Tunable LLP via Energy Transfer between Na2ây(Zn1âxGax)GeO4

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Tunable LLP via Energy Transfer Between Na (Zn Ga)GeO Sosoloid Host and Emission Centers With the assistance of Zn Vacancies Ting Wang, Wenjuan Bian, Dacheng Zhou, Jianbei Qiu, Xuhui Xu, and Xue Yu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b03909 • Publication Date (Web): 03 Jun 2015 Downloaded from http://pubs.acs.org on June 8, 2015

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The Journal of Physical Chemistry

Tunable LLP via Energy Transfer between Na2-y(Zn1-xGax)GeO4 Sosoloid Host and Emission Centers with the Assistance of Zn Vacancies Ting Wanga , Wenjuan Biana, Dacheng Zhoua,b, Jianbei Qiua,b, Xue Yu*a,b, Xuhui Xu*a,b a

School of Materials Science and Engineering, Kunming University of Science and Technology, Xuefu RD, Kunming 650093, PR

China b

Key Laboratory of Advanced of Materials Yunnan Province, Kunming 650093, PR China

*E-mail address: [email protected]; [email protected] *Tel: 86-871-5188856. Fax: 86-871-5188856.

ABSTRACT: A series of sodium zinc gallogermanate sosoloid Na2-y(Zn1-xGax)GeO4: yTb3+ (x=0, 0.1, 0.12, 0.15, 0.18, and 0.20; y= 0, 0.01, 0.02, 0.03, and 0.04) samples were successfully synthesized and their luminescence properties were investigated. It was found that Zn vacancies could be the predominant contribution to the enhancement of PL and LLP intensities, when Ga3+ ions were introduced as solute atoms substituted the crystal sites of Zn2+ in Na2ZnGeO4 host. Zn vacancies act not only as an exciton energy-level participated in the PL process, but also as trapping centers contributed to the LLP process. Furthermore, the energy transfer processes from Na2(Zn0.8Ga0.2)GeO4 host to Tb3+ ions in PL and LLP were identified. Accordingly, the LLP colors changed from blue to green with increasing concentration of Tb3+, and the mechanism of which was proposed. Besides, after irradiation with 800 nm femtosecond pulsed laser, a blue upconversion LLP phenomenon was clearly observed in Na2(Zn0.8Ga0.2)GeO4 for the first time. Analysis suggested that multiphoton absorption process was dominant in this upconversion luminescence, and subsequently the carriers excited into conduction band were captured by the trapping centers. Then the realization of LLP was due to the thermal stimulated recombination of holes and electrons at traps. Our results indicated that the

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blue-green emitting phosphor of Na2(Zn0.8Ga0.2)GeO4: Tb3+ could be a new member in the family of LLP materials, of which could also provide potential applications in the fabrication of optical memory. INTRODUCTION Long-lasting phosphorescence (LLP) is a phenomenon in which emission remaining visible for an appreciable time from seconds to hours after the stoppage of excitation (typically ultraviolet light and visible light).1-3 Such phosphors have long been of great research interest and are nowadays commercialized as night or dark environment vision materials for wide range of applications such as safety signage, night-vision surveillances, displays, decorations, and in vivo bio-imaging.4-6 It is generally recognized that LLP properties are closely related to the defects in the phosphors. The defects can trap the electrons or holes during the excitation process, and the trapped carriers are released under thermal stimulation to recombine at emission centers, which lead to the characteristic emission of the emission centers. Therefore, the emission wavelength of a persistent phosphor is mainly determined by the emitter, whereas the persistence intensity and time are controlled by the defects created or improved by the inequivalent doping. Establishing suitable defects in the host and improving the capability of storing incident energy could be achieved through inequivalent substitution, such as doping ions, usually transition metal ions or rare earth ions, taking the crystal site of a matrix which has different valence.7,8 Up to now, the best performance of LLP phosphors has been reported in Eu2+, Dy3+ co-doped alkaline earth aluminates, which exhibits persistent luminescence more than


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20h in the green or blue region.9,10 However, the synthetic temperature of these materials is usually too high (>1200oC), which disadvantages the development of the applications for the expensive cost.11 Furthermore, although several mechanisms about the LLP phenomenon have been suggested, the detailed information about the carriers capture and release was not clarified or reached a consensus. Therefore, designing a facile approach to develop novel afterglow persistent phosphor with excellent intensity and lifetime for extending the family of LLP materials and exploring the nature of persistence luminescence process is essentially important and interesting. At present, gallogermanates have been developed as host matrix of LLP phosphors due to their low synthesis temperature, high stability and reasonable conductivity. Since 2012, zinc gallogermanates, such as Zn3Ga2Ge2O10 and Zn1+xGa2-2xGexO4 are investigated as white-bluish LLP phosphors for the self-activated properties,12,13 which inspire us to explore novel zinc gallogermanate phosphor as potential excellent LLP materials. In this work, sodium zinc gallogermanate sosoloid of Na2(Zn1-xGax)GeO4 was designed based on Na2ZnGeO4 host matrix. It was found that both the PL and LLP intensities of the native emission in Na2(Zn1-xGax)GeO4 could be enhanced greatly with increasing concentration of Ga3+. Besides, the energy transfer from Na2(Zn0.8Ga0.2)GeO4 host to Tb3+ ions in PL and LLP processes were affirmed, which contributes to the tunable LLP color from blue to green region. In addition, after irradiation with 800 nm femtosecond pulsed laser, a blue upconversion LLP phenomenon was observed, and the multiphoton



