Tunable Luminescence Contrast of Na0.5Bi4.5Ti4O15:Re (Re = Sm

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Tunable luminescence contrast of Na Bi TiO :Re (Re=Sm, Pr, Er) photochromics by controlling the excitation energy of luminescent centers Qiwei Zhang, Yao Zhang, Haiqin Sun, Wei Geng, Xusheng Wang, Xihong Hao, and Shengli An ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11825 • Publication Date (Web): 28 Nov 2016 Downloaded from http://pubs.acs.org on November 29, 2016

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ACS Applied Materials & Interfaces

Tunable luminescence contrast of Na0.5Bi4.5Ti4O15:Re (Re=Sm, Pr, Er) photochromics by controlling the excitation energy of luminescent centers Qiwei Zhang,† Yao Zhang, Haiqin Sun,*† Wei Geng, Xusheng Wang, ‡ Xihong Hao,*† Shengli An† † School of Materials and Metallurgy, Inner Mongolia University of Science and Technology, 7# Arerding Street, Kun District, Baotou 014010, China ‡ Functional Materials Research Laboratory, School of Materials Science and Engineering, Tongji University, 4800 Caoyang Road, Shanghai 201804, China ABSTRACT High luminescent switching contrast of photochromic materials is extremely important in improving the sensitivity and resolution of optical switches and high-density optical data storage devices. To date, conventional methods, such as tuning absorption and emission bands based on electron or resonance energy transfer mechanisms in well-known organic photochromic molecules or compounds, have routinely been adopted to tune luminescent switching behavior. However, these strategies and mechanisms are not effectively applied to luminescence switching in inorganic materials because their crystal structures differ strongly from those of organic materials. In this paper, we report a new method to significantly tune the luminescent switching contrast by modifying the excitation energy of luminescent centres in a newly synthesized photochromism material: Na0.5Bi4.5Ti4O15:Re (Re=Sm, Pr, Er). A significant enhancement of luminescence switching contrast was achieved when the luminescent centres were excited by low energy photons at a given irradiation wavelength, intensity and time, compared with high excitation energy photons. The trend “the lower the excitation energy, the higher the luminescence switching contrast” is universal in different rare earth ion-doped Na0.5Bi4.5Ti4O15 ferroelectrics. The changes in the luminescent switching contrast based on excitation energy are ascribed to nonradiative 1

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energy transfer from the luminescent centre to the colour centre by dipole-dipole interactions according to Dexter theory. This possible utilization of excitation energy at lower energy levels is usually less destructive to both information recording and the recording material itself during luminescent readout processes while achieving higher luminescence switching contrast. KEYWORDS: luminescent switching contrast, optical data storage, photochromism, nonradiative energy transfer, luminescent readout

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INTRODUCTION Luminescent photochromic materials, usually accompanied by a change of luminescence emission intensity upon photochromic reactions, have been attracting an increasingly fundamental and practical interest because of their potential applications in optical switches and high-density optical data storage for optoelectronic devices.1,2 The ability to modulate luminescence emission intensity has been proven to be a promising method to achieve non-destructive luminescence readout in practical “writing” and “erasing” processes of optical memory.3,4 To date, there are at least six different mechanisms to explain luminescent switches of well-known organic photochromic compounds, such as photoinduced conjugation and polarity changes, photoinduced electron transfer, luminescence resonance energy transfer, and photoinduced filter effects, among others.5 Of these, the electron and resonance energy transfers from the luminescent to the photochromic components are two key mechanisms.6 Scheme 1 presents the spectral requirements (luminescent and absorption spectra) for luminescence photoswitching based on the energy transfer. The overlapping of the emission band and the absorption band would lead to luminescent quenching.7 The quenching degree or the photoinduced luminescence switching contrast can be generally estimated from the luminescent emission intensity ratio before and after photochromism, which plays an essential role in improving the sensitivity and signal-to-noise of optical storage.8 Therefore, tailoring luminescence switching contrast is one of the most widely exploited and studied areas in optical memory materials.

Scheme 1. Absorption, excitation and emission spectral requirements for luminescence switching based on the energy transfer between luminescence centres and photochromic components.

