UV/VUV switch-driven color-reversal effect for Tb-activated phosphors

The remarkable narrow-band emission of trivalent lanthanide-doped phosphors excited by the vacuum ultraviolet (VUV) radiation lines of Xe atoms/Xe2 molecules at 147/172 nm are extensively investigated in the development of plasma display panels and Hg-free fluorescent lamps, which are frequently used in our daily lives. Numerous solid materials, particularly Tb3+-doped oxides, such as silicates, phosphates and borates, are efficient green/blue sources with color-tunable properties. The excitation wavelength and rare earth concentration are usually varied to optimize efficiency and the luminescent properties. However, some underlying mechanisms for the shift in the emission colors remain unclear. The present study shows that a UV/VUV switch systematically controls the change in the phosphor (Ba3Si6O12N2:Tb) photoluminescence from green to blue, resulting in a green emission when the system is excited with UV radiation. However, a blue color is observed when the radiation wavelength shifts to the VUV region. Thus, a configurational coordinate model is proposed for the color-reversal effect. In this model, the dominant radiative decay results in a green emission under low-energy UV excitation from the 5D4 state of the f–f inner-shell transition in the Tb system. However, under high-energy VUV excitation, the state switches into the 5D3 state, which exhibits a blue emission. This mechanism is expected to be generally applicable to Tb-doped phosphors and useful in adjusting the optical properties against well-known cross-relaxation processes by varying the ratio of the green/blue contributions.


INTRODUCTION
Inorganic-material-based phosphors have been extensively investigated for their applications in electronic illustrations, such as backlighting sources of liquid-crystal displays, plasma display panels and white light-emitting diodes (WLEDs) 1,2 . In particular, phosphors are important components of these displays, which have chemical durability and efficient luminescent properties. Compared with the traditional incandescent lamp and mercury-vapor lamp, WLED has attracted considerable attention because of its highly applicable value for our daily lives and wide feasibility for use in commercial products 3,4 . The most common WLED strategy is to combine blue InGaN chips and Y 3 Al 5 O 12 :Ce 3+ (YAG:Ce) phosphor in addition to employing three light-emitting diode (LED) chips in red, green and blue, which partially converts the original blue radiation into the complementary yellow color, yielding cool white light. This cool white light, which is based on using a single phosphor, is suitable for everyday applications only if the poor color rendition index (CRI) and high-correlated color temperature are bearable 5,6 . The requirement for high CRI cannot be satisfied using this approach because the insufficient spectral components cannot entirely cover the visible region. Recently, a novel LED device, in which white light is produced by an ultraviolet (UV) chip with red, green and blue phosphors, can achieve a high CRI of up to 90 7,8 . To pursue the above purposes, new phosphors adopted for UV excitation must be developed.
Oxonitridosilicates, which are formally derived from typical oxosilicates by partially substituting oxygen with nitrogen to form rigid Si(O,N) 4 tetrahedra, have excellent chemical, physical, mechanical and thermal stabilities for application in WLEDs [9][10][11] . With additional structural possibilities, nitrogen can be a triple or even quadruple connecting atom in the tetrahedral network. Therefore, the diversity of oxonitridosilicates is superior to that of oxosilicates because only terminal-and simple-bridging oxygen are available in oxosilicates 12 .
A system of Ba 3 Si 6 O x = 6, 9, 12, 15 N y = 6, 4, 2, 0 :M (M = Eu 2+ , Ce 3+ ) has been reported because of its high luminescent properties and easy synthesis 13 N 2 has not been reported. In contrast, Tb-activated phosphors are widely studied as green or blue luminescent candidates in the development of UV/vacuum UV (VUV)-excited applications [17][18][19][20][21][22][23][24][25] . The relative intensities of 5 D 3 / 5 D 4 emissions strongly depend on the Tb concentration through the cross-relaxation process, which result in a color change from blue to green 26 . However, studies on the systematic photoluminescence excitations (PLEs) from UV to VUV radiation are rare, and related studies are also limited. Therefore, a Ba 3 Si 6 O 12 N 2 material with constant 3.7% Tb doping at the Ba site was synthesized and characterized in the current study. The principal relationship between the different excitation energies and the luminescence mechanism was investigated.