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absorption processes were expected. On the basis of the experimental results, the mechanism of the observed LLP process was proposed in brief. EXPERIMENTAL Na2-y(Zn1-xGax)GeO4: yTb3+ (x=0, 0.1, 0.12, 0.15, 0.18, and 0.20; y=0, 0.01, 0.02, 0.03, and 0.04) samples were synthesized by the conventional high temperature solid state

reaction.

The

stoichiometry

amounts

of

Na2CO3(A.R),

GeO2(A.R),

ZnO(99.99%), Tb4O7(99.99%), Ga2O3(99.99%), and Mn2CO3(99.99%) were mixed in an agate mortar with ethanol. After fully grinding, the mixtures were put into crucibles and calcined at 900oC for 3 h. After cooling to room temperature naturally, the as-obtained samples were ground into a powder for the following measurements. The crystalline structures of the prepared powders were investigated by X-ray diffraction (XRD) with Ni-filter Cu Kα radiation （ λ= 0.154056 nm) at a scanning stepping of 0.02o. The XRD data were collected in the range of 10o to 60o by applying a D8ADVANCE/Germany Bruker X-ray diffractometer. The absorption spectra were obtained using the UV-visible spectrophotometer. The photoluminescence excitation (PLE), emission (PL) spectra and LLP were recorded by using a Hitachi F-7000 fluorescence spectrophotometer. The PL decay curves were measured by FS980 fluorescence spectrophotometer. The LLP decay curves measurements were measured with a PR305 long afterglow instrument after the sample was irradiated by UV light (254 nm) for about 10 min. The thermoluminescence (TL) curves were measured with a FJ-427 A TL meter (Beijing Nuclear Instrument Factory). Weight of the measured samples was constant (0.002 g). Prior to the TL measure, the samples were first


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exposed to the radiation from UV light (254 nm) for about 10 min, then heated from room temperature to 550K with a rate of 1K/s. For the femtosecond laser LLP spectra, the sample was first irradiated by the focused laser, a regeneratively amplified 800nm Ti: sapphire laser system with 1 kHz repetition rate and approximately 45fs pulse duration was used as an irradiation source. The laser beam (65mW) was focused into the center of the sample by an optical lens with a focal length of 100 mm along the longitudinal direction of the sample, and the corresponding LLP spectrum was recorded from the sample side by a spectrophotometer of Ocean Optics USB2000+. RESULTS AND DISCUSSION The purity of all the prepared samples was systematically checked by XRD measurements. Fig. 1(a-g) show the typical XRD patterns of Na2-y(Zn1-xGax)GeO4: yTb3+ (x=0, 0.1, and 0.15; y= 0.01, 0.02, and 0.04) samples. Clearly, all the diffraction peaks could be exactly indexed to the phase of Na2ZnGeO4 registered in JCPDS file No. 37-0255, which indicates that the sosoloid are formed and Tb3+ doped samples are identified as a single phase. Based on the relationship between effective ionic radii and different coordination number (CN), Ga3+ [0.47Å, CN =4] is proposed to occupy the regular Zn2+ [0.59Å, CN=4] site rather than Ge4+ [0.38Å, CN=4] sites, while Tb3+ [0.76Å, CN=4] locates the regular Na+ [0.99Å, CN=4] site. In order to investigated the occupancy situation, the XRD result of Na2(Zn0.8Ga0.2)GeO4 is refined by the Rietveld method as shown in Fig. 1h. The black solid lines and red crosses are calculated patterns and experimental patterns, respectively. The black short vertical lines show the position of the Bragg reflections of the calculated pattern.



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The difference between the experimental and calculated patterns is plotted by the green line at the bottom. The reliability parameters of refinement are Rwp=18.06%, and χ2=1.28, which verifies the phase purity of the as-prepared sample. Fig. 1i illustrates the ball and stick model and the polyhedron model of Na2ZnGeO4. The sodium atoms occupy two different sites (denoted as Na(I) and Na(II)), while the oxygen atoms occupy four different sites (denoted as O(I)-O(IV)). Since cation atoms are surrounded by an average of four oxygen atoms in this host as shown in Fig.1i, the normal tetrahedral structures are formed. The polyhedron model demonstrates that Na2ZnGeO4 actually has a layered framework structure built by corner-connected tetrahedra. The Na(I)O4, Na(II)O4, ZnO4 and GeO4 tetrahedra connect in a corner-sharing manner to form alternating Na(I)O4-GeO4, Na(II)O4-ZnO4 chains along the b direction; the parallel chains join each other to generate the tetrahedra layers in the a-b plane; and then the tetrahedra layers stack along the c-axis by corner-connecting to construct the three-dimensional framework.14