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Current studies on luminescence modulation mainly focus on organic photochromic molecules or compounds.9,10 A universal approach is to alter their absorption and emission characteristics by designing a photochromic molecule that incorporates a fluorophore, for example, photochromic metal complexes depending on characteristic electronic transitions (π-π* transition of the ligand, metal-to-ligand charge transfer, and f-f transition in lanthanides) based on electronic or energy transfer mechanisms.11-13 In contrast, studies on luminescence modulation of inorganic photochromic materials are comparatively rare; limited success has been achieved in switching the luminescence of inorganic solid-state materials with high contrast and reproducibility.14-16 We recently reported the luminescence-switching phenomenon of photochromic ferroelectrics (Na0.5Bi2.5Nb2O9, NBN) by introducing luminescent lanthanide ions.17-19 A simple model of luminescence modulation based on the free or trapped charge carrier and intervalence-charge transfer (IVCT) was proposed by our groups, in which, the luminescent quenching behavior was mainly attributed to the energy transfer of electrons from rare earth ions to vacancy-related defects (colour centres). However, the switching mechanism and the interplay process between photochromism and luminescence in inorganic materials remain obscure because of their notably different crystal structures from organic materials. Meanwhile, the mechanisms of luminescent modulation mentioned above are not successfully applied to the behaviour of optically induced luminescence switching in inorganic materials. In these cases, systematically investigating luminescence modulation behaviour is necessary to advance the basic understanding of the photochemical and photophysical properties of inorganic compounds and helpful in the search for new approaches to effectively improve luminescence switching contrast. In this paper, we report a series of new inorganic photochromic materials, Na0.5Bi4.5Ti4O15:Re (Re=Sm, Pr, Er), which exhibit a reversible conversion between luminescent and photochromic behaviour. In this system, we found a dipole-dipole coupling reaction that was strongly related to the excited energy of luminescent centres, which plays a critical role in modulating the luminescent switching degree. Low excitation energies (long wavelengths) induce a higher luminescence switching contrast than do high excitation energies (short wavelengths) at a given irradiation wavelength, intensity and time, the contrast inherently depending on the interaction time and intensity between dipoles. The trend “the higher the luminescence switching contrast, the lower the excitation energy” is clearly observed in different rare earth ion-doped Bi4.5Na0.4Ti4O15 4


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ferroelectrics. For the purpose of photoswitching luminescence with high contrast, this possible utilization of excitation wavelengths at lower energy is usually less destructive to both information recording and the recording material itself during luminescent readout processes.

RESULTS AND DISCUSSION XRD analyses of rare earth-doped Na0.5Bi4.5Ti4O15 samples (Na0.5Bi4.5Ti4O15:Re=Sm, Pr, and Er) reveal only a single phase bismuth layer-type structure with m=4 within the composition studied here, as shown in Figure S1. These diffraction peaks can be well indexed by a standard PDF card of Na0.5Bi4.5Ti4O15 (PDF#01-074-1317), similar to results reported in the literature.20,21 The strongest diffraction peak is found to be the (119) orientation, aligning with the fact that the most intense reflections of bismuth layer structures are the (112m+1). Compared with pure Na0.5Bi4.5Ti4O15, rare earth doping induces the shift of diffraction peaks to higher angles, clearly observed in the Sm-doped sample. These shifts are related to the rare earth substituted ionic radii (RSm=0.958 Å, RPr=0.990 Å, REr=0.890 Å, Coordination number (CN)=6) being smaller than those of Bi3+ (1.03 Å, CN=6) or Na+ ions (1.02 Å, CN=6). To provide additional information on rare earth ion substitution in Na0.5Bi4.5Ti4O15 hosts, we used the Raman scattering to investigate the vibrational properties of all samples. Figure 1 shows the room temperature Raman spectra recorded in the frequency range of 50-650 cm-1. Three broad scattering bands, at 50-190, 190-430, and 430-650 cm-1, are visible for all compositions, and each broad band clearly contains some individual Raman peaks. To obtain the exact positions of the Raman peaks, the measured Raman data are fitted to individual Lorentz components; the fitted results are listed in Table S1. Ten major bands, centred at ~68, 83, 108, 145, 229, 270, 341, 494, 543, and 572 cm-1, are observed and labeled ν1 to ν10, respectively, for all samples. As shown in Ref.,21,22,23 the phonon modes below 200 cm-1 (ν1, ν2, ν3 and ν4) originate from the vibrations of A-site ions in the pseudo-perovskite layer. The modes at high frequency are generally believed to come from the vibrations of TiO6 octahedra, while the modes of ν6 and ν9 (or ν10) are attributed to the torsional bending vibrations and stretching vibrations, assigned to the F2g and Eg (splitting into B2g and B3g modes) characters, respectively. As seen in Table S1, the ν1, ν2 and ν3 bands are drastically influenced by the substitution of rare earth ions for Bi or Na ion, showing a visible low-frequency shift for Sm, Pr and Er doping, whereas no obvious frequency shift is observed for the high frequency bands (ν6, ν7, 5



