Introduction

Eu2+-activated phosphors usually exhibit broadband luminescence under external excitations1,2,3. Most of these emissions are attributed to the dipole-allowed 4f65d→4f7 transitions associated with relatively large transition probabilities and short radiative lifetimes (0.4−1.8 μs)3. In these phosphors, the emissions from the localized 4f65d states exhibit typical Stokes shifts in the range of 0.1−0.5 eV3.

Besides the normal Eu2+ 4f65d→4f7 transitions, some Eu2+-activated phosphors exhibit extraordinarily broad emission bands associated with abnormally long radiative lifetimes (typically >2 μs) and extremely large Stokes shift (typically >0.6 eV)4,5,6,7,8,9. This type of emission was described as anomalous luminescence by McClure and Pedrini4. In their model, the lowest 4f65d states of Eu2+ generally lie inside the host conduction band and the excitation of a Eu2+ ion results in its autoionization, followed by trapping of a conduction electron close to the ionized Eu2+ ion. As a result, a Eu-trapped exciton becomes the emitting state. The transition between the exciton-like state and the 4f7 ground state of Eu2+ is forbidden by the dipole selection rule in the anomalous luminescent phosphors4; therefore the transition probabilities for both anomalous luminescence and its corresponding excitation are generally small, resulting in a long radiative lifetime and an weak (often undetectable) exciton excitation band. Moreover, in anomalous luminescent phosphors, the delocalization of the lowest excited state (i.e., the exciton-like state) induces the ligand ions around the Eu2+ to undergo a considerably larger relaxation than for the normal case of Eu2+, which is reflected in the emission spectrum by an extremely large Stokes shift4,6.

There is also a special class of Eu2+-activated phosphors that exhibit the characteristics of both normal 4f65d→4f7 transitions (e.g., large transition probabilities for emission and excitation) and anomalous luminescence (e.g., very large bandwidth and Stokes shift of emission)10,11,12,13,14,15. Based on the relationship between the luminescence and host crystalline structure in this special class of Eu2+-activated phosphors, Poort et al. attributed this type of emission to 4f65d→4f7 transitions and suggested that the linear chain structure of large cations in the lattice (depending on the values of lattice parameters) contributed to this special luminescence10. However, with the emergence of more and more novel Eu2+-activated phosphors, in which the chain structure of the large cations is usually absent in the hosts11,12,13,14,15, Poort's explanation is no longer adequate for this special type of broadband emissions.

In this study, we identify and investigate a Eu2+ broadband emission associated with both a large transition probability and an extremely large Stokes shift (0.92 eV) in a new concentrated europium aluminate phosphor, hexagonal EuAl2O4 (h-EAO). We attribute this unusual emission to the delocalized nature of the excited electrons in the 4f65d emitting level. That is, a ground-state 4f7 electron is excited to the lowest 4f65d state, from where the excited electron may delocalize to the host continuum state according to a small probability, followed by radiative emission from the 4f65d state. This investigation provides new insights into a unique type of Eu2+ luminescence in solids whose emission exhibits both a very large bandwidth and an extremely large Stokes shift.

Results

Photoluminescence (PL) at room temperature and low temperature

The h-EAO phosphor was fabricated by carbothermal reaction and evaporation of a mixture of Eu2O3, Al2O3 and graphite powders at 1450°C in a flowing argon environment in a tube furnace system. The detailed fabrication process was described in ref. 16. The carbothermal reaction produced a weakly reducing environment which facilitated the formation of effective Eu2+ luminescence centers in the concentrated h-EAO phosphor16,17. The h-EAO phosphor is nanoribbon-like with typical widths of 200–1000 nm, thicknesses of 50–300 nm and lengths of 50–500 μm16. The h-EAO nanoribbons exhibit a hexagonal space lattice with a new set of lattice parameters of a = b = 6.15 Å and c = 10.57 Å, as determined by synchrotron X-ray Laue microdiffraction16 and further confirmed by synchrotron X-ray microcrystallographic measurements (see Supplementary Information, including Supplementary Table 1 and Supplementary Fig. 1, for the determination of the unit cell structure of h-EAO). The X-ray microdiffraction results showed that each nanoribbon in the phosphor is a single crystal with composition EuAl2O4 and no evidence was found to suggest the presence of second phases or inclusions embedded in the single crystals. X-ray absorption near edge structure measurement on Eu L3 edge showed that the Eu ions in h-EAO are predominantly in the divalent state, i.e., Eu2+ (ref. 16).