Fig. 1 X-ray diffraction patterns of Na2-y(Zn1-xGax)GeO4: yTb3+ (a-g); Powder X-ray


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diffraction spectra refinement of Na2(Zn0.8Ga0.2)GeO4 (h); The ball and stick model and the polyhedron model of Na2ZnGeO4 (i). The density functional theory calculations of Na2(Zn0.8Ga0.2)GeO4 sosoloid host based on crystal structure refinement are employed and shown in Fig. 2a and b. The local density approximation (LDA) is chosen for the theoretical basis of the density function. Na2(Zn0.8Ga0.2)GeO4 possessed an indirect band-gap of about 2.67 eV with the valence band (VB) maximum and the conduction band (CB) minimum located at the G and Y point of the Brillouin zone, respectively. Fig. 2c shows the total and partial density of states of Na2(Zn0.8Ga0.2)GeO4. It could be concluded that the CB of Na2ZnGeO4 consists of the orbitals of Ge 4s24p2 , whereas the VB is dominated by the orbitals of Na 2s22p6, O 2s22p4, Zn 3d10 and Ge 4s24p2. It is expected that the value of the calculated band gap of Na2(Zn0.8Ga0.2)GeO4 will be smaller than the experimental one as the LDA underestimates the size of the band gap.14 Fig. 2d plots the experimental absorption spectrum of Na2(Zn0.8Ga0.2)GeO4, which exhibits a broad and strong absorption in the range from 200-300 nm. The energy band gaps of sample can be calculated according to the equation that had been widely adopted for crystalline semiconductors:15-17 (αhν)=A(hν-Eg)2/n

(1)

Here, α, ν, A, Eg, and n are the absorption coefficient, incident light frequency, constant, band gap, and an integer (normally equal to 1, 2, 4, 6), respectively. The n value is decided to be 2 for Na2(Zn0.8Ga0.2)GeO4, indicating that the observed optical transition of which should be indirect.14 The α and ν values at the steep edges of the



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absorption spectra are used to construct the plots of (αhν)2/n against photon energy. As depicted in the inset of Fig. 2d, the curve for Na2(Zn0.8Ga0.2)GeO4 shows a linear region with n=2. The band gaps of samples can be determined by intersection of the extrapolated linear portion of the plots with the energy axis, and the value is calculated to be 3.49 eV.

Fig. 2 Energy band structure of Na2(Zn0.8Ga0.2)GeO4 crystal (a); Total density of states of

Na2(Zn0.8Ga0.2)GeO4

(b);

Total

band

partial

density

of

state

of

Na2(Zn0.8Ga0.2)GeO4 (c); Absorption spectrum of Na2(Zn0.8Ga0.2)GeO4, the inset shows (αhν)2/n～hν plots for hν Na2(Zn0.8Ga0.2)GeO4 (d). Fig. 3a shows the PLE and PL spectra of Na2ZnGeO4 and Na2(Zn0.8Ga0.2)GeO4 phosphors, respectively. Under the excitation at 262 nm, samples of Na2ZnGeO4 with and without Ga3+ doping show a purple-blue broad band ranging from 310 to 700 nm (centered at 407 nm). The broad nonsymmetrical band can be readily Gaussian-resolved into two bands located 405 and 464 nm. Besides, the emission


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intensity of Na2(Zn0.8Ga0.2)GeO4 is remarkable higher than that of Na2ZnGeO4 host. Similar photoluminescence located in the blue region has been observed in Li2ZnGeO4, Zn2GeO4 powders.18,19 The broad emission is considered to be the recombination of donors ( VO and Zni ) and acceptors ( VZn'' ) which are generated as intrinsic defects in host lattice at high temperature.20 Therefore, the blue emission in Na2ZnGeO4 and Na2(Zn0.8Ga0.2)GeO4 could be originated from the transitions of VO and Zni to VZn'' . Monitoring the emission at 407 nm, the PLE spectra show a broad excitation band with the maximum at 262 nm. The energy of excitation peak is higher than that of the energy gap calculated above, therefore, the electrons in the VB can be excited to the CB directly under 262 nm excitation. In consideration of the fact that Ga3+ ion replaces the Zn2+ site in Na2ZnGeO4 host matrix, two Ga3+ ions would substitute for three Zn2+ ions to keep the electroneutrality of the compound. Therefore, more charge compensating lattice defects of VZn''

are inevitable formed, which

contributes to the enhancement of the PL intensity observed in Fig. 3a. Fig. 3b portrays the PL of Na2(Zn1-xGax)GeO4 (x=0.10, 0.12, 0.15, 0.18, and 0.20) samples excited at 262 nm. With the concentration of Ga3+ increased, the PL intensities increase and reach the maximum when x=0.18, and then decreases due to the concentration quenching. Besides, it is noticed that the PL spectra of Na2(Zn1-xGax)GeO4 exhibit similar shape and position to that of Na2ZnGeO4. It indicates that the emission of Na2(Zn1-xGax)GeO4 are ascribed to the intrinsic defects ( VZn'' , VO and Zni ) as discussed previous. Therefore, with the increases number of VZn''

as the concentration of Ga3+ increases, higher PL intensities are achieved as



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observed in Fig. 3b. Above results provide further evidences that the defects ( VO , ''

Zni , and VZn ) act as the excition energy level involved in the PL process.