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ν9 and ν10) compared to pure Na0.5Bi4.5Ti4O15. This implies that the rare earth ion substitution changes the A-site cation vibrations, and these ions would prefer to occupy the A-sites of pseudo perovskite-like layers, rather than the (Bi2O2)2+ layers.24

Figure 1. Room temperature Raman spectra of pure and Sm, Pr and Er-doped samples. To achieve different excitation and emission wavelengths, a series of activator ions (Sm3+, Pr3+, Er3+) was used to exploit the effect of different excitation energies on the luminescence switching contrast in the Na0.5Bi4.5Ti4O15 host, as shown in Figure 2. All activator ions at a dopant concentration of 1 mol% were homogeneously diffused into the Na0.5Bi4.5Ti4O15 host by a high temperature solid-state reaction method. The micrographs of different ion-doped samples in Figure 2 show dense and uniform microstructures and exhibit typical plate-like structures of bismuth layer materials.25 Excitation (monitored by 603 nm) and emission spectra (excited by 407 nm) of Na0.5Bi4.5Ti4O15 with Sm doping are shown in Figure 2a. The excitation spectrum exhibits a strong excitation band and three weak bands ranging from 400 nm to 500 nm, located at 407 nm, 420 nm, 464 nm, and 479 nm, which are ascribed to typical f-f electronic transitions from the 6

H5/2→4G7/2 (407 nm), 6H5/2→4P5/2 (420 nm), 6H5/2→6P5/2 (464 nm) and 6H5/2→4I11/2 (479 nm)

transitions, respectively. Under 407 nm excitation, three obvious emission bands are observed: a green emission at 566 nm, a red emission at 603 nm, and a deep red emission at 650 nm. These narrow and sharp bands can be assigned to the 4G5/2 to 6HJ (J=5/2, 7/2, 9/2) transitions.26 Upon 450 nm excitation, the Pr3+-doped Na0.5Bi4.5Ti4O15 sample shows a strong red emission with a single peak centred at 613 nm arising from the well-known 1D2→3H4 transition.27 When 6


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monitoring at 613 nm, there are three strong sharp peaks (centred at 450, 477 and 492 nm) and a weak broad peak (centred at 365 nm), of which the broad band is assigned to the valence-to-conduction transition according to the position of the absorption edge in the diffuse reflectance spectrum, which is different from the intra-4f transitions of Pr3+ from the 3H4 absorption to the excited states 3PJ (J=0,1,2).28,29 Compared with the Pr3+ and Sm3+ activator ions, Er3+ doping induces a strong green emission located at 550 nm (4S3/2→4I15/2) and a weak green emission at 525 nm (2H11/2→4I15/2) with an excitation of 487 nm. Monitored at 550 nm, one can see clearly that the excitation spectrum consists of two narrow absorption bands: a much weaker band at 450 nm and a strong band at 487 nm (Figure 2c).30 Furthermore, it is clear that activator ion-doped samples have at least two different excitation levels, and the emission spectral patterns and central wavelengths demonstrate almost no change upon different excitation wavelengths except for their relative intensities.

