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Cr3+ doped Ca14Zn6Ga10O35: A near-infrared long persistent luminescence phosphor
Journal of Luminescence 180 (2016) 251–257
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Full Length Article
Cr3 þ doped Ca14Zn6Ga10O35: A near-infrared long persistent luminescence phosphor Fuqiang Sun n, Rongrong Xie, Li Guan, Canyuan Zhang School of Pharmacy, Guangdong Pharmaceutical University, Guangzhou 510006, China
art ic l e i nf o
a b s t r a c t
Article history: Received 27 March 2016 Received in revised form 2 July 2016 Accepted 18 August 2016 Available online 23 August 2016
The near-infrared persistent phosphors have attracted increasing attention due to the potential application in in vivo imaging. Exploring new materials to realize efficient near-infrared persistent luminescence is a goal of general concern. Ca14Zn6Ga10O35:Cr3 þ phosphor gives a near-infrared long persistent luminescence over 3 h with the 650–750 nm emission range after the short UV-irradiation. The afterglow behaviors such as the persistent luminescence, the trap depth distribution, the trap types and the underlying mechanism for persistent luminescence, were comprehensively surveyed by means of thermoluminescence methods and electron spin resonance spectra. In addition, Al dopant was introduced to redeploy the trap distribution in Ca14Zn6Ga10O35:Cr3 þ phosphor. Compared with the wellknown phosphor ZnGa2O4:Cr3 þ , Ca14Zn6Ga10O35:Cr3 þ is proved to be a new NIR persistent phosphor potentially suitable for in vivo imaging due to its 650–750 nm emission range. & 2016 Elsevier B.V. All rights reserved.
Keywords: Persistent phosphor Near-infrared phosphorescence Optical materials Cr3 þ
1. Introduction Long persistence luminescence (LPL) is the occurrence of luminescence which can last for the several minutes or hours after the stoppage of excitation. To date, long persistent phosphors (LPPs) have been received more attention and interests due to much wide application, e.g. , such as safety signage, displays, and exit signs, optical energy media, thermal sensors and in vivo bioimaging [1–6]. Especially for the application in in vivo bio-imaging, LPPs in the near infrared (NIR) region (650–1350 nm) have been developed as a promising category of luminescent labels, since their emission lifetime is sufficiently long to permit late timegated imaging [7–9]. In such a case, auto-fluorescence from tissue organic components during imaging, resulting in an extraordinary high signal-to-noise ratio, can be completely avoided [10,11]. Thus, a LPP probe is expected to open up the possibility of advanced optical imaging with high resolution and weak disturbance for factually assessing the structural and functional processes in cells, tissues, and other complex systems [12–15]. For NIR luminescence, trivalent chromium ion (Cr3 þ ) is an ideal activator in hosts since its 3d electron configuration allows a narrow-band emission (ca. 700 nm) due to the spin-forbidden 2 E-4A2 transition, or a broad emission (650–1600 nm) ascribed to the spin-allowed 4T2-4A2 transition, which strongly depends on the n
Corresponding author. Fax: þ 86 20 39352141. E-mail address: [email protected] (F. Sun).
http://dx.doi.org/10.1016/j.jlumin.2016.08.043 0022-2313/& 2016 Elsevier B.V. All rights reserved.
crystal-field environment of the host [16]. In design of NIR phosphors, Cr3 þ is a favorable choice of activation center since the LPL of Cr3 þ -activated phosphors, ranging from about 600 to 1300 nm in NIR region, meets the requirement of in vivo bioimaging. The past few years have witnessed great progress in the development of the LPP probe in in vivo bioimaging, with the main focus on the Cr3 þ activated gallate materials [11–20]. The Zn1 þ xGa2 2xGexO4 system, i.e., a solid solution between the normal ZnGa2O4 and inversed Zn2GeO4 spinle structure has been attracting much attention since the LPL of Cr3 þ -activated Zn1 þ xGa2-2xGexO4 phosphors and established gallates as the preferred material system for the fabrication of Cr3 þ -activated NIR LPP [21]. For example, Pan et al. achieved a super-long NIR afterglow emission time of 360 h in Zn1.5GaGe0.5O4: Cr3 þ (x¼ 0.5) phosphor [17]; Allix et al. gained a considerable improvement of NIR long-persistent luminescence in germanium and tin substituted ZnGa2O4:Cr3 þ [18]. Besides La3Ga5GeO14:Cr3 þ [19], LiGa5O8:Cr3 þ [20], the LPL of Cr3 þ -activated phosphors, such as Zn1.5GaSn0.5O4:Cr3 þ [6], Ca3Ga2Ge3O12:Cr3 þ [21], were investigated due to the strong ability of Cr3 þ to substitute for Ga3 þ in octahedral coordination. As far as we know, the Cr3 þ -doped gallate phosphors have a common ground, in which a octahedral site is occupied by Cr3 þ . In other words, Cr3 þ can be better accommodated and stabilized in an octahedral lattice site. This gives us a hint to explore the ideal Cr3 þ -doped host. Therefore, we select the Cr3 þ ions as the activators and the gallate (Ca14Zn6Ga10O35) as the target host due to its unique crystal structure [22]. The basic structural units of Ca14Zn6Ga10O35 (CZGO) are composed of [Ga/ZnO6] octahedron,
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Fig. 1. (a) View of the structure of CZGO from c-projection; (b) GaO4 tetrahedron and GaO6 octahedron are shown, respectively.
[ZnO4] tetrahedron and [GaO4] tetrahedron, as shown in Fig. 1. It can be seen that GaO4 and ZnO4 tetrahedra (partial disorder) share vertices to form a three-dimensional and additional Ga and Ca in octahedral voids. Thus, this structure can be designed to provide ideal cation sites and coordination polyhedra for Cr3 þ . Supposing that Cr3 þ could be doped into the host, it tends to substitute for gallium sites in GaO6 as a result of matched size and charge. Indeed, Ca14Zn6Ga10O35 was firstly prepared and its crystal structure was reported by S. Ya. Istomin [22], Ca14Zn6M10O35: Mn4 þ (M¼ Al3 þ and Ga3 þ ) phosphors were reported to present a deep red emission [23]. In our previous work, we prepared its isomorphic material Ca14Zn6Al10O35 and found Cr3 þ doped Ca14Zn6Al10O35 phosphor gave an excellent NIR LPL [24]. In this paper, we prepared the Cr3 þ -doped gallate phosphor Ca14Zn6Ga10O35:Cr3 þ (CZGO:Cr) by the simple solid-state method, and highlighted the NIR LPL. The shape, height, and location of the TL glow curves were investigated in order to obtain the information on the trap density and depth of traps and the kinetics of the trapping and detrapping processes. Furthermore, we modified the composition around Cr3 þ by adjusting the Ga/Al content in order to regulate the trap depth. Besides, a study in regard to trap types can be done by performing lowtemperature ESR experiments. The Cr3 þ -doped gallate phosphor might be a potential alternative to Cr3 þ -activated gallate materials for bio-imaging applications.