After irradiation by a 254 nm UV lamp for 10 min, the LLP phenomenon is clearly observed from Na2(Zn1-xGax)GeO4 samples (Fig. 3c). The LLP intensities gradually increase with increasing concentration of Ga3+. It implies that VZn'' , acting as the trapping center, plays a significant role on the LLP phenomenon. Furthermore, the obvious LLP concentration quenching of the sosoloid is not observed in the current doping level.

Fig. 3 PLE and PL spectra of Na2ZnGeO4 (red lines) and Na2(Zn0.8Ga0.2)GeO4 (black lines), the PL of Na2(Zn0.8Ga0.2)GeO4 is Gaussian-resolved into two bands ( dotted lines) located at 405 and 464 nm (a); The PL of Na2(Zn1-xGax)GeO4 (x=0.10, 0.12, 0.15, 0.18, and 0.20) under the excitation at 262 nm (b); The LLP spectra Na2(Zn1-xGax)GeO4 (x=0, 0.10, 0.12, 0.15, 0.18, and 0.20) (c); LLP decay curves of Na2(Zn1-xGax)GeO4 (x=0, 0.10, 0.12, 0.15, 0.18, and 0.20) (d).


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The LLP decay curves of Na2(Zn1-xGax)GeO4 (x=0, 0.10, 0.12, 0.15, 0.18, and 0.20) are measured at room temperature and displayed in Fig. 3d. The afterglow decay curves consist of a rapid decay process initially and a slow decay process, which infer the existence of various traps with different depths. Since the lattice defects, acted as trap centers, play an essential role for energy storage in afterglow phosphors, longer persistent time is obtained in those phosphors with increasing concentration of Ga3+. Hence, it can be safe to say that the incorporation of Ga3+ creates more defects ( VZn'' ) in Na2(Zn1-xGax)GeO4 phosphors, which contributes to the significant prolonged LLP decay time. Fig. 4a exhibits the PL spectra of Na2-y(Zn0.8Ga0.2)GeO4: yTb3+ (y=0, 0.01, 0.02, 0.03, and 0.04) and Na1.98ZnGeO4: 0.02Tb3+ samples, respectively. The profiles of the PL spectra are identical in Na2-y(Zn0.8Ga0.2)GeO4: yTb3+ phosphors. However, compared with Na1.98ZnGeO4: 0.02Tb3+, the PL intensities of Na2-y(Zn0.8Ga0.2)GeO4: yTb3+ are enhanced significantly. Besides, the intensities of the blue emission band (407 nm) decrease remarkably with the concentration of Tb3+ ions increase in Na2-y(Zn0.8Ga0.2)GeO4: yTb3+ phosphors. Moreover, the weak green emission peak located around 550 nm, attributed to the 5D4-7F5 transition of Tb3+ ions,21 is observed. The emission intensities of Tb3+ increase initially and reach the maximum at y=0.02, and then decrease due to concentration quenching, as shown in Fig. 4b. These results indicate the energy transfer between Na2(Zn0.8Ga0.2)GeO4 host and Tb3+ can be expected. One of the essential conditions usually required for energy transfer is the overlap between the donor emission and the acceptor excitation spectra.22 To clarify



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the

energy

transfer

process,

Fig.

4c

illustrates

the

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PLE

spectrum

of

Na2(Zn0.8Ga0.2)GeO4: Tb3+ and PL spectrum of Na2(Zn0.8Ga0.2)GeO4 phosphors. A significant spectral overlap between the emission band of Na2(Zn0.8Ga0.2)GeO4 and the excitation band of Tb3+ (4f-4f transition) is observed in Fig. 4c, therefore, it can be speculated that energy transfer from Na2(Zn0.8Ga0.2)GeO4 host to Tb3+ ions happens. In order to further illustrate the energy transfer process, the fluorescence decay curves of Na2-y(Zn0.8Ga0.2)GeO4: yTb3+ (y=0, 0.01, 0.02, 0.03, and 0.04) phosphors (λex=262nm, λem=407nm) samples are recorded as shown in Fig. 4d. The decay curves of the Na2-y(Zn0.8Ga0.2)GeO4: yTb3+ samples can be well fitted by the second-order exponential decay mode as the following equation:23 I  A1 exp(t /  1 )  A2 exp(t /  2 )

(2)

Where I is the luminescence intensity, A1 and A2 are fitting parameters, t is the time, τ1 and τ2 are the luminescence lifetimes. On the basis of the equation (2), the average decay time τ of Na2(Zn0.8Ga0.2)GeO4 sosoloid can be calculated by the following equation:



A1 12  A2 22 A1 1  A2 2

(3)

The effective lifetime values are calculated to be 48.2, 41.7, 39.1, 36.3, and 31.8 µs, respectively. The decay time of Na2(Zn0.8Ga0.2)GeO4 host decreases with increasing concentration of Tb3+, which strongly confirms the existence of the energy transfer process from Na2(Zn0.8Ga0.2)GeO4 host to Tb3+.