Figure 2. Schemes of rare earth-doped Na0.5Bi4.5Ti4O15 material design, SEM images of the synthesized ceramics, and the excitation and emission spectra of (a) Sm (λex=407 nm, λem=603 nm), (b) Pr (λex=450 nm, λem=613 nm) and (c) Er (λex=487 nm, λem=550 nm)-doped samples. Figure 3 presents the colouration and emission spectra changes of Sm3+, Pr3+ and Er3+-doped Na0.5Bi4.5Ti4O15 samples before and after light irradiation. Under irradiation of a hand-held near-UV laser diode (LD, 407 nm, 200 mW) for 5 s, all samples exhibit interesting photochromic behaviours, with their colours quickly changing from an initial pale yellow to dark grey at room temperature. Similar colour changes are also observed under sunlight or simulated solar energy (AM 1.5G) irradiation. Most importantly, the colour change is completely reversible, and the 7



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samples can fully recover to their initial colour through a thermal stimulus with a temperature of 200 oC for 10 min. Further, a change of luminescence emission intensity is also observed with the photochromic reaction. When the samples reach their photostationary state under near-UV 407 nm light irradiation for 5 s, the emission intensity clearly decreases, showing luminescence quenching behaviour. For instance, the emission intensity (4G5/2→6H7/2) at 603 nm under 407 nm excitation decreases to 57% of the initial value for the Sm3+-doped sample. Correspondingly, the emission intensity decreases to 42% and 17% of the initial values of the Pr3+-doped sample (1D2→3H4) at λex=450 nm and the Er3+-doped sample (4S3/2→4I15/2) at λex=487 nm, respectively. In addition, no spectral shift occurs before and after irradiation. Generally, the quenching, sometimes called the luminescence switching contrast (∆Rt), can be estimated from the luminescent emission intensity ratio before and after photochromism. It is defined by the formula ∆Rt=(R0-Rt)/R0×100 (%), where R0 and Rt are the emission intensity before and after irradiation. The obtained ∆Rt values of the Sm3+, Pr3+, Er3+-doped samples are 42.56%, 57.67% and 82.87%, respectively. The quenching luminescence intensity recovers to its initial state upon a heat treatment of 200 oC for 10 min. The quenching effect can be ascribed to an efficient energy transfer from the luminescent centres to photochromic components.31

Figure 3. Photoluminescence emission spectra changes for the Na0.5Bi4.5Ti4O15: Re (Re=Sm, Pr, Er) samples before and after irradiation; the insets are the photographs of ceramic samples. As mentioned above, the photochromic behaviour is directly related to the visible light absorption. In Figure 4, we show the UV-vis diffuse reflectance spectra of pure Na0.5Bi4.5Ti4O15 and different ion-doped samples before and after irradiation. The reflectance spectral curves and absorption peaks are similar to those that have been reported.19 The broad absorption bands near 8


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390 nm (~3.19 eV) of all samples correspond to the valence-conduction transition of the host. The transition is a direct transition and can be described by the Kubelka-Munk function (K-M) K/S=(1-R)2/2R, where K and S are the absorption coefficient and scattering coefficient, respectively. R is the relative reflectance intensity.32 Using this formula, the band gap width (Eg) is obtained by extrapolating the absorption edge onto the energy axis, as presented in Figure S2. Compared to the spectrum of pure Na0.5Bi4.5Ti4O15, the Eg values of Sm, Pr or Er-doped samples change slightly. However, several weak, narrow absorption bands appear in the Pr and Er-doped samples, which arise from the absorption of 4f bands of rare earth ions, in agreement with the excitation and emission peaks in Figure 2b and 2c. Under near-UV laser diode (LD, 407 nm, 200 mW) irradiation, the reflectance intensities of samples dramatically decrease in the visible light region from 400 nm to 800 nm and almost saturate with an irradiation time of 5 s; further increasing the irradiation time only slightly changes the reflectance intensity, with no remarkable change of reflectance spectra in the broad absorption bands, including Eg values (Table S2). In contrast, the reflectance intensity after irradiation can be easily returned to its initial stage after a heat treatment of 200 oC for 10 min, which is consistent with the photochromic process from pale yellow to dark grey. Such a broad absorption covering the entire visible region almost overlaps the emission spectra of the samples, allowing an energy transfer from luminescence centres to photochromic components, leading to the luminescence quenching.33 The changes of reflectance intensity before and after irradiation, ∆Abs.=R(0)-R(t), where R(0) and R(t) are the reflectance intensity before and after irradiation, can be used to analyse the positions of the strongest absorption peaks, as shown in the insets of Figure 4. The strongest absorption (∆Abs.) occurs at 526 nm, 528 nm, 544 nm and 506 nm for pure Na0.5Bi4.5Ti4O15 and Sm, Pr and Er-doped samples, respectively.