2. Experimental procedures 2.1. Synthesis The samples were prepared by a simple solid state reaction. CaCO3 (99.99%), Ga2O3 (99.99%), ZnO (99.99%) and Cr2O3 (99.99%) were used as starting materials. After the raw materials were weighed according to the composition of Ca14Zn6Ga10(1 x)O35: xCr3 þ (CZGO:xCr) (x ¼0, 0.05%, 0.15%, 0.25%, 0.35%, 0.50%, and 1%, etc.), the powders were mixed and milled thoroughly for 1 h in an agate mortar and prefired at 900 °C for 8 h in air. After ground again, the mixtures were sintered at 1250 °C for 10 h. After being cooled down to room temperature naturally, the as-prepared powder samples were obtained.
2.2. Characterization of samples The phase purity of the prepared phosphors was measured by an X-ray diffractometer with Cu Ka radiation (wavelength¼0.15406 nm) at 36 kV tube voltage and 20 mA tube current. The excitation and emission spectra of all the samples were measured by a Hitachi F-7000 Fluorescence Spectrophotometer equipped with a 150 W xenon lamp as excitation source. The decay curves were measured by a GFZF-2A single-photo-counter system. The thermoluminescence (TL) spectrum was measured with a FJ-427A1 thermoluminescence meter. Prior to the persistent luminescence spectra, decay curve and glow curve measurements, the samples were excited for 1 min by a 15 W low-pressure mercury discharge lamp (254 nm). Electron spin resonance spectra were recorded by using a Bruker A 300 ESR spectrometer operating at X-band microwave frequency for studying paramagnetic defects. NIR imaging was performed with a modified imaging system including a Germany Pco Dicam camera as the signal collector.
3. Results and discussion 3.1. Phase characterization The XRD patterns of the samples, CZGO:xCr, are shown in Fig. 2. A single phase of CZGO:xCr is obtained and all the diffraction peaks are in good agreement with the Ca14Zn6Ga10O35 (JCPDF#580031), indicating that Cr3 þ ions are successfully dissolved into the CZGO host lattice while maintaining the crystal structure intact. When Cr3 þ is doped into the CZGO host, it tends to substitute for the hexacoordinated Ga3 þ sites (GaO6) since the size and charge between Ga3 þ (r ¼0.62 Å with CN ¼6) and Cr3 þ (r ¼0.62 with CN ¼6) are absolutely identical; however, the tetracoordinated Ga3 þ sites (GaO4) is hardly occupied by the doped Cr3 þ ions because Cr3 þ could not be stabilized in a tetrahedral lattice site [25,26]. To further analyze the crystal structure details of the as-prepared powder, Rietveld refinements of CZGO:Cr sample were performed based on the initial structural mode of CZGO. The calculated and experimental results as well as their differences in the XRD refinement of CZGO:Cr are shown in Fig. 2 (b). The results further demonstrate that the Cr3 þ -doped CZGO sample does not generate any impurity or secondary phases. The
F. Sun et al. / Journal of Luminescence 180 (2016) 251–257
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Fig. 3. PLE (λem ¼711 nm) and PL (λex ¼254 nm) spectra of CZGO:0.5%Cr at room temperature. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Fig. 2. (a) XRD patterns of CZGO:xCr (x¼ 0.05%, 0.15%, 0.25%, 0.35% and 0.50%); (b) observed (crosses) and calculated (solid lines) XRD patterns of the Rietveld refinement of the as-prepared CZGO:Cr sample.
refinement started with the crystallographic data of CZGO and it converged with the residual values of Rp ¼ 8.68%, Rwp ¼6.69%, Rexp ¼4.52% and GOF ¼ 2.51. All the refinement factors (Rp, Rwp and Rexp are less than 10%) revealed a good quality of fitting. For CZGO: Cr, it belongs to a cubic system with the space group F23(196), the lattice parameters were fitted to be a ¼15.075(2) Å, a/b¼ 1, b/c ¼1, V¼ 3425.87 Å3, and Z¼4. The refining results confirm Cr3 þ prefers to substitute for the gallium site in GaO6 octahedron rather than that in GaO4 tetrahedron as well. 3.2. Photoluminescence properties Photoluminescence excitation and emission (PLE and PL) spectra should be analyzed first, in order to identify which activator is taking part in the luminescent process. The PLE spectrum of Cr3 þ -doped gallate phosphor CZGO:Cr (Fig. 3, black solid) exhibits four broad excitation bands with maximum at 254, 304, 408, and 542 nm. The excitation band at 254 nm is attributed to the contribution of both host absorption and the O2 -Cr3 þ charge transfer band [27]. The other three band at 304 nm (4A2g-4T1g (4P)), 408 nm (4A2g-4T1 (4F)) and 542 nm (4A2g-4T2g (4F)) originate from the 3d intrashell transitions of Cr3 þ . Excitation of dispersion of CZGO:Cr powders gives a NIR emission at 711 nm (Fig. 3, red solid) due to the 2Eg-4A2g transition in CZGO host. Under different excitation wavelengths such as 304, 408 and 542 nm, CZGO:Cr phosphor exhibits the NIR emission with the same shapes and positions except for the intensity of 2Eg-4A2g transition in Fig. 3(b) (red solid). The emission consists of one
Fig. 4. (a) The afterglow emission and (b) images of CZGO:0.5%Cr at different time, respectively.