Under blue light or ultraviolet excitation, the h-EAO phosphor exhibits intense orange emission. Figure 1a shows the normalized PL emission and excitation spectra of h-EAO at room temperature, as well as the low-energy side of the diffuse reflectance absorption spectrum. At room temperature, the shapes of the emission and excitation spectra are independent of the corresponding excitation and monitoring wavelengths, respectively (Supplementary Fig. 2a). The emission spectrum contains a Gaussian-shaped emission band peaking at 645 nm (~1.93 eV) with a full width at half maximum (FWHM) of 0.45 eV. The PL excitation spectrum in Fig. 1a contains two excitation bands: a high-energy band peaking at 365 nm (~3.40 eV) and a low-energy band peaking at 435 nm (~2.85 eV). The low-energy 435 nm excitation band starts at ~540 nm (~2.3 eV), which coincides with the onset of the diffuse reflectance absorption spectrum and conforms to the “mirror image” of the 645 nm emission band (dashed curve in Fig. 1a), indicating that the 435 nm excitation band corresponds to the lowest excited state in the phosphor. Thus, the Stokes shift of the emission can be determined to be 0.92 eV, which is equal to the energy difference between the intensity maxima in the lowest excitation band and the emission band.

Figure 1
figure 1

PL emission and excitation spectra of h-EAO phosphor.

(a), Emission (red), excitation (blue) and diffuse reflectance absorption spectra of h-EAO phosphor acquired at room temperature. The dashed line curve represents the “mirror image” reflection of the emission spectrum about the zero-phonon line energy E0. (b), Normalized emission and excitation spectra of h-EAO acquired at 77 K. The difference between the emission spectra excited by 355 nm and 460 nm is represented by an additional emission band peaked at 550 nm.

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In contrast to the room-temperature luminescence, at 77 K the shapes of the emission and excitation spectra become dependent on their respective excitation and monitoring wavelengths (Fig. 1b). A Gaussian-shaped emission band peaking at 645 nm is obtained under 460 nm excitation; the spectral profile of this emission band is similar to that obtained at room temperature except for reduced FWHM of the emission (0.38 eV). Under the excitation of 355 nm light, however, a shoulder appears on the high-energy side of the 645 nm emission band (for comparison, the 645 nm emission spectra acquired at 77 K and 300 K under 355 nm excitation are re-displayed in Supplementary Fig. 2b). The difference between the normalized emission spectra excited under 355 nm and 460 nm light is represented by an additional small band peaked at 550 nm (Fig. 1b). Moreover, at 77 K, the excitation spectrum obtained while monitoring the high-energy emission shoulder (e.g., at 530 nm; by selecting this wavelength we can monitor the 550 nm emission band and avoid the overlap with the 645 nm band) shifts to a shorter wavelength than the excitation spectrum of the 645 nm emission (Fig. 1b). Both the emission and excitation spectra suggest the existence of two different Eu2+ luminescent sites in h-EAO. We call them Eu1 site and Eu2 site and they are responsible for the 645 nm and 550 nm broadband emissions at 77 K, respectively, as indicated in Fig. 1b.