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Figure 4. Reflectance spectra of the Na0.5Bi4.5Ti4O15:Re (Re=Sm, Pr, Er) samples before and after 407 nm light irradiation; the insets are the differences of reflectance spectra (∆Abs.). In addition, the luminescence switching reversibility is performed under alternate 407 nm light irradiation (obtained by F-4600) and a thermal stimulus of 200 oC for 10 min. The luminescence switching contrasts (∆Rt) of all samples as a function of irradiation time during 5 colouration and decolouration cycles are displayed in Figure 5. The luminescence emission intensity is recorded at the position of the strongest emission peaks, which was 603 nm for Sm doping, 613 nm for Pr doping, and 550 nm for Er doping. With irradiation times gradually increasing 10 s, 20 s, 30 s, 1 min and 2 min, the curves of the colouration process exhibit an exponential decay, where the emission intensity quickly diminishes to 70.14%, 61.68% and 34.39% of the initial intensity within 10 seconds for the Sm, Pr and Er-doped samples, respectively, and gradually saturates. The dynamic change of emission intensity with irradiation time can be well fitted by the exponential relaxation model proposed by Dachraoui et al.34,35 After the colouration sample undergoes a heat treatment of 200 oC for 10 min, the emission intensity or the ∆Rt values would fully recover to their initial stage. During five colouration-decolouration cycles, no obvious change of the ∆Rt value is observed during a given irradiation time, showing excellent reversibility.

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Figure 5. Luminescence switching contrast (∆Rt) dependence on irradiation time for (a) Sm (λex=407 nm), (b) Pr (λex=450 nm) and (c) Er (λex=487 nm)-doped samples over five irradiation cycles by alternating 407 nm light irradiation and thermal stimulus. According to the results discussed above, the luminescent quenching degree upon photochromic reactions strongly depends on the emission and absorption behaviour, except for the irradiation wavelength, time and intensity. Specifically, the emission and absorption bands can be designed to overlap, inducing the energy transfer of the excitation energy from the activator ions to the photochromic matrix. Intramolecular electron transfer and resonance energy transfer are two very important mechanisms to modulate luminescence switching in organic molecules or compounds.6,7 However, these mechanisms are not well suited to inorganic photochromism materials as these materials possess very different structures from organic materials, as well as structure-induced different photochromic and luminescence mechanisms. Figure 6 presents a schematic diagram of luminescence modulation upon photochromic reactions in the present study. Rare earth ion doping is not the cause of inducing photochromic phenomenon in the Na0.5Bi4.5Ti4O15 matrix; they are simply considered luminescent centres to produce luminescence emission. The appearance of a great deal of Bi, Na, and oxygen vacancies (VBi, VNa, VO) caused by alkali element (Bi, Na) volatilization during high temperature sintering are responsible for the photochromism.36 These defects can trap electrons and holes, forming so-called colour centres (Figure 6a and 6d).37,38 Free charge carriers (electrons or holes) caused by near-UV light irradiation of 407 nm are trapped by colour centres in the Na0.5Bi4.5Ti4O15 matrix, showing a broad absorption in the visible light region (Figure 4). The statements above have been confirmed by ICP and XPS analysis in our previous work.20 At low temperatures, these charge carriers are stable and escape with difficulty from these 11



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traps even though they are at a higher irradiation energy (E>Eg=3.15 eV). At an elevated temperature 200 oC, these charge carriers would obtain enough energy to escape from the traps and return to their initial stages, suggesting that these defect levels are caused by deep levels far from the edges of the conduction band and valence bands; the trap depth depends on the absorption of light observed in Figure 4. In addition, no afterglow is observed during the decolouration process, which agrees with the results of thermoluminescence (TL) measured from room temperature to 350 oC (Figure S3), meaning that non-radiative transitions occur.

Figure 6. Schematic representation of luminescence modulation upon photochromic reactions, (a) color center formation in pure materials, (b) luminescent center formation in doping materials, (c) the coupling interaction between the color center and luminescent center after 407 nm light irradiation, and their energy levels correspond to the (d), (e) and (f), respectively.

To verify the strong dependence of colour centres on temperature, the strongest excitation and emission peaks of all samples are collected to investigate the changes of luminescent emission intensity (∆Rt) under different thermal stimulus temperatures, as shown in Figure 7. The treatment temperature plays an important role in the decolouration process. The decolouration degrees or the ∆Rt values exhibit an exponential change with increasing temperature from 50 to 200 oC with a similar temperature interval (Figure S4). According to the decay trend, the luminescence emission intensity or the absorption spectra can almost fully recover to their initial states at 200 oC.