strong line and the other weak shoulder line located at 699.7 nm (1.42 103 cm 1) and 706.1 nm (1.41 103 cm 1), respectively. Note that the NIR emission (2Eg-4A2g) is electronic dipole forbidden in the octahedral GaO6 but gains intensity by the activation of vibronic modes in the centers, resulting in the appearance of the Stokes and anti-Stokes lines [28,29]. As far as we know, a host material with lower crystal symmetry exhibits stronger zerophonon line (ZPL) emission intensity. Compared with the classic R1, R2 and N2 lines classically observed from ZnGa2O4:Cr3 þ [30,31], these two narrow zero-phonon lines observed at 1.42 103 cm 1 and 1.41 103 cm 1, labeled respectively as R2 and R1, correspond to the 2Eg-4A2 transitions of Cr3 þ occupying the octahedral site (m (D3h) trigonal site symmetry) of an ideal spinel structure. The Stokes and Anti-Stokes parts of the vibronic side bands of the R lines are not observed to the expected positions at room temperature. 3.3. Long-persistent phosphorescence of CZAO:Cr Not only the steady-state emission spectrum during excitation, but also the afterglow emission spectrum needs to be investigated. As shown in Fig. 4, the afterglow spectra of CZGO:0.5%Cr are the same in shape and position to the emission spectrum, indicating that Cr3 þ ions take part in the persistent luminescence; To more visually present the afterglow emission of CZGO:0.5%Cr, the digital
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exponential function: t t IðtÞ ¼ I 1 exp þI 2 exp
τ1
τ2
ð1Þ
where I(t) is the persistent luminescence intensity at time t ; I1 and I2 are constants, τ1 and τ2 are the decay times for the short and long decay components, respectively. In other words, these decay curves consist of a fast and a consequent slow decay with a long decay tail, implying the existence of various trap depths. The energy distribution of traps will be discussed in the following Section 3.4. In order to evaluate the phosphorescence of CZGO:Cr phosphor, a comparison between CZGO:Cr and the well-known phosphor ZnGa2O4:Cr (ZGO:Cr) was made, as shown in the inset of Fig. 5. We can see that the persistent luminescence of CZGO:Cr phosphor is superior to that of ZGO:Cr, indicating its potential application for in vivo imaging. 3.4. Trap properties of CZGO:Cr
Fig. 5. (a) Phosphorescence intensity after 0.5 min monitored at 711 nm emission as a function of excitation wavelength for CZGO:0.5%Cr (sphere) along with the PLE spectrum of CZGO:0.5%Cr (dotted); (b) LPP decay curves of CZGO:0.5% Cr (x ¼0.1%, 0.2%, 0.5% ,1% and 2%) and inset shows the comparison of decay curves between CZGO:Cr sample and ZnGa2O4:Cr phosphor.
images (inset in Fig. 4) of CZGO:0.5%Cr are recorded at different time intervals after being irradiated by using the a xenon lamp. To better understand the effectiveness of different excitation wavelengths (energies) for NIR LPP, the relation between LPP intensity and excitation wavelengths was investigated. Fig. 5(a) shows the persistent emission intensities (I0.5 min) as a function of the different excitation (I0.5 min, recorded at 0.5 min after ceasing irradiation, was used as the references). The PLE spectrum of CZGO:0.5%Cr is also presented in Fig. 5(a) for comparison. From Fig. 5(a), one can see that the persistent luminescence can only be less excited in the wavelength region from 400 to 600 nm, but can be effectively achieved by UV (250–350 nm) illumination. The reason for this phenomenon is that electrons are barely pumped into the conduct band under visible light (400–600 nm) excitation, and then are impossibly captured by the electron traps. In order to prove this material can be promising one for bio-imaging, LPP decay curves were recorded monitored at 711 nm emission after irradiation by a 254-nm UV light for 3 min. LPP decay curves of CZGO:xCr (x ¼0.1%, 0.2%, 0.5% ,1% and 2%) are shown in Fig. 5(b). From Fig. 5(b), we can see that the optimal concentration of CZGO: xCr phosphors for persistent luminescence is about 0.5%. All the afterglow decay curves can be well fitted by the double-
In order to conduct a prospective study to explore the nature of traps and hence to evaluate the trap properties, diverse and systematic investigation was performed. Generally, the thermoluminescence (TL) method is considered as an efficient tool to probe the traps, such as the depth, types and density of traps in host material [25,26]. The appearance of a peak in the emitted light intensity (glow curves) indicates the presence of a charge carrier trap in this material. The temperature at which this peak is located is a measure for the trap depth, since it is the temperature at which enough thermal energy is available for the trapped charge carriers to be released and recombine at luminescent centers. Generally, the shape, height and location of the peak contain an vital information on the number and depth of the traps and the kinetics of the trapping and detrapping processes. Clearly, knowing the depth and density of traps is crucial when trying to understand the mechanism of persistent luminescence, and when developing new afterglow materials. Furthermore, how the trap depth distribution presents is also vital to reveal the mysteries behind the persistent phosphors. To gain insight into the trapping and detrapping processes involved in the persistent luminescence of CZGO:Cr, we conducted a series of TL measurements by varying the excitation time after duration and the thermal cleaning temperature. Fig. 6(a) shows the influence of the excitation duration on the TL glow curves for CZGO:Cr. Obviously, the TL intensity increased with an increase in the excitation duration from 10 to 300 s, indicating that a larger number carries are captured by the traps at longer excitation duration. The location of the maximum of the TL glow curves shows a slight towards lower temperatures. For shorter excitation duration (60 s), the peak maximum is located at 370 K, while it shifts to 348 K for longer duration (360 s). This shift presents that the trap system is more complicated than a single trap obeying the first order kinetics. To study the presence of a trap depth distribution, it is interesting to vary the temperature at which the sample is excited. If a phosphor is excited at a higher temperature with a continuous trap depth distribution, only deeper fractions of the traps are filled. The shallower traps are immediately bleached due to the increased thermal energy available [32,33]. TL glow curves of CZGO:Cr sample were collected after preheated at different preheating temperature in Fig. 6(b). This sample was first irradiated for 3 min at 300 K and then heated to a certain temperature (Tstop) to partially clean the occupied traps. From Fig. 6(b), the gradual deepening of the trap depth for increasing Tstop proves the presence of a continuous trap distribution in CZGO:Cr phosphor. The shape, height, and location of the TL glow curves contain information on the trap density and
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Fig. 6. (a) TL intensity of CZGO:0.5%Cr after 254 nm excitation with various duration (15, 30 and 120 s) at a heating rate of 5 K/s; (b) TL intensity of CZGO:0.5%Cr for different excitation temperatures. CZGO:0.5%Cr sample was excited at different excited temperature under 254 nm irradiation.