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Fig. 4 PL spectra of Na2-y(Zn0.8Ga0.2)GeO4: yTb3+ (y=0, 0.01, 0.02, 0.03, and 0.04) under the excitation at 262 nm (a); The dependence of the relative emission intensities of Na2(Zn0.8Ga0.2)GeO4 host at 407 nm and Tb3+ at 550 nm (b); PLE spectrum of Na2(Zn0.8Ga0.2)GeO4: Tb3+ and PL spectrum of Na2(Zn0.8Ga0.2)GeO4 (c); The fluorescence decay curves of Na2-y(Zn0.8Ga0.2)GeO4: yTb3+ (y=0, 0.01, 0.02, 0.03, and 0.04) phosphors ( λem=407nm, λex=262nm) (d). After been irradiated by 254 nm UV light for 10 min, the LLP spectra of the samples Na2-y(Zn0.8Ga0.2)GeO4: yTb3+ (y=0, 0.01, 0.02, 0.03, and 0.04) and Na1.98ZnGeO4: 0.02Tb3+ phosphors are recorded and demonstrated in Fig. 5a. Compared

with

the

Na1.98ZnGeO4:

0.02Tb3+,

the

LLP

intensities

of

Na2-y(Zn0.8Ga0.2)GeO4: yTb3+ are enhanced greatly as shown in Fig. 5a. Besides, the green emission peaks centered at 491 and 550 nm of Tb3+ are detected, and it implies that Tb3+ ions also act as an LLP emission centers. The LLP intensities of



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Na2(Zn0.8Ga0.2)GeO4 decrease remarkably with increasing concentration of Tb3+, while the green emission intensities of Tb3+ at 550 nm enhance gradually and reach the maximum value at y=0.03, which lead to the LLP color change accordingly. The results indicate that the LLP energy transfer process from the host to Tb3+ occurs, and it is obvious that the energy transfer efficiency of LLP is more significant compared with PL process. The phosphorescence CIE chromaticity coordinates of samples are calculated from the phosphorescence spectrum using chromaticity coordinate calculation methods based on the CIE1931 (Commission International de I’Eclairage France) system and marked in Fig. 5b. It is found that the chromaticity coordinates of the corresponding samples varies from (0.1478, 0.1805) to (0.2230, 0.3448). The chromaticity coordinate points and LLP photographs of these samples are depicted in Fig. 5b, which describes the LLP colors vary from blue to green in Na2-y(Zn0.8Ga0.2)GeO4: yTb3+ (y=0, 0.01, 0.02, 0.03, and 0.04) samples. The LLP decay curves of Na2-y(Zn0.8Ga0.2)GeO4: yTb3+ (y=0.01, 0.02, 0.03, and 0.04) samples are shown in Fig. 5c. Clearly, the concentration of Tb3+ has a great influence on the LLP duration. Na1.97(Zn0.8Ga0.2)GeO4: 0.03Tb3+ sample exhibits the LLP with the highest relative intensity and the longest lasting time. Therefore, the optimal concentration of Tb3+ in Na2(Zn0.8Ga0.2)GeO4 for LLP is confirmed to be about y=0.03. LLP is strongly governed by the slow liberation of trapped charge carriers under thermal stimulation in the host lattice at room temperature,15 and it is widely


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recognized that LLP is caused by the energy exchange processes from traps centers to emission centers. Therefore, the LLP properties are critically depended on the traps introduced in the host lattice. TL glow curve is one of efficient tools to get some useful information about the traps for the exploration of LLP mechanism.24 TL curves of Na2-y(Zn0.8Ga0.2)GeO4: yTb3+ (y=0, 0.01, 0.02, 0.03, and 0.04) and Na2ZnGeO4 phosphors are recorded as shown in Fig. 5d. Both Na2(Zn0.8Ga0.2)GeO4 (red dot) and Na2ZnGeO4 (black dot) show an asymmetry broad TL band between 300 and 400 K with the peak predominating at 331 K (TA). The TL results suggest that the defects in the two samples are basically the same. VZn''

 and GaZn are introduced, when the

sodium zinc gallogermanate sosolid is formed as the solute atom of Ga3+ are added into Na2(Zn1-xGax)GeO4. The positively charged substitution defect of

 Ga Zn

can

capture electrons, while the negatively charged defect of VZn'' as the intrinsic defect of the host has the ability to capture holes. Since the TL band at 331 K (TA) is observed in both Na2(Zn0.8Ga0.2)GeO4 and Na2ZnGeO4, the TL band at 331 K is mainly ascribed to the intrinsic defect VZn'' rather than