Characterization methods Synchrotron X-ray diffraction patterns with wavelength of λ = 0.774907 Å were recorded using a Debye-Scherrer camera installed at the BL01C2 beamline of the National Synchrotron Radiation Research Center, Taiwan. X-ray Rietveld profile refinements of the structural models and texture analysis were performed using the General Structure Analysis System software 27 . 29 Si solid-state nuclear magnetic resonance spectrum was recorded on a wide-bore 14.1 Tesla Bruker Avance III NMR spectrometer (Germany), equipped with a 4-mm doubleresonance magic-angle-spinning probe. The Larmor frequency for 29 Si was 119.24 MHz. High-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) images were obtained via a JEOL JEM-2011 microscope (USA) operated at 200 kV. Synchrotron VUV photoluminescence (PL) and PLE spectra were obtained using the same synchrotron source at the BL03A beamline. The excitation spectra were recorded by scanning a 6 m cylindrical grating monochromator with a grating of 450 grooves mm − 1 over a wavelength range of 100-350 nm. A CaF 2 plate served as a filter to remove the high-order light from the synchrotron. The emission from the phosphor was analyzed with a 0.32 m monochromator and then detected in a photon-counting mode. PL and PLE spectra were collected using a FluoroMax-3 spectrophotometer (USA) equipped with a 150 W Xe lamp and a Hamamatsu R928 photo-multiplier tube (Japan).

RESULTS AND DISCUSSION
The X-ray Rietveld refinement of Ba 2.89 Si 6 O 12 N 2 :Tb 0.11 (BSON:Tb) is shown in Figure 1a, including the observed, calculated and difference profiles, and the relative Bragg reflection markers. Supplementary  Table S1 presents the crystallographic data, which are consistent with the lattice constants, reflection conditions and cell parameters of a previous study 28 . The results indicated that the compound is in pure phase, the data are reliable and the powder sample is crystallized into a trigonal structure with a P3 (no. 147) space group. In the inset of Figure 1a, the peak at − 60 to − 90 ppm indicates that Si 4+ cations in the structural lattices are coordinated by oxide and nitride to form the  Figure 1c, the crystal-lattice spacing is 6.482 Å, which is consistent with the pure BSON result (6.468 Å). Figure 1d shows the crystal structure of 2 × 2 × 3 unit cells, viewed along the [010] direction. The BSON structure consists of barium and SiO 3 N-tetrahedral layers. The distance is 6.5 Å in these layers, as shown in Figure 1e.  Figure 1f). The Ba ions are located at two independent crystallographic sites. One Ba ion is surrounded by six oxygen ions in site 1, whereas the other Ba ion (in site 2) is also coordinated by six oxygen ions, but has an additional capped nitrogen ion. The coordination of these two Ba sites results in the formation of a slightly distorted octahedral structure, as shown in Figure 1g. Therefore, the Tb activators occupy these sites by replacing the Ba ions. The coordination symmetry of the surrounding anions is significant to the degeneracy of the activator 5d level in the 4f → 5d transition shown below because the Tb activators are not in a perfect octahedral crystal-field environment. As heterovalent and homovalent substitutions, the PLE and PL spectra of BSON:Ce and BSON:Ce, Li are very similar, except in terms of intensity, as shown in Supplementary Fig. S1. The lattice structure retains electric neutrality through vacancies, defects and anions (O 2 − and N 3 − ), although Tb 3+ activators are introduced into the Ba 2+ sites 30,31 . Based on the structure refinement, the crystallographic data (Supplementary Table  S1) demonstrated that BSON:Tb can maintain a perfect structure without charge compensation.
The emission spectrum of the Tb activator normally presents two typical sets of intense line systems from the 5 D 4 → 7 F J (J = 3-6, 620-465 nm) and 5 D 3 → 7 F J (J = 3-6; 465-375 nm) transitions, which result in green and blue emissions, respectively. The dominant set usually corresponds to 5 D 4 → 7 F J transitions (green set) in most cases [32][33][34] , whereas the 5 D 3 → 7 F J transitions (blue set) are difficult to obtain as a primary emission because of the depopulation of the 5 D 3 state. This phenomenon can be elucidated by the direct feeding of the excited energy from the 5d level to the 5 D 4 state 35 . The crossrelaxation process between neighboring Tb activators also results in considerable quenching from the 5 D 3 to the 5 D 4 state, as normally expected in many cases with high-Tb-dopant concentrations 26 . These two mechanisms are responsible for the predominant green emissions of numerous Tb-doped phosphors. In this study, an unprecedented effect was further investigated by performing a color reversal between the green and blue sets through the control of the relative contributions of both colors in a Tb-doped phosphor under synchrotron radiation excitation at different wavelengths. The experiment had two requirements: (1) a moderate Tb concentration to ensure the initial appearance of a green emission and (2) changeable excitation energy from the UV to VUV range.