Fig. 7. (a) Initial rise analysis of the corresponding TL glow curves; (b) Trap depth distribution of CZGO:0.5%Cr sample.
depth of traps and the kinetics of the trapping and detrapping processes. An initial rising method, proposed by Van den Eeckhout et al. [33], is further applied to the measured TL curves to uncover how the trap depth of CZGO:0.5%Cr phosphor distributes. Briefly, this method for estimating trap depths starts from a assumption that the concentration of trapped electron on the low-temperature side of the TL curves remains relatively constant; only a tiny fraction of the charge carriers can escaped under the small amount of thermal energy available. Thus, the TL intensity (I(t)) can be approximately expressed as: E IðtÞ ¼ C exp ð2Þ kT where C is the constant including the frequency factor s, and k is the Boltzmann constant [33]. By plotting the TL curves as ln(I) vs 1000/T, i.e., in an Arrhenius plot, the shallowest trap depth can be readily estimated by the slope of a fitted straight section at the low-temperature side. The transformed Arrhenius plots for each curves in Fig. 6(b) were presented in Fig. 7(a). Obviously, all the plots give a straight section in the low-temperature side, indicating that our assumption is valid for the initial rising analysis. From the fitted slope, the depth of the shallowest trap can be obtained after different thermal cleaning temperature. As shown in Fig. 7(b), the estimated trap depths gradually deepens from 0.685 eV to 0.720 eV, demonstrating the continuous distribution of the trap depth, thus. In Fig. 7(b), the trap density between two adjacent depths can be roughly calculated from the difference between the
integrated intensities of the corresponding two TL curves. The depth of TL band are calculated to be close to the ideal trap depth (0.6–0.8 eV) for the release at room temperature of the energy storage [32–34]. Thus, CZGO:Cr phosphor can present a long NIR persistent luminescence. As far as we know, effects, either intrinsic or extrinsic defects or both of them, play an important role in energy storage of persistent luminescence. Therefore, understanding of defect properties is indispensable for further development and applications of certain potential persistent phosphors. In the structure of Ca14Zn6Ga10O35, there are much empty voids, which could be partially occupied by additional oxygen atoms upon substitution of Zn2 þ by Ga3 þ . As compared to other defects, these defects require much lower formation energy (0.9 eV) according to Pandey et al. [35]: 0 : Zn Zn þ Ga XGa -ZnGa þ GaZn
ð3Þ
Then, the charge balance can be ensured by the oxygen vacancies (V0O ) and additional charge-balanced pairs of Zn0Ga =Ga:Zn will be created. These oxygen vacancies are introduced into the structure of Ca14Zn6Ga10O35 as a part of four tetrahedral (Zn/Ga)O4 groups sharing common vertex. This creates a situation where even a minor change leads to considerable anion and cation disordering resulting in a GaO6 octahedra [22]. To demonstrate the above vacancies, a study in regard to trap types can be done by performing ESR experiments. ESR spectra of CZGO:0.5%Cr3 þ sample at different time intervals (30 min, 1 h and 3 h) after being irradiated by using an xenon lamp for 3 min were measured in
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Fig. 8. Electron spin resonance of CZGO:0.5%Cr sample measured at 100 K after 254 nm irradiation at different interval time of 30 min, 1 h and 2 h after the stoppage of irradiation.
Fig. 10. The schematic diagram of the phosphorescence mechanism in CZGO:Cr. ( represents electrons and ○ refers holes; dotted arrow shows carrier relaxation or thermal motion).
toward higher temperature might be caused by the variation in the energy level location of the conduct band due to the substitution of Al for Ga in this phosphors [36]. However, it is difficult to estimate the independent effect of an increasing energy gap between the conduct band and the trap site level, especially considering the disordered variation in persistent time. Thus, additional investigation is recommended in the future. 3.5. Possible mechanism of long persistent luminescence
Fig. 9. Normalized TL glow curves as a function of Al doping concentration after 254 nm irradiation at room temperature.
Fig. 8. For comparison, the ESR signals of CZGO host were also measured in Fig. 8. Signals with g ¼1.99 and g ¼3.96 can be detected after the irradiation. However, we only detect a signals with g ¼1.99 of CZGO host after the irradiation. Considering the value of the weak signal is close to the g-factor (g ¼2) of single electron ion, it should be attributed to the antisite defects such Zn0Ga and Ga:Zn that can capture an electron under UV irradiation [31]. The stronger signal in the ESR spectra might be attributed to the isolated Cr3 þ centers at strongly distorted octahedral sites. Thus, this afterglow phenomenon mainly originates from the extrinsic defects caused by the doped Cr3 þ ions. Engineering a suitable trap depth is essential for achieving room-temperature persistent phosphorescence. A trap which is too shallow will result in a very short afterglow; if a trap is too deep no charge carriers can escape at room temperature and no persistent luminescence will be observed unless the temperature is raised. The TL experiments are useful for evaluating trap depths. The shift of a TL peak to higher temperature usually indicates that the ratio of deep traps to total traps has increased. Fig. 9 shows the normalized TL curves for CZG1 xO:0.5%Cr, xAl samples measured 1 min after irradiation ceased. It is clear that the TL peaks shift to a higher temperature from 365 to 386 K with increasing Al3 þ concentration; that is, the proportion of deep traps grows, indicating that Al dissolution is beneficial in adjusting trap depths. The TL peaks of CZG1 xO:0.5%Cr, xAl phosphors monotonically shifting
To understand the dynamical process in persistent luminescence, a schematic graph based on the above results is proposed and illustrated in Fig. 10. Just for illustration purpose, the Cr3 þ energy levels are placed into the middle of the forbidden zone to discuss the afterglow luminescence. After irradiation with the ultraviolet light , the ground electrons of Cr3 þ are excited along path 1 to three different energy levels of excited-states (4T1(te2)), (4T1(t2e)) and (4T2). The majority of excited electrons will be back and then relax to 2E level in non-radiative way. The subsequent jumping of electrons to the ground levels 4A2 of Cr3 þ and recombination of holes lead to the characteristic emission of Cr3 þ ions (process numbered 2 and 3). These processes present photoluminescence emission of CZGO:Cr3 þ . However, the residual minority excited electrons at the excited-state of 4T1 (te2) relax to the lower end of the conduction band and then are captured by the electron traps through the non-radiative (path 4 and 5). In the initial stage of the afterglow luminescence, the electrons captured by traps escape thermally via the conduction and are transferred to the activators Cr3 þ (process numbered 4, 2 and 3), resulting in the initial persistent emission. Subsequently, the electrons trapped in deeper traps might tunnel to the shallower traps. The slow reverse tunneling recombination for electrons released from deeper traps probably causes a weak but long persistent luminescence (process numbered 4, 2 and 3).
4. Conclusion In conclusion, a novel near-NIR persistent phosphor CZAO:Cr3 þ was prepared by the simple solid-state reaction. CZAO:Cr phosphor gives a long persistent NIR luminescence, which makes it potential candidates for their application in security, dark/night vision, or medical imaging. The doped Cr3 þ ions prefer to occupy the octahedral sites of Ga3 þ in GaO6; the afterglow duration can
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last for 3 hour. The NIR LPP can be effectively excited by UV light (250–350 nm) but is hardly achieved by visible light (400– 600 nm); the estimated trap depth for the TL glow peaks is about 0.684 and 0.748 eV. At last, the persistent luminescence mechanism of CZAO:Cr is illustrated and discussed on the basis of the experimental results.