Spectral identification of two Eu2+ luminescent sites

To confirm the presence of two Eu2+ sites in h-EAO, we measured wavelength dependent luminescence decays under the excitation of a 355 nm pulsed laser at 77 K. A set of luminescence decay curves were recorded at 77 K by monitoring emission wavelengths between 550−690 nm in 10 nm steps (Supplementary Fig. 3). Under the assumption that there exist two Eu2+ sites, we fitted the decay curves using a double exponential formula I(t,λ) = A1(λ) × exp(-t/τ1) + A2(λ) × exp(-t/τ2), where I(t,λ) represents the total emission intensity at time t after the excitation pulse and at the monitoring wavelength λ; A1(λ) and A2(λ) are the fractional PL emission intensities contributed by Eu1 site and Eu2 site, respectively, at the monitoring wavelength λ; τ1 and τ2 are the luminescence lifetimes of Eu1 site and Eu2 site, respectively. It is expected that once the lifetimes τ1 and τ2 are determined, the existence of Eu1 site and Eu2 site can then be confirmed. At the same time, the PL spectral contributions from the two sites can be determined using the A2(λ)/A1(λ) ratios. Our fitting results showed that the luminescence decay curves monitored between 630−690 nm can be well reproduced by a series of monoexponential functions (i.e., A2(630−690) = 0) with the fitted lifetime τ1 = 1.27 μs (Supplementary Fig. 3). As an example, the decay curve monitored at 650 nm and its fitted profile are given in Fig. 2a. In contrast to the monoexponential decay curves obtained between 630−690 nm, the decay curves monitored between 550−620 nm can be well reproduced by a series of double-exponential functions with the fitted lifetime τ2 = 0.57 μs and a fixed τ1 (1.27 μs) (Supplementary Fig. 3). The A1(550−620) and A2(550−620) values were obtained in the same fitting. As an example, the decay curve monitored at 550 nm and its fitted profile are also shown in Fig. 2a. The A2(λ)/A1(λ) ratios are then plotted as a function of emission wavelengths (550−690 nm) in Fig. 2b (solid ball curve), along with the emission spectra from the two sites at 77 K. The monoexponential decay behavior (A2/A1 = 0) between 630−690 nm is consistent with the absence of the emission from the Eu2 site in this long-wavelength emission region (Fig. 2b). In the range of 550−620 nm, the A2/A1 value increases as the wavelength decreases, which corresponds to the increase of the 550 nm emission contribution and the decrease of the 645 nm emission contribution; this verifies the existence of the additional 550 nm emission band from Eu2 site at 77 K.

Figure 2
figure 2

Spectral identification of Eu2+ luminescent sites in h-EAO.

(a), Normalized luminescence decay curves of h-EAO monitored at 650 nm and 550 nm emissions at 77 K. The excitation source is a 355 nm Nd:YAG pulsed laser. The red solid lines are the fitted profiles. The black dashed lines are the decomposed decay compositions. Inset is a zoomed-in view of the initial stage of the decays for the two emission wavelengths. (b), Plot of PL spectral distribution (A2/A1) over two Eu2+ sites (Eu1 and Eu2) as a function of emission wavelengths at 77 K (solid balls). The line curves are the PL emission spectra from Eu1 and Eu2 sites at 77 K, which are the same as the emission curves in Fig. 1b.

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We attribute the 550 nm emission band and its corresponding excitation spectrum at 77 K (Fig. 1b) to the 4f65d↔4f7 transitions of the Eu2+ luminescent center (Eu2 site) due to the spectral similarity between the current spectra and the reported spectra of Eu2+-activated green aluminate phosphors3. However, the lifetime of the 550 nm emission (τ2, 0.57 μs) is much shorter than the typical radiative lifetimes of the green emissions from the Eu2+-activated green aluminate phosphors (~1.2 μs)17. This can occur because the 550 nm emission from Eu2 site might be partially quenched by the energy transfer between Eu2 site and Eu1 site even at 77 K. That is, the lifetime of 0.57 μs at 77 K may not represent the radiative lifetime of Eu2 site. Indeed, the small spectral overlap between the 550 nm emission band (Eu2 site) and the 435 nm excitation band (Eu1 site) at 77 K (Fig. 1) makes it possible for the energy transfer to occur from the Eu2 site to the Eu1 site. Moreover, the decay curve monitored at 650 nm (Eu1 site only) exhibits a build-up at the initial decay stage (0.34−0.46 µs, the shaded area in the inset of Fig. 2a), while during that same time period the decay curve monitored at 550 nm (dominated by Eu2 site) exhibits a faster decay. The different initial decays at the 650 nm and 550 nm emissions provide unambiguous evidence for energy transfer from the Eu2 site to the Eu1 site. At higher temperature, e.g., room temperature, more efficient energy transfer from the high-energy Eu2 site to the low-energy Eu1 site is expected due to the stronger spectral overlap. Thus, the energy transfer can cause the complete quenching of the 550 nm emission band at room temperature (Fig. 1a).