A series of PL spectra of the Ba 2.89 Si 6 O 12 N 2 :Tb 0.11 phosphor is shown in Figure 2a. The phosphor was excited under several specific synchrotron radiation wavelengths: 254, 234, 211 and 147 nm (according to the excitation peaks in the PLE spectra). The characteristic fine structure in the PL spectra is caused by the splitting of the 2S+1 L J states as a result of Russell-Saunders coupling. Each specific 5 D J = 3, 4 to 7 F J = 3-6 transition is labeled in the inset, and the corresponding energy levels are plotted in Supplementary Fig. S2. The most intense transitions in the green and blue transition sets are 5 D 4 → 7 F 5 and 5 D 3 → 7 F 6 , respectively. By contrast, PLE spectra monitored at 542 ( 5 D 4 → 7 F 5 ), 478 ( 5 D 4 → 7 F 6 ), 435 ( 5 D 3 → 7 F 4 ) and 412 nm ( 5 D 3 → 7 F 5 ) wavelengths are shown in Figure 2b (2) and was determined as 42 786 cm − 1 (233 nm), which was consistent with the experimental data (234 nm). The calculation scheme is shown in Supplementary  Fig. S3.
Another broad band (130-200 nm) monitored at 435/412 nm ( 5 D 3 → 7 F 4,5 ; blue sets) belongs to a mixed band composed of the host-absorption band (HAB) and the O 2p /N 2p to Tb 4f charge-transfer band transitions. Furthermore, the host-absorption band from the undoped Ba 3 Si 6 O 12 N 2 sample is at~180 nm. This value agrees well with the calculated band gap of 6.9 eV using density functional theory calculation 16,28 . The detailed discussion and experimental results are depicted in the supporting information and Supplementary Fig. S4. The charge-transfer band position of Tb 3+ in Ba 3 Si 6 O 12 N 2 can be predicted by Jørgensen's expression 40 : where χ opt (X) is the optical electronegativity of the ligand ion (similar to Pauling's electronegativity), and χ uncorr (M) can be calculated using Su's expression 41 : where E 0 Tb (Tb 3+ → Tb 2+ ) was reported as − 3.7 eV 41 . Therefore, χ uncorr (Tb) can be predicted to be 0.95. The E CT values were predicted as 64 500 and 52 500 cm − 1 , which correspond to 155 and 190 nm, respectively. χ opt (O) and χ opt (N) are~3.1 and B2.7, respectively.
A possible mechanism for the overall effects of UV/VUV-pumped Tb-doped phosphor is proposed and depicted by a configurational coordinate model shown in Figure 3a. A moderate Tb-activator concentration is used in the Ba 2.89 Si 6 O 12 N 2 :Tb 0.11 phosphor (Tb occupancy of~3.7 atom%) to suppress the high cross-relaxation probability (i.e., to retain a certain number of excited electrons at the 5 D 3 state). Radiative decay partly occurs from the 5 D 3 to the 7 F J = 3-6 states. Nevertheless, the green set remains the leading transition because the direct feeding from the 5d level to the 5 D 4 state [path (I) in Figure 3a] remains the dominant process (shown in 254 nm of the excited PL spectrum). The green transition gradually declines with increasing excited radiation energy from 254 to 147 nm, and the blue transition concurrently grows toward the opposite direction. Increased radiation energy from 254 to 211 nm leads to enhanced probability of the excited electrons to move down to the 5 D 3 state by crossing the intersection point between the 5d level and the 5 D 3 state [path (II) in Figure 3a].