Acknowledgments This work was financially supported by the Natural Science Foundation of Guangdong province (No. S2013040013904).
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Full Length Article
Cr3 þ doped Ca14Zn6Ga10O35: A near-infrared long persistent luminescence phosphor Fuqiang Sun n, Rongrong Xie, Li Guan, Canyuan Zhang School of Pharmacy, Guangdong Pharmaceutical University, Guangzhou 510006, China
art ic l e i nf o
a b s t r a c t
Article history: Received 27 March 2016 Received in revised form 2 July 2016 Accepted 18 August 2016 Available online 23 August 2016
The near-infrared persistent phosphors have attracted increasing attention due to the potential application in in vivo imaging. Exploring new materials to realize efficient near-infrared persistent luminescence is a goal of general concern. Ca14Zn6Ga10O35:Cr3 þ phosphor gives a near-infrared long persistent luminescence over 3 h with the 650–750 nm emission range after the short UV-irradiation. The afterglow behaviors such as the persistent luminescence, the trap depth distribution, the trap types and the underlying mechanism for persistent luminescence, were comprehensively surveyed by means of thermoluminescence methods and electron spin resonance spectra. In addition, Al dopant was introduced to redeploy the trap distribution in Ca14Zn6Ga10O35:Cr3 þ phosphor. Compared with the wellknown phosphor ZnGa2O4:Cr3 þ , Ca14Zn6Ga10O35:Cr3 þ is proved to be a new NIR persistent phosphor potentially suitable for in vivo imaging due to its 650–750 nm emission range. & 2016 Elsevier B.V. All rights reserved.
Keywords: Persistent phosphor Near-infrared phosphorescence Optical materials Cr3 þ
1. Introduction Long persistence luminescence (LPL) is the occurrence of luminescence which can last for the several minutes or hours after the stoppage of excitation. To date, long persistent phosphors (LPPs) have been received more attention and interests due to much wide application, e.g. , such as safety signage, displays, and exit signs, optical energy media, thermal sensors and in vivo bioimaging [1–6]. Especially for the application in in vivo bio-imaging, LPPs in the near infrared (NIR) region (650–1350 nm) have been developed as a promising category of luminescent labels, since their emission lifetime is sufficiently long to permit late timegated imaging [7–9]. In such a case, auto-fluorescence from tissue organic components during imaging, resulting in an extraordinary high signal-to-noise ratio, can be completely avoided [10,11]. Thus, a LPP probe is expected to open up the possibility of advanced optical imaging with high resolution and weak disturbance for factually assessing the structural and functional processes in cells, tissues, and other complex systems [12–15]. For NIR luminescence, trivalent chromium ion (Cr3 þ ) is an ideal activator in hosts since its 3d electron configuration allows a narrow-band emission (ca. 700 nm) due to the spin-forbidden 2 E-4A2 transition, or a broad emission (650–1600 nm) ascribed to the spin-allowed 4T2-4A2 transition, which strongly depends on the n
Corresponding author. Fax: þ 86 20 39352141. E-mail address: [email protected] (F. Sun).
http://dx.doi.org/10.1016/j.jlumin.2016.08.043 0022-2313/& 2016 Elsevier B.V. All rights reserved.
crystal-field environment of the host [16]. In design of NIR phosphors, Cr3 þ is a favorable choice of activation center since the LPL of Cr3 þ -activated phosphors, ranging from about 600 to 1300 nm in NIR region, meets the requirement of in vivo bioimaging. The past few years have witnessed great progress in the development of the LPP probe in in vivo bioimaging, with the main focus on the Cr3 þ activated gallate materials [11–20]. The Zn1 þ xGa2 2xGexO4 system, i.e., a solid solution between the normal ZnGa2O4 and inversed Zn2GeO4 spinle structure has been attracting much attention since the LPL of Cr3 þ -activated Zn1 þ xGa2-2xGexO4 phosphors and established gallates as the preferred material system for the fabrication of Cr3 þ -activated NIR LPP [21]. For example, Pan et al. achieved a super-long NIR afterglow emission time of 360 h in Zn1.5GaGe0.5O4: Cr3 þ (x¼ 0.5) phosphor [17]; Allix et al. gained a considerable improvement of NIR long-persistent luminescence in germanium and tin substituted ZnGa2O4:Cr3 þ [18]. Besides La3Ga5GeO14:Cr3 þ [19], LiGa5O8:Cr3 þ [20], the LPL of Cr3 þ -activated phosphors, such as Zn1.5GaSn0.5O4:Cr3 þ [6], Ca3Ga2Ge3O12:Cr3 þ [21], were investigated due to the strong ability of Cr3 þ to substitute for Ga3 þ in octahedral coordination. As far as we know, the Cr3 þ -doped gallate phosphors have a common ground, in which a octahedral site is occupied by Cr3 þ . In other words, Cr3 þ can be better accommodated and stabilized in an octahedral lattice site. This gives us a hint to explore the ideal Cr3 þ -doped host. Therefore, we select the Cr3 þ ions as the activators and the gallate (Ca14Zn6Ga10O35) as the target host due to its unique crystal structure [22]. The basic structural units of Ca14Zn6Ga10O35 (CZGO) are composed of [Ga/ZnO6] octahedron,
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Fig. 1. (a) View of the structure of CZGO from c-projection; (b) GaO4 tetrahedron and GaO6 octahedron are shown, respectively.
[ZnO4] tetrahedron and [GaO4] tetrahedron, as shown in Fig. 1. It can be seen that GaO4 and ZnO4 tetrahedra (partial disorder) share vertices to form a three-dimensional and additional Ga and Ca in octahedral voids. Thus, this structure can be designed to provide ideal cation sites and coordination polyhedra for Cr3 þ . Supposing that Cr3 þ could be doped into the host, it tends to substitute for gallium sites in GaO6 as a result of matched size and charge. Indeed, Ca14Zn6Ga10O35 was firstly prepared and its crystal structure was reported by S. Ya. Istomin [22], Ca14Zn6M10O35: Mn4 þ (M¼ Al3 þ and Ga3 þ ) phosphors were reported to present a deep red emission [23]. In our previous work, we prepared its isomorphic material Ca14Zn6Al10O35 and found Cr3 þ doped Ca14Zn6Al10O35 phosphor gave an excellent NIR LPL [24]. In this paper, we prepared the Cr3 þ -doped gallate phosphor Ca14Zn6Ga10O35:Cr3 þ (CZGO:Cr) by the simple solid-state method, and highlighted the NIR LPL. The shape, height, and location of the TL glow curves were investigated in order to obtain the information on the trap density and depth of traps and the kinetics of the trapping and detrapping processes. Furthermore, we modified the composition around Cr3 þ by adjusting the Ga/Al content in order to regulate the trap depth. Besides, a study in regard to trap types can be done by performing lowtemperature ESR experiments. The Cr3 þ -doped gallate phosphor might be a potential alternative to Cr3 þ -activated gallate materials for bio-imaging applications.
