was not clearly observed. If happened, it should be close to the foot mountain of C band in the right hand. So, it was estimated to be 235 nm. The calculated values of the parameters F0,
z
, G, and Rthaccording to Eqs. (4)–(7) with theavailable parameters of free Sb3 þis presented inTable 1.
5. Conclusions
In summary, the photoluminescence of YBO3: Sb3 þ was
investigated systematically under the VUV and UV excitation at variant temperature conditions. The site-selective luminescence of YBO3: Sb3 þwas revealed by exciting/monitoring with different
wavelengths and confirmed by the YBO3activated with multiple
Sb3 þ concentrations. More importantly, the superposition of the
Jahn–Teller effect and sites effect on the luminescence of YBO3:
Sb3 þwas discriminated from the hyperfine structure in excitation
spectra which were measured on the synchrotron radiation instrument at 14 K low temperature. Although Bi3 þ and Sb3 þ has the same ns2 electronic configuration, the luminescence of
0.00 0.03 0.06 0.09 0.12 0.15 0 50 100 150 200 250 300 350 (d) λex= 262 nm, λem= 460 nm (c) λex= 262 nm, λem= 400 nm (b) λex= 252 nm, λem= 460 nm
Counts per second, a.u.
Time, millisecond
(a) λex= 252 nm, λem= 400 nm
Fig. 12. The lifetime of YBO3: Sb3 þupon 400 and 460 nm emission by respectively exciting with either 252 or 262 nm.
Table 1
Values of the parameters F0,z, G, and Rth(eV).
WC WB WA F0 z G Rth Free Sb3 þ 11.90 9.00 8.27 10.25 0.67 1.59 57.35 Ref.[21] YBO3: Sb3 þ 5.9161 5.2870 4.9695 5.5117 0.2755 0.3625 21.5584 300 K 5.8603 5.2421 4.9301 5.4629 0.2708 0.3562 21.5583 22 K
Sb3 þin YBO
3host distinguish from that of Bi3 þsignificantly. The
strong electron–lattice interaction between Sb3 þand YBO
3host
will affect the energy transfer from Sb3 þto other activators, if it were served as a sensitizer. Accordingly, appropriate measures, such as the replacement of Y3 þ by a heavy Lu3 þone, should be
taken to suppress the electron–lattice interaction in order to improve luminescence. More researches are undertaking.
Acknowledgments
The authors acknowledge the financial support from the National High-Tech R&D Program of China (863 program) for semiconductor lighting application and demonstration of ‘‘Ten cities with all LED lamps’’ (ss2013AA030114), the National Natural Science Foundation of China (Nos. 51002043, 51172086, 51272081, and 61076040), the Science Foundation for Excellent Young Scholars of the Ministry of Education of China (No. 20090111120001), the Science and Tech-nology Program of Anhui Province (No. 12010202004), the China Postdoctoral Science Foundation (Nos. 20090450802 and 2012T50568), the Fundamental Research Funds for the Central Universities (2012HGQC0033), and the Student Innovation Training Program of Hefei University of Technology (Nos. 2012CXCY071 and 2012CXCY044).
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![Fig. 1 in Ref. [25] . The eightfold Y 3 þ in C](https://thumb-ap.123doks.com/thumbv2/9libinfo/7899369.155954/5.892.68.437.797.1079/fig-ref-eightfold-y-þ-c.webp)