 Ga Zn

. For the Tb3+ doped samples, a

predominate band centered at 331 K and a relatively weak band at 390 K (TB) are observed. It is commonly considered that the lower and higher temperature of the TL band is related to the shallower and deeper traps, respectively.25,26 The suitable TL peak is situated slightly above the room temperature (323-393 K) for the excellent LLP performance.27,28 Therefore, the TL bands centered at 331 K and 390 K can be responsible for the blue and green LLP emissions. With increasing concentration of Tb3+, the TL intensities of 331 K band decrease, meanwhile the intensities of 390 K



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band increase obviously. Evidently, the incorporation of a slight amount of Tb3+ ions promotes the generation of traps at 390 K. It indicates that Tb3+ ions not only act as luminescent centers, but also introduce foreign trap centers contributing to the LLP process. The relative intensities of TL band at 390 K are dependent on the concentration of Tb3+, suggests that the defects such as

 TbNa

and

' V Na

served as traps

can be generated as the incorporation of Tb3+ in Na2(Zn0.8Ga0.2)GeO4 phosphor 3

 ' ( 3Na  Tb ). That is to say, with the addition of Tb3+, holes traps   TbNa  2VNa

and electrons traps states of

 ' TbNa / V Na

 TbNa

' V Na

are introduced. Furthermore, The existence of the defect

, which served as the bridge from Na2(Zn0.8Ga0.2)GeO4 host to Tb3+

in LLP process, leading to the energy transfer efficiency of LLP is more significant compared with PL process as observed in Fig. 5.

Fig. 5 LLP spectra of Na2-y(Zn0.8Ga0.2)GeO4: yTb3+ (y= 0, 0.01, 0.02, 0.03, and 0.04) and Na1.98ZnGeO4: 0.02Tb3+ samples (a); The phosphorescence chromaticity


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coordinates of Na2-y(Zn0.8Ga0.2)GeO4: yTb3+ samples (y= 0, 0.01, 0.02, 0.03, and 0.04) and their LLP photographs (b); LLP decay curves of Na2-y(Zn0.8Ga0.2)GeO4: yTb3+ (y= 0.01, 0.02, 0.03, and 0.04) samples (c); TL spectra of Na2ZnGeO4 and Na2-y(Zn0.8Ga0.2)GeO4: yTb3+ (y=0, 0.01, 0.02, 0.03, and 0.04) samples (d). To study the transition and captured processes of carriers, 800 nm femtosecond laser has been acted as the excitation light source, and the upconversion LLP process is realized as shown in Fig. 6. It is clear that the LLP spectrum of Na2(Zn0.8Ga0.2)GeO4 is similar to the PL spectrum as depicted in Fig. 4a, after the removal of the 800 nm femtosecond laser, thereby suggesting the LLP comes from the same emission state independent of the excitation wavelength. Since the broad LLP band centered at 420 nm is derived from the native emission of the Na2(Zn0.8Ga0.2)GeO4 sosoloid, the possibility of the presence of multiphoton process when the crystal is irradiated at 800 nm are taken into consideration. Three-photons absorption in Na2(Zn0.8Ga0.2)GeO4 sosoloid is proposed, which includes two possible mechanisms, one of which is direct three-photon absorption and the other is two-(one-) photon absorption followed by one-(two-) photon absorption from the excited level.29-31 To identify the process, the single-photon absorption spectrum of Na2(Zn0.8Ga0.2)GeO4 sosoloid is measured and shown in the inset of Fig. 6. There is no linear absorption at the one-photon energy of 800 nm femtosecond radiation in Na2(Zn0.8Ga0.2)GeO4 samples. Consequently, there is highly unlikely for one-photon absorption followed by the two-photon absorption process. Another possible process is a two-photon absorption followed by an excited-state one-photon absorption in this case, the absorption spectrum should have



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Page 18 of 24

an intermediate state corresponding to the absorption peak at about 400 nm. Although the absorption at 400 nm is relative weak, this process can’t be excluded. Besides, the three-photon energy of the 800 nm radiation is found to be in good agreement with the strong UV absorption band, thus, three-photon simultaneous absorption can be expected. The multiphoton-excited processes indicate that the carriers can be excited to CB and relaxed to the excited levels, and then captured by trapping centers. Subsequently, the realization of LLP was due to the thermal stimulated recombination of holes and electrons at traps.

Fig. 6 LLP spectrum of Na2(Zn0.8Ga0.2)GeO4 phosphor after the removal of the femtosecond

laser.