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Figure 7. Luminescence switching contrast (∆Rt) dependence on irradiation time under different thermal stimulus temperatures for Sm (λex=407 nm), Pr (λex=450 nm) and Er (λex=487 nm)-doped samples. The luminescence switching upon photochromic reactions indicates an energy transfer between luminescence centres and colour centres. The detailed transfer process can be described according to photoluminescence and photochromism mechanisms. In the photoluminescence process, the sharp spectral lines of Sm, Pr, and Er-doped Na0.5Bi4.5Ti4O15 (Figure 3) are obviously associated with the weak intra-4f electron transitions. Before irradiation, the excited electrons nonradiatively relax to low levels by thermal processes of vibrational relaxation and collisional quenching, and recombine to ground states accompanied by strong photon emission. The electron transition processes are strictly not influenced by the hosts,28,37 as shown in Figure 6b and 6e. Therefore, the electron transfer between luminescence centres and colour centres is not large. After irradiation of 407 nm, the electrons from luminescence centres are excited to high energy levels by specific wavelengths, and the excited energy is transferred to colour centres by a resonance energy transfer due to emission spectra overlapping with absorption spectra, accompanied by electron transitions from excited sates to the ground state (Figure 6c). Finally, the luminescence quenching behaviour is clearly observed (Figure 3), but the quenching degree greatly depends on the colouration process. During the energy transfer process, the energy of the luminescence centre as a donor is absorbed by the color centre as an acceptor through a dipole-dipole interaction, as reported by Port et al.38 In the interaction model (Figure 6c and 6f), the electrons in an excited state and the central rear earth ions can be represented by a dipole, as well as the electrons and colour centres. The dipole-dipole interaction relies on the electron transitions from both luminescent centres and color 13



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centres, including the excited energy and moving velocity. According to Dexter theory,39 the trend “the faster the velocity of electrons, the shorter the interaction time between two dipoles” is general, which leads to a smaller energy transfer to another dipole, suggesting lower luminescence changes. This theory is consistent with our experimental results on the changes of luminescence switching contrast (∆Rt) along with different excited energies from luminescent colours (Sm, Pr, Er), as shown in Figure 8. We can clearly observe a strong dependence of the luminescence switching contrast on the excitation energy of the activator ions. For example, the ∆Rt value of the Sm doped sample increases to 80.46% (λex=479 nm) from 38.54% (λex=407 nm) under irradiation of 407 nm for 2 min. Correspondingly, the ∆Rt values are 23.10% (λex=365 nm), 50.50% (λex=450 nm), 60.95% (λex=477 nm) and 61.85% (λex=492 nm) for the Pr-doped sample and 65.08% (λex=450 nm) and 78.91% (λex=487 nm) for the Er-doped sample. As seen from these results, the luminescence quenching of the samples is apparently enhanced with the increase of the excitation wavelengths from UV to visible light.

Figure 8. Luminescence switching contrast (∆Rt) dependence on irradiation time under different excitation wavelengths for Sm, Pr and Er-doped samples. Raman spectroscopy is a powerful technique to gain insight into mechanisms to tune luminescence contrast by the dipole-dipole interaction mentioned above. In Figure 9, we show the Raman spectra of all samples before and after 407 nm light irradiation. The fitting results are listed in Table S1. First, the intensities of all Raman peaks (ν1 to ν10) decrease under light irradiation. The decrease for the Raman peaks located at low frequencies (below 400 cm-1) is more obvious than that at high frequencies. According to Raman analysis of the scattering intensity from the Placzek theory,40-42 14


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2

σ (ν 0 ) = K R L(ν 0 )ν 0 (ν 0 −ν f )

3

α ρσ (ν ∑ ρσ

0

)