2. Experimental procedures 2.1. Synthesis The samples were prepared by a simple solid state reaction. CaCO3 (99.99%), Ga2O3 (99.99%), ZnO (99.99%) and Cr2O3 (99.99%) were used as starting materials. After the raw materials were weighed according to the composition of Ca14Zn6Ga10(1 x)O35: xCr3 þ (CZGO:xCr) (x ¼0, 0.05%, 0.15%, 0.25%, 0.35%, 0.50%, and 1%, etc.), the powders were mixed and milled thoroughly for 1 h in an agate mortar and prefired at 900 °C for 8 h in air. After ground again, the mixtures were sintered at 1250 °C for 10 h. After being cooled down to room temperature naturally, the as-prepared powder samples were obtained.
2.2. Characterization of samples The phase purity of the prepared phosphors was measured by an X-ray diffractometer with Cu Ka radiation (wavelength¼0.15406 nm) at 36 kV tube voltage and 20 mA tube current. The excitation and emission spectra of all the samples were measured by a Hitachi F-7000 Fluorescence Spectrophotometer equipped with a 150 W xenon lamp as excitation source. The decay curves were measured by a GFZF-2A single-photo-counter system. The thermoluminescence (TL) spectrum was measured with a FJ-427A1 thermoluminescence meter. Prior to the persistent luminescence spectra, decay curve and glow curve measurements, the samples were excited for 1 min by a 15 W low-pressure mercury discharge lamp (254 nm). Electron spin resonance spectra were recorded by using a Bruker A 300 ESR spectrometer operating at X-band microwave frequency for studying paramagnetic defects. NIR imaging was performed with a modified imaging system including a Germany Pco Dicam camera as the signal collector.
3. Results and discussion 3.1. Phase characterization The XRD patterns of the samples, CZGO:xCr, are shown in Fig. 2. A single phase of CZGO:xCr is obtained and all the diffraction peaks are in good agreement with the Ca14Zn6Ga10O35 (JCPDF#580031), indicating that Cr3 þ ions are successfully dissolved into the CZGO host lattice while maintaining the crystal structure intact. When Cr3 þ is doped into the CZGO host, it tends to substitute for the hexacoordinated Ga3 þ sites (GaO6) since the size and charge between Ga3 þ (r ¼0.62 Å with CN ¼6) and Cr3 þ (r ¼0.62 with CN ¼6) are absolutely identical; however, the tetracoordinated Ga3 þ sites (GaO4) is hardly occupied by the doped Cr3 þ ions because Cr3 þ could not be stabilized in a tetrahedral lattice site [25,26]. To further analyze the crystal structure details of the as-prepared powder, Rietveld refinements of CZGO:Cr sample were performed based on the initial structural mode of CZGO. The calculated and experimental results as well as their differences in the XRD refinement of CZGO:Cr are shown in Fig. 2 (b). The results further demonstrate that the Cr3 þ -doped CZGO sample does not generate any impurity or secondary phases. The
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Fig. 3. PLE (λem ¼711 nm) and PL (λex ¼254 nm) spectra of CZGO:0.5%Cr at room temperature. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Fig. 2. (a) XRD patterns of CZGO:xCr (x¼ 0.05%, 0.15%, 0.25%, 0.35% and 0.50%); (b) observed (crosses) and calculated (solid lines) XRD patterns of the Rietveld refinement of the as-prepared CZGO:Cr sample.
refinement started with the crystallographic data of CZGO and it converged with the residual values of Rp ¼ 8.68%, Rwp ¼6.69%, Rexp ¼4.52% and GOF ¼ 2.51. All the refinement factors (Rp, Rwp and Rexp are less than 10%) revealed a good quality of fitting. For CZGO: Cr, it belongs to a cubic system with the space group F23(196), the lattice parameters were fitted to be a ¼15.075(2) Å, a/b¼ 1, b/c ¼1, V¼ 3425.87 Å3, and Z¼4. The refining results confirm Cr3 þ prefers to substitute for the gallium site in GaO6 octahedron rather than that in GaO4 tetrahedron as well. 3.2. Photoluminescence properties Photoluminescence excitation and emission (PLE and PL) spectra should be analyzed first, in order to identify which activator is taking part in the luminescent process. The PLE spectrum of Cr3 þ -doped gallate phosphor CZGO:Cr (Fig. 3, black solid) exhibits four broad excitation bands with maximum at 254, 304, 408, and 542 nm. The excitation band at 254 nm is attributed to the contribution of both host absorption and the O2 -Cr3 þ charge transfer band [27]. The other three band at 304 nm (4A2g-4T1g (4P)), 408 nm (4A2g-4T1 (4F)) and 542 nm (4A2g-4T2g (4F)) originate from the 3d intrashell transitions of Cr3 þ . Excitation of dispersion of CZGO:Cr powders gives a NIR emission at 711 nm (Fig. 3, red solid) due to the 2Eg-4A2g transition in CZGO host. Under different excitation wavelengths such as 304, 408 and 542 nm, CZGO:Cr phosphor exhibits the NIR emission with the same shapes and positions except for the intensity of 2Eg-4A2g transition in Fig. 3(b) (red solid). The emission consists of one
Fig. 4. (a) The afterglow emission and (b) images of CZGO:0.5%Cr at different time, respectively.