Surprisingly, the blue set rapidly becomes the primary emission when the excitation energy reaches 147 nm in the VUV range. This process can be attributed to a mechanism by which the electrons of the host lattice in the valence band (VB) are excited to the conduction band (CB) by high-energy radiation (147 nm) and then relax to the charge-transfer state (CTS) [path (III) in Figure 3a UV/VUV switch-driven color-reversal effect CC Lin et al is considered the possible driving force for the color-reversal effect. The band gap between VB and CB (host-lattice absorption) was evaluated at 6.9 eV. This value indicates that the route can be switched on or off (i.e., as a color-tunable switch) by controlling the excited wavelength at approximately this energy. Figure 3b plots the ratio of the blue set to the green set, ΣI( 5 D 3 )/I( 5 D 4 ) (i.e., the ratio of the entirely integrated emission bands of 5 D 3 → 7 F J = 3-6 to that of 5 D 4 → 7 F J = 3-6 ) according to the emission spectra of the BSON:Tb excited by different wavelengths (254,234,211,190,170 and 147 nm), as shown in Supplementary Fig. S5. Under 211-254-nm-UV excitation, this ratio is~0.5. However, the ratio increases rapidly to 4.6 under 147-nm-VUV excitation, indicating that additional emission in the 5 D 3 → 7 F J = 3-6 process can be estimated when the excitation energy exceeds 6.5 eV. This extra energy can be attributed to the electrons donated from the host VB because the onset energy for this curve is at~6.5 eV. This value closely matches the band gap of the host lattice, suggesting that the excited electrons of the 5 D 3 state radiative decay are from both the 5d level of the Tb activator and the host-lattice CB. This dual accumulation leads to an exponential growth in the ΣI( 5 D 3 )/I( 5 D 4 ) ratio, and the systematic color reversal between blue and green sets is also observed in the Commission Internationale de l'Eclairage coordinate, as shown in Supplementary Table S2 and Supplementary Fig. S6. Therefore, the color-reversal effect can be practically manipulated by a UV/VUV switch with a wavelength of~190 nm. The PLE spectra can be classified into two types. The first type monitors the emission from the 5 D 4 state under 542/478 nm, and the other type monitors the emission from the 5 D 3 state under 435/412 nm. Two findings supporting the proposed mechanism can be discovered in the PLE spectra. First, the onset wavelength of the broad band (o200 nm) and the onset energy of Figure 3b match well with the band gap value. The coincidence of both experimental and hypothetical analysis strongly supports the mechanism. Therefore, this onset can represent a switch function in the proposed route. Moreover, this wide band within the VUV region is observed only in the 5 D 3 -type PLE spectra, but disappears in the 5 D 4 -type, suggesting that cross-interaction might not occur between CTS and the 5 D 4 state. All the excited electrons from the host-lattice VB simply emit blue light and then respond to an exponential climb on the ΣI( 5 D 3 )/I( 5 D 4 ) curve. Second, the other evidence for the mechanism is the shape change of the 4f → 5d transition band in the 200-280 nm range. The direct feeding to the 5 D 4 state is considerable and leads to a low quantity of electrons that can be accommodated in the 5 D 3 state. Numerous studies have reported that the excited electrons in a low-temperature environment have higher probability to populate at lower energy vibrational states of the five 5d-orbitals, leading to narrow peaks, and respond to a higher resolution for excitation detection because of the relaxation process of the thermally stable phonons at individual 5d states without cross-interacting with each other 43,44 . A similar phenomenon can be observed in the 5 D 3 -type excitation spectra. The low population of 5 D 3 state electrons narrows down the bandwidth of the 4f-5d excitation transition in the PLE spectrum. The reason for this is that the electron population among each 5d-orbital might be limited to the localized vibrational state distribution because of the few electrons in the 5 D 3 state, whereas the excited energy scans until the UV region (4200 nm). As a result, the 5d level can be clearly resolved at 211, 234 and 254 nm. Although, the Tb ions are situated in nearly octahedral coordination spheres, the crystal-field strength splits the five 5d-orbitals into three different degeneracies rather than two theoretical octahedral 5d states (e.g., e g and t 2g ). The detailed discussion is depicted in the supporting information. However, the excitation signal of 5 D 3 -type rapidly increases compared with that of 5 D 4 -type, whereas the excited energy scans until the VUV region (o200 nm). These findings demonstrate that the CTS simply interacts with the 5 D 3 state rather than the 5 D 4 state.

CONCLUSIONS
In summary, a color-reversal effect, which is a specific transition route that acts as a switch of green/blue emissions, was observed in a Ba 2.89 Si 6 O 12 N 2 :Tb 0.11 phosphor. This effect is expected to be generally exhibited by all phosphors doped with Tb activators. In addition, the same effect may also be applicable to other lanthanide f-f inner-shell transition systems that are sensitive to cross-relaxation mechanisms.