(1)

the Raman scattering intensity (IR) is proportional to the Raman scattering cross sections (RSCSs) (σ(ν0)) of vibrational modes. According to Eq. (1), σ(ν0) is also proportional to the square of the polarizability. In Eq. (1), K is a constant, and νf is the Raman wavenumber. αρσ (ν0) is the component of the Raman polarizability tensor, which can be defined as follows:43

 f µ ρ r r µσ i f µσ r r µ ρ i   (α ρσ ) = ∑  +  hωri + hω0 − iΓr  r  hω ri − hω0 − iΓr   where

f µρ r

(2)

is the ρ-th component of the transition dipole moment related to the transition f

to r, and µρ is being the dipole moment operator in the ρ direction. ω0 is the angular frequency of the incident radiation and ωri is the angular frequency associated with the transition from an initial state i of Raman transition to any state r. Γr is a damping factor correlated to the lifetime of the state r. In our experimental situation, an electron transition from low energy to high energy after irradiation implies a modification of the dipole moment, as discussed above, coming from luminescent centres and colour centres. The electron is excited by a low energy photon, which would induce a smaller dipole moment than a high energy photon, indicating the decrease of Raman peak intensity,44 as observed in Figure 8. A schematic diagram illustrating the modulation mechanism for luminescent switching is shown in Figure 10. Therefore, the difference of Raman intensities before and after irradiation provides evidence for the modifications of photon-induced dipole moments. Second, these phonon modes (ν1 to ν10) do not have obvious frequency shifts for rare earth samples, but for pure Na0.5Bi4.5Ti4O15 the phonon modes at low frequencies exhibit a visible frequency shift to lower frequencies after light irradiation, suggesting that the appearance of colour centres in pure Na0.5Bi4.5Ti4O15 has a significant influence on the vibrational modes; the detailed causes of this require further deep investigation.

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Figure 9. Raman spectra of the Na0.5Bi4.5Ti4O15:Re (Re=Sm, Pr, Er) samples before and after 407 nm light irradiation.

Figure 10. Schematic diagram of the energy transfer process between luminescence centres and colour centres. The energy transfer process between luminescence centres and colour centres above can also be analysed by the Dexter energy transfer probability according to the energy transfer model, analogous to energy transfer from a sensitizer to an activator in photoluminescent materials.45 The energy transfer probability (PSA) from a sensitizer (S) to an activator (A) can be approximated as follows:39

PSA =

2π 〈 S , A∗ H SA S ∗ , A〉 h

2

∫g

S

( E )g A ( E )dE

(3)

where HSA is the interaction Hamiltonian, and ħ is the Planck constant. < S,A* and S*,P> are the ground state and excitation state, respectively. The integral represents the spectral overlap. gS(E) and gA(E) stand for the normalized spectral functions for emission and absorption, respectively. E 16


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is the transfer energy. From the equation, the energy transfer probability is proportional to the spectral overlap and inversely proportional to the distance (R) between the activator and sensitizer, PSA∝R-n (n=6, 8…., depending on the type of the interaction), as shown in Figure 10. In the present study, two features are clearly observed: (i) a significant spectral overlap between the emission of activators and the absorption of photochromic components, and (ii) a significant decrease in luminescence intensity along with the colouration process, suggesting the occurrence of nonradiative energy transfer. In addition, for the exchange interaction from the dipole-dipole interaction, the interaction intensity and time between dipoles are related to energy transfer. Generally, strong interactions produce greater energy transfer. Low excitation energy would result in longer interaction times. Correspondingly, the energy absorption from low excitation energy is far higher than that of high excitation energy, showing high energy transfer probability, thus leading to high luminescence switching contrast, as observed in Figure 8. This method of tuning luminescence switching contrast by modifying the excitation energy may be applied to other photochromic materials. The possible extension of the excitation energy to lower energies is also less destructive to information recording while maintaining high luminescence switching contrast.

CONCLUSIONS In the present study, a series of novel photochromism materials based on Na0.5Bi4.5Ti4O15 ferroelectrics with different rare earth ion dopants (Sm, Pr, Er) were fabricated by a solid-state reaction. These materials exhibit dual characteristics of luminescence and photochromism. Luminescence modulation based on photochromic reactions is realized, and the degree of the luminescence modulation shows a strong dependence on the excitation energy of luminescent centres. When the luminescent centres are excited by longer wavelengths, the luminescence switching contrast is clearly higher than with shorter wavelengths. According to Dexter theory, the significant change of luminescent switching contrast can be ascribed to nonradiative energy transfer from luminescent centres to colour centres by a dipole-dipole interaction. These results could be used as a guide to tailor the luminescent switching of photochromic materials for optical data storage applications. EXPERIMENTAL SECTION 17