strong line and the other weak shoulder line located at 699.7 nm (1.42 103 cm 1) and 706.1 nm (1.41 103 cm 1), respectively. Note that the NIR emission (2Eg-4A2g) is electronic dipole forbidden in the octahedral GaO6 but gains intensity by the activation of vibronic modes in the centers, resulting in the appearance of the Stokes and anti-Stokes lines [28,29]. As far as we know, a host material with lower crystal symmetry exhibits stronger zerophonon line (ZPL) emission intensity. Compared with the classic R1, R2 and N2 lines classically observed from ZnGa2O4:Cr3 þ [30,31], these two narrow zero-phonon lines observed at 1.42 103 cm 1 and 1.41 103 cm 1, labeled respectively as R2 and R1, correspond to the 2Eg-4A2 transitions of Cr3 þ occupying the octahedral site (m (D3h) trigonal site symmetry) of an ideal spinel structure. The Stokes and Anti-Stokes parts of the vibronic side bands of the R lines are not observed to the expected positions at room temperature. 3.3. Long-persistent phosphorescence of CZAO:Cr Not only the steady-state emission spectrum during excitation, but also the afterglow emission spectrum needs to be investigated. As shown in Fig. 4, the afterglow spectra of CZGO:0.5%Cr are the same in shape and position to the emission spectrum, indicating that Cr3 þ ions take part in the persistent luminescence; To more visually present the afterglow emission of CZGO:0.5%Cr, the digital
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exponential function: t t IðtÞ ¼ I 1 exp þI 2 exp
τ1
τ2
ð1Þ
where I(t) is the persistent luminescence intensity at time t ; I1 and I2 are constants, τ1 and τ2 are the decay times for the short and long decay components, respectively. In other words, these decay curves consist of a fast and a consequent slow decay with a long decay tail, implying the existence of various trap depths. The energy distribution of traps will be discussed in the following Section 3.4. In order to evaluate the phosphorescence of CZGO:Cr phosphor, a comparison between CZGO:Cr and the well-known phosphor ZnGa2O4:Cr (ZGO:Cr) was made, as shown in the inset of Fig. 5. We can see that the persistent luminescence of CZGO:Cr phosphor is superior to that of ZGO:Cr, indicating its potential application for in vivo imaging. 3.4. Trap properties of CZGO:Cr
Fig. 5. (a) Phosphorescence intensity after 0.5 min monitored at 711 nm emission as a function of excitation wavelength for CZGO:0.5%Cr (sphere) along with the PLE spectrum of CZGO:0.5%Cr (dotted); (b) LPP decay curves of CZGO:0.5% Cr (x ¼0.1%, 0.2%, 0.5% ,1% and 2%) and inset shows the comparison of decay curves between CZGO:Cr sample and ZnGa2O4:Cr phosphor.
images (inset in Fig. 4) of CZGO:0.5%Cr are recorded at different time intervals after being irradiated by using the a xenon lamp. To better understand the effectiveness of different excitation wavelengths (energies) for NIR LPP, the relation between LPP intensity and excitation wavelengths was investigated. Fig. 5(a) shows the persistent emission intensities (I0.5 min) as a function of the different excitation (I0.5 min, recorded at 0.5 min after ceasing irradiation, was used as the references). The PLE spectrum of CZGO:0.5%Cr is also presented in Fig. 5(a) for comparison. From Fig. 5(a), one can see that the persistent luminescence can only be less excited in the wavelength region from 400 to 600 nm, but can be effectively achieved by UV (250–350 nm) illumination. The reason for this phenomenon is that electrons are barely pumped into the conduct band under visible light (400–600 nm) excitation, and then are impossibly captured by the electron traps. In order to prove this material can be promising one for bio-imaging, LPP decay curves were recorded monitored at 711 nm emission after irradiation by a 254-nm UV light for 3 min. LPP decay curves of CZGO:xCr (x ¼0.1%, 0.2%, 0.5% ,1% and 2%) are shown in Fig. 5(b). From Fig. 5(b), we can see that the optimal concentration of CZGO: xCr phosphors for persistent luminescence is about 0.5%. All the afterglow decay curves can be well fitted by the double-
In order to conduct a prospective study to explore the nature of traps and hence to evaluate the trap properties, diverse and systematic investigation was performed. Generally, the thermoluminescence (TL) method is considered as an efficient tool to probe the traps, such as the depth, types and density of traps in host material [25,26]. The appearance of a peak in the emitted light intensity (glow curves) indicates the presence of a charge carrier trap in this material. The temperature at which this peak is located is a measure for the trap depth, since it is the temperature at which enough thermal energy is available for the trapped charge carriers to be released and recombine at luminescent centers. Generally, the shape, height and location of the peak contain an vital information on the number and depth of the traps and the kinetics of the trapping and detrapping processes. Clearly, knowing the depth and density of traps is crucial when trying to understand the mechanism of persistent luminescence, and when developing new afterglow materials. Furthermore, how the trap depth distribution presents is also vital to reveal the mysteries behind the persistent phosphors. To gain insight into the trapping and detrapping processes involved in the persistent luminescence of CZGO:Cr, we conducted a series of TL measurements by varying the excitation time after duration and the thermal cleaning temperature. Fig. 6(a) shows the influence of the excitation duration on the TL glow curves for CZGO:Cr. Obviously, the TL intensity increased with an increase in the excitation duration from 10 to 300 s, indicating that a larger number carries are captured by the traps at longer excitation duration. The location of the maximum of the TL glow curves shows a slight towards lower temperatures. For shorter excitation duration (60 s), the peak maximum is located at 370 K, while it shifts to 348 K for longer duration (360 s). This shift presents that the trap system is more complicated than a single trap obeying the first order kinetics. To study the presence of a trap depth distribution, it is interesting to vary the temperature at which the sample is excited. If a phosphor is excited at a higher temperature with a continuous trap depth distribution, only deeper fractions of the traps are filled. The shallower traps are immediately bleached due to the increased thermal energy available [32,33]. TL glow curves of CZGO:Cr sample were collected after preheated at different preheating temperature in Fig. 6(b). This sample was first irradiated for 3 min at 300 K and then heated to a certain temperature (Tstop) to partially clean the occupied traps. From Fig. 6(b), the gradual deepening of the trap depth for increasing Tstop proves the presence of a continuous trap distribution in CZGO:Cr phosphor. The shape, height, and location of the TL glow curves contain information on the trap density and
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Fig. 6. (a) TL intensity of CZGO:0.5%Cr after 254 nm excitation with various duration (15, 30 and 120 s) at a heating rate of 5 K/s; (b) TL intensity of CZGO:0.5%Cr for different excitation temperatures. CZGO:0.5%Cr sample was excited at different excited temperature under 254 nm irradiation.