The

inset

shows

one-photon

absorption

spectrum

of

Na2(Zn0.8Ga0.2)GeO4 phosphor and the photographs after the removal of the femtosecond laser. The above results clearly show that Na2(Zn0.8Ga0.2)GeO4 has excellent persistent luminescence properties. All of the experiments indicate that there exist highly efficient trapping levels in this material. Fig. 7 provides a possible mechanism of Na2(Zn0.8Ga0.2)GeO4: Tb3+, which should be treated as a qualitative analysis. For Na2(Zn0.8Ga0.2)GeO4 phosphor, the electrons in the VB are excited to the CB (step 1)


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with UV irradiation in Na2(Zn0.8Ga0.2)GeO4 phosphor. Subsequently, a part of electrons nonradiative relaxation from CB to Zni and VO levels. Meanwhile, the holes are created in the VB, and a portion of which jump from VB to VZn'' (TA) by the thermal radiation. Then the recombination of electrons and holes results in the generation of the blue emission of Na2(Zn0.8Ga0.2)GeO4 host (step 2). After the stoppage of the irradiation, the holes are released from the TA under the effect of the thermal radiation. The direct recombination of electrons and holes (step 2) contributes to the LLP phenomenon. With the co-doping of Tb3+, electrons traps of

 TbNa

and

' holes traps of VNa are generated. Under the excitation at UV light, the step 1 and 2

happen. Then, the electrons are transferred from the Zni and

VO

levels to the 5D3

level of Tb3+ (step 3), and after the nonradiative relaxation to the 5D4 level, the energy relaxes through the transitions from 5D4 to 5F5 and 5F6 levels and contributes to the emission located at 491 and 550 nm, respectively (step 6). Meanwhile, a part of electrons are trapped from Zni and

VO

 level to electrons traps of TbNa (step 4),

and a part of holes are trapped from VZn'' level to holes traps of VNa' (step 6). After  the UV light is switched off, the electrons are released from the TbNa to 5D4 and 5D4

levels, and holes are released from VNa' to 5F5 and 5F6 levels of Tb3+ (step 7) under the effect of the thermal radiation, afterwards, the electrons jump to 5F5 and 5F6 levels and combines with holes accompanying with the LLP emission from Tb3+ (step 8). Thus,  ' after UV light is turned off, the defect states of TbNa , served as the bridge from / VNa

Na2(Zn0.8Ga0.2)GeO4 host to Tb3+, leading to the energy transfer efficiency of LLP is more significant compared with PL process. Notably, a new approach of tunable LLP



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can be realized via persistent energy transfer from host-sensitized to emitting centers with the assistance with traps. Here, we adopted Mn2+ as the emitting centers in the Na2(Zn0.8Ga0.2)GeO4 host lattice. As we expected, tunable LLP are also observed (Fig. S1).

Fig. 7 Schematic energy level diagram of Na2(Zn0.8Ga0.2)GeO4: Tb3+. CONCLUSION: A novel sodium zinc gallogermanate sosoloid of Na2(Zn0.8Ga0.2)GeO4: Tb3+ has been synthesized by the traditional high-temperature solid-state method. The structure and luminescence properties of Na2(Zn0.8Ga0.2)GeO4: Tb3+ are investigated. By introducing Ga3+ ions, the PL and LLP intensities in Na2(Zn1-xGax)GeO4 sosoloid are enhanced significantly, indicating that the defects of VZn'' are related to the native blue emission of this sosoloid. The energy transfer from Na2(Zn0.8Ga0.2)GeO4 host to Tb3+ is affirmed by spectra overlap and decay curves. With increasing concentration of Tb3+, the LLP colors are changed from blue to green region. Tb3+ ions not only act as luminescent centers, but also introduce foreign trap centers. The existed trap centers, acted as the bridge, promotes the energy transfer from Na2(Zn0.8Ga0.2)GeO4 host to


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Tb3+ effectively in LLP process. A new approach of tunable LLP color was approached via persistent energy transfer from host-sensitized to emitting centers with assistance of trapping centers in this work. In addition, a blue LLP phenomenon of Na2(Zn0.8Ga0.2)GeO4 sosoloid is observed after irradiation with 800 nm femtosecond pulsed laser for the first time, which suggests Na2(Zn0.8Ga0.2)GeO4 could also be useful in the fabrication of optical memory. Acknowledgments Project supported by the National Nature Science Foundation of China (61308091, 11204113, 61265004 and 51272097), The open project of Chongqing Key Laboratory of Micro/Nano Material Engineering and technology(KFJJ1401), The Young Talents Support Program of Faculty of Materials Science and Engineering, Kunming University of Science and Technology( 14078342 ). Supporting Information: LLP spectra of Na2(Zn0.8-ZGa0.2)GeO4: zMn2+ (z= 0, 0.05, and 0.10) after ceasing the UV light and their LLP photographs.(Fig. 1S). This information is available free of charge via the Internet at http://pubs.acs.org

References: 1. Yu, X.; Wang, T.; Xu, X. H.; Zhou, D. C.; Qiu, J. B. Yellow Photo-Stimulated Long Persistent Luminescence in Strontium Silicate Phosphor. ECS Solid State Lett. 2014, 3, R4-06 2. Zhang, B. H.; Yu, X.; Wang, T.; Cheng, S.; Qiu, J. B.; Xu, X. H. Photostimulated and Long Persistent Luminescence Properties from Different Crystallographic Sites of β‐Sr2SiO4: Eu2+, R3+ (R= Tm, Gd). J. Amer. Ceram. Soc. 2015, 98, 171-77 3. Allix, M.; Chenu, S.; Véron, E.; Poumeyrol, T.; Kouadri-Boudjelthia, El. A.; Alahrache, S.; et al. Considerable Improvement of Iong-Persistent Luminescence in