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EXPERIMENTAL PROCEDURE Material synthesis procedure: (Na0.5Bi0.5)1-xRexBi4Ti4O15 (x=0.01, Re=Sm, Pr, Er) ferroelectric ceramics were prepared by the conventional solid-state reaction method. High purity Na2CO3 (Alfa Aesar, 99.5%), Bi2O3 (Alfa Aesar, 99.975%), TiO2 (Alfa Aesar, 99.6%), Sm2O3 (Tianjin Chemical Reagent, 99.9%), Pr6O11 (Tianjin Chemical Reagent, 99.9%) and Er2O3 (Tianjin Chemical Reagent, 99.9%) powders were used as starting materials. These powders were mixed in alcohol, dried and calcined at 875 oC for 4 h in air. The calcined powders were then mixed in alcohol. After drying, these powders were granulated with 8 wt% polyvinyl alcohol binder and pressed into disk-shaped pellets (15 mm diameter, 1 mm thick). The green pellets were heated to 550 oC for 6 h at a rate of 1 oC/min and then sintered at 1100 oC for 2 h in air. Structure and properties characterization: X-ray diffraction (XRD, D8 Advanced, Bruker, Germany) and scanning electron microscopy (SEM, Quanta 400, CENESIS) was adopted to identify the phase composition and microstructure of samples. The luminescence spectra of ceramic samples were recorded by an F-4600 luminescence spectrometer (HITACHI, Japan) with a xenon lamp, which was used for all tested irradiation wavelengths, intensities and times The irradiation intensity was modified by changing the slit width of the excitation wavelength. In the present paper, an excitation split of 5.0 nm and an excitation wavelength of 407 nm in the F-4600 were used to irradiate samples. During luminescence measurements, the detailed parameters were as follows: scanning rate of 240 nm/min, PMT voltage of 700 V, response time of 0.1 s, and scanning wavelength range from 500 nm to 700 nm. In addition, a laser diode (LD) of 407 nm and 200 mW was used to obtain the saturated colouration. The photochromic reaction process was obtained by the a UV-visible spectrophotometer (U-3900, HITACHI, Japan). The luminescent quenching process was described by measuring the changes of luminescent emission intensity with irradiation time. For luminescent modulation behaviour with temperature, the sample was first irradiated for 10 s by a laser diode (407 nm and 200 mW) to the saturated colouration and then heated to a stable temperature (50 oC, 100 oC, 150 oC, and 200 oC) for 10 min, after which it was measured for the luminescent spectrum changes with irradiation time. Raman spectra of samples before and after 407 nm irradiation (LD) for 5 s were performed with a HORIBA XploRA system (HORIBA Jobin Yvon Company, Paris, France). The exciting source was the 785 nm line. 18


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Thermoluminescence (TL) curves were recorded on a thermoluminescence-dosimeter (FJ427A1, Beijing Nuclear Instrument Factory). ASSOCIATED CONTENT Supporting Information XRD patterns of Na0.5Bi4.5Ti4O15:Re (Re=Sm, Pr, Er) samples, the absorption spectra for all samples obtained by K-M functions, thermoluminescence (TL) glow curves, luminescence switching contrast (∆Rt) dependence on thermal stimulus temperature, and fitting results of Raman spectra for all samples before and after irradiation are available. These materials are available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] E-mail: [email protected] NOTES The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the Natural Science Foundation of China (No. 51562030), the Natural Science Foundation of Inner Mongolia (No. 2015BS0503), the Innovation Fund of Inner Mongolia University of Science and Technology (No. 2016YQL01, 2015QNGG04) and the Program for Innovative Research Team in Universities of Inner Mongolia Autonomous Region (NMGIRT-A1605). References (1) Hasegawa, Y.; Nakagawa, T.; Kawai, T. Recent Progress of Luminescent Metal Complexes with Photochromic Units. Coord. Chem. Rev. 2010, 254, 2643–2651. (2) Jiang, G. Y.; Wang, S.; Yuan, W. F.; Jiang, L.; Song, Y. L.; Tian, H.; Zhu, D. B. Highly Fluorescent

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Tunable Luminescence Contrast of Na0.5Bi4.5Ti4O15:Re (Re = Sm

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