Fig. 7. (a) Initial rise analysis of the corresponding TL glow curves; (b) Trap depth distribution of CZGO:0.5%Cr sample.
depth of traps and the kinetics of the trapping and detrapping processes. An initial rising method, proposed by Van den Eeckhout et al. [33], is further applied to the measured TL curves to uncover how the trap depth of CZGO:0.5%Cr phosphor distributes. Briefly, this method for estimating trap depths starts from a assumption that the concentration of trapped electron on the low-temperature side of the TL curves remains relatively constant; only a tiny fraction of the charge carriers can escaped under the small amount of thermal energy available. Thus, the TL intensity (I(t)) can be approximately expressed as: E IðtÞ ¼ C exp ð2Þ kT where C is the constant including the frequency factor s, and k is the Boltzmann constant [33]. By plotting the TL curves as ln(I) vs 1000/T, i.e., in an Arrhenius plot, the shallowest trap depth can be readily estimated by the slope of a fitted straight section at the low-temperature side. The transformed Arrhenius plots for each curves in Fig. 6(b) were presented in Fig. 7(a). Obviously, all the plots give a straight section in the low-temperature side, indicating that our assumption is valid for the initial rising analysis. From the fitted slope, the depth of the shallowest trap can be obtained after different thermal cleaning temperature. As shown in Fig. 7(b), the estimated trap depths gradually deepens from 0.685 eV to 0.720 eV, demonstrating the continuous distribution of the trap depth, thus. In Fig. 7(b), the trap density between two adjacent depths can be roughly calculated from the difference between the
integrated intensities of the corresponding two TL curves. The depth of TL band are calculated to be close to the ideal trap depth (0.6–0.8 eV) for the release at room temperature of the energy storage [32–34]. Thus, CZGO:Cr phosphor can present a long NIR persistent luminescence. As far as we know, effects, either intrinsic or extrinsic defects or both of them, play an important role in energy storage of persistent luminescence. Therefore, understanding of defect properties is indispensable for further development and applications of certain potential persistent phosphors. In the structure of Ca14Zn6Ga10O35, there are much empty voids, which could be partially occupied by additional oxygen atoms upon substitution of Zn2 þ by Ga3 þ . As compared to other defects, these defects require much lower formation energy (0.9 eV) according to Pandey et al. [35]: 0 : Zn Zn þ Ga XGa -ZnGa þ GaZn
ð3Þ
Then, the charge balance can be ensured by the oxygen vacancies (V0O ) and additional charge-balanced pairs of Zn0Ga =Ga:Zn will be created. These oxygen vacancies are introduced into the structure of Ca14Zn6Ga10O35 as a part of four tetrahedral (Zn/Ga)O4 groups sharing common vertex. This creates a situation where even a minor change leads to considerable anion and cation disordering resulting in a GaO6 octahedra [22]. To demonstrate the above vacancies, a study in regard to trap types can be done by performing ESR experiments. ESR spectra of CZGO:0.5%Cr3 þ sample at different time intervals (30 min, 1 h and 3 h) after being irradiated by using an xenon lamp for 3 min were measured in
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Fig. 8. Electron spin resonance of CZGO:0.5%Cr sample measured at 100 K after 254 nm irradiation at different interval time of 30 min, 1 h and 2 h after the stoppage of irradiation.
Fig. 10. The schematic diagram of the phosphorescence mechanism in CZGO:Cr. ( represents electrons and ○ refers holes; dotted arrow shows carrier relaxation or thermal motion).
toward higher temperature might be caused by the variation in the energy level location of the conduct band due to the substitution of Al for Ga in this phosphors [36]. However, it is difficult to estimate the independent effect of an increasing energy gap between the conduct band and the trap site level, especially considering the disordered variation in persistent time. Thus, additional investigation is recommended in the future. 3.5. Possible mechanism of long persistent luminescence
Fig. 9. Normalized TL glow curves as a function of Al doping concentration after 254 nm irradiation at room temperature.
Fig. 8. For comparison, the ESR signals of CZGO host were also measured in Fig. 8. Signals with g ¼1.99 and g ¼3.96 can be detected after the irradiation. However, we only detect a signals with g ¼1.99 of CZGO host after the irradiation. Considering the value of the weak signal is close to the g-factor (g ¼2) of single electron ion, it should be attributed to the antisite defects such Zn0Ga and Ga:Zn that can capture an electron under UV irradiation [31]. The stronger signal in the ESR spectra might be attributed to the isolated Cr3 þ centers at strongly distorted octahedral sites. Thus, this afterglow phenomenon mainly originates from the extrinsic defects caused by the doped Cr3 þ ions. Engineering a suitable trap depth is essential for achieving room-temperature persistent phosphorescence. A trap which is too shallow will result in a very short afterglow; if a trap is too deep no charge carriers can escape at room temperature and no persistent luminescence will be observed unless the temperature is raised. The TL experiments are useful for evaluating trap depths. The shift of a TL peak to higher temperature usually indicates that the ratio of deep traps to total traps has increased. Fig. 9 shows the normalized TL curves for CZG1 xO:0.5%Cr, xAl samples measured 1 min after irradiation ceased. It is clear that the TL peaks shift to a higher temperature from 365 to 386 K with increasing Al3 þ concentration; that is, the proportion of deep traps grows, indicating that Al dissolution is beneficial in adjusting trap depths. The TL peaks of CZG1 xO:0.5%Cr, xAl phosphors monotonically shifting
To understand the dynamical process in persistent luminescence, a schematic graph based on the above results is proposed and illustrated in Fig. 10. Just for illustration purpose, the Cr3 þ energy levels are placed into the middle of the forbidden zone to discuss the afterglow luminescence. After irradiation with the ultraviolet light , the ground electrons of Cr3 þ are excited along path 1 to three different energy levels of excited-states (4T1(te2)), (4T1(t2e)) and (4T2). The majority of excited electrons will be back and then relax to 2E level in non-radiative way. The subsequent jumping of electrons to the ground levels 4A2 of Cr3 þ and recombination of holes lead to the characteristic emission of Cr3 þ ions (process numbered 2 and 3). These processes present photoluminescence emission of CZGO:Cr3 þ . However, the residual minority excited electrons at the excited-state of 4T1 (te2) relax to the lower end of the conduction band and then are captured by the electron traps through the non-radiative (path 4 and 5). In the initial stage of the afterglow luminescence, the electrons captured by traps escape thermally via the conduction and are transferred to the activators Cr3 þ (process numbered 4, 2 and 3), resulting in the initial persistent emission. Subsequently, the electrons trapped in deeper traps might tunnel to the shallower traps. The slow reverse tunneling recombination for electrons released from deeper traps probably causes a weak but long persistent luminescence (process numbered 4, 2 and 3).
4. Conclusion In conclusion, a novel near-NIR persistent phosphor CZAO:Cr3 þ was prepared by the simple solid-state reaction. CZAO:Cr phosphor gives a long persistent NIR luminescence, which makes it potential candidates for their application in security, dark/night vision, or medical imaging. The doped Cr3 þ ions prefer to occupy the octahedral sites of Ga3 þ in GaO6; the afterglow duration can
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last for 3 hour. The NIR LPP can be effectively excited by UV light (250–350 nm) but is hardly achieved by visible light (400– 600 nm); the estimated trap depth for the TL glow peaks is about 0.684 and 0.748 eV. At last, the persistent luminescence mechanism of CZAO:Cr is illustrated and discussed on the basis of the experimental results.
Acknowledgments This work was financially supported by the Natural Science Foundation of Guangdong province (No. S2013040013904).
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