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Germanium and Tin Substituted ZnGa2O4. Chem. Mater. 2013, 25, 1600-06 4. Liu, F.; Yan, W. Z.; Chuang, Y.; Zhen, Z. P.; Xie, J.; Pan, Z. W. Photostimulated Near-Infrared Persistent Luminescence as a New Optical Read-Out from Cr3+-Doped LiGa5O8. Sci. rep. 2013, 3, 1-9 5. Sharma, S.; Gourie,r D.; Viana, B.; Scherman, D.; Richard, C.; Maldiney, T.; et al.; Persistent Luminescence in Nanophosphors for Long Term in-Vivo Bio-Imaging. SPIE BiOS. 2015, 93370I-93370I-6 6. Van den Eeckhout, K.; Smet, P. F.; Poelman, D. Persistent Luminescence in 2+ Eu -Doped Compounds: a Review. Mater. 2010, 3, 2536-66 7. Li, Y.; Zhou, S. F.; Li, Y.; Sharafudeen, K.; Ma, Z. J.; Dong, G. P.; et al. Long Persistent and Photo-Stimulated Luminescence in Cr3+-Doped Zn-Ga-Sn-O Phosphors for Deep and Reproducible Tissue Imaging. J. Mater. Chem. C 2014, 2, 2657-63 8. Zhang, B.; Xu, X. H.; Li, Q. Y.; Wu, Y. M.; Qiu, J. B.; Yu, X. Long Persistent and Optically Stimulated Luminescence Behaviors of Calcium Aluminates with Different Trap Filling Processes. J. Solid State Chem. 2014, 217, 136-41 9. Matsuzawa, T.; Aoki, Y.; Takeuchi, N.; Murayama, Y. A New Long Phosphorescent Phosphor with High Brightness, SrAl2O4: Eu2+, Dy3+. J. Electrochem. Soc. 1996, 143, 2670-73 10. Clabau, F.; Rocquefelte, X.; Jobic, S.; Deniard, P.; Whangbo, M.; Garcia, A.; et al. Mechanism of Phosphorescence Appropriate for the Long-Lasting Phosphors Eu2+-Doped SrAl2O4 with Dodopants Dy3+ and B3+. Chem. Mater. 2005, 17, 3904-12 11. Jin, Y.; Hu, Y.; Fu, Y.; Ju, G.; Mu, Z.; Chen, R.; et al. Preparation, Design, and Characterization of the Novel Long Persistent Phosphors: Na2ZnGeO4 and Na2ZnGeO4: Mn2+. J. Amer. Ceram. Soc. 2015, 98, 1555-1561 12. Pan, Z. W.; Lu, Y. Y.; Liu, F. Sunlight-Activated Long-Persistent Luminescence in the Near-Infrared from Cr3+-Doped Zinc Gallogermanates. Nat. Mater. 2012, 11, 58-63 13. Ren, J.; Xu, X. Q.; Zeng, H. D.; Chen, G. R.; Kong, D. S; Gu, C. J.; et al. Novel Self-Activated Zinc Gallogermanate Phosphor: The Origin of its Photoluminescence. J. Amer. Ceram. Soc. 2014, 97, 3197-201 14. Li, X. K.; Ouyang, S. X.; Kikugawa, N.; Ye, J. H. Novel Ag2ZnGeO4 Photocatalyst for Dye Degradation under Visible Light Irradiation. Appl. Cata. A: General. 2008, 334, 51-58 15. Zeng, W.; Wang, Y. H.; Han, S. C.; Chen, W. B.; Li, G. Investigation on Long-Persistent Luminescence of Ca2BO3Cl: Eu2+, Ln3+ (Ln= Nd, Dy, Er). Opt. Mate. 2014, 36, 1819-21 16. Yin, J.; Zou, Z. G.; Ye, J. H. A Novel series of the New Visible-Light-Driven Photocatalysts MCo1/3Nb2/3O3 (M= Ca, Sr, and Ba) with Special Electronic Structures. J. Phys. Chem. B 2003, 107, 4936-41 17. Zeng, W.; Wang, Y. H.; Han, S. C.; Chen, W. B.; Li, G.; et al. Design, Synthesis and Characterization of a Novel Yellow Long-Persistent Phosphor: Ca2BO3Cl: Eu2+, Dy3+. J. Mater. Chem. C 2013, 1, 3004-11 18. Jin, Y. H.; Hu, Y. H.; Duan, H.; Chen, L.; Wang, X. J. The Long Persistent Luminescence Properties of Phosphors: Li2ZnGeO4 and Li2ZnGeO4: Mn2+. RSC Adv.


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Contents graphic

LLP spectra of Na2-y(Zn0.8Ga0.2)GeO4: yTb3+ (y= 0, 0.01, 0.02, 0.03, and 0.04) and Na1.98ZnGeO4: 0.02Tb3+ samples. Accordingly, the LLP colors changed from blue to green with increasing concentration of Tb3+.


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Tunable LLP via Energy Transfer between Na2ây(Zn1âxGax)GeO4

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