Localized/delocalized characters of the 645 nm emitting state

The above results reveal that the 645 nm emission band from the Eu1 site is the dominant emission band at 77 K and the only emission band at room temperature. The nature of the emitting state corresponding to the 645 nm emission band can be initially revealed by its radiative lifetime whose value (1.27 µs) is close to the typical value (~1.1 µs) of the normal Eu2+ 4f65d→4f7 emission in solids17. This indicates that the wave function of the emitting state (i.e., the lowest excited state) should contain dominant 4f65d compositions. Meanwhile, the presence of a distinct PL excitation band at 435 nm indicates that there is a large transition moment between the 4f7 ground state and the lowest excited state, which further confirms the dominant 4f65d character of the emitting state18. Thus, the 645 nm emission may originate from the lowest 4f65d level of Eu2+ luminescent center (Eu1 site) whose eigenfunction is a localized 4f65d composition. However, both the FWHM (0.38 eV at 77 K and 0.45 eV at room temperature) and the Stokes shift (0.92 eV) of the 645 nm emission are significantly larger than those reported previously in Eu2+-activated aluminate phosphors3. These extraordinarily large values suggest that the 645 nm broadband emission is uncharacteristic of the normal Eu2+ 4f65d→4f7 transition. Based on the above discussion, we deduce that the emitting state corresponding to the 645 nm emission band is essentially a 4f65d composition but probably not a normal localized state.

To further understand the unusual 645 nm emission, we conducted photoionization experiments based on thermoluminescence (TL) excitation spectroscopy. The measurements were carried out by illuminating h-EAO at 77 K for 10 min with monochromatic light whose wavelength was tuned between 320–540 nm in 10 nm steps. If the excitation energy is high enough to promote the 4f7 ground state electron to a delocalized state, the delocalized electron possibly will be captured by electron traps (such as disordered oxygen vacancies, which are inevitably created in reducing synthesis environment19) at 77 K and subsequently released during heating, followed by the recombination with the ionized luminescent center. Figure 3a shows a set of TL curves monitored at 645 nm; each was recorded from 77 to 380 K at a linear heating rate of 4 K/s. These TL curves show similar spectral shapes but with varied TL intensities. A typical TL spectrum contains an asymmetric broadband ranging from 90 to 290 K and peaking at ~135 K, indicating that the traps are distributed over a wide range of energies below the host conduction band in h-EAO. According to previous studies20,21,22, the delocalization energy of the excited electron at 77 K can be determined from the TL creation spectrum, i.e., the plot of TL integrated intensities versus the illumination wavelengths, as shown by the red ball curve in Fig. 3b (Note that the effect of thermally activated ionization is eliminated at 77 K). For comparison, the PL excitation spectrum at 77 K monitoring the 645 nm emission is also presented. Figure 3b shows that the onset of the electron delocalization energy coincides with the onset of the lowest PL excitation energy at ~500 nm, revealing the delocalized character of the lowest excited state of the Eu1 site at 77 K. Taking the luminescence performances of Eu1 site into account, the optical excitation of the lowest excited state in h-EAO at 77 K should be followed by an autoionization process18. This means that the lowest excited state (i.e. the emitting state) is a mixed state consisting of dominant localized 4f65d states and complementary delocalized conduction-band-like states. In other words, the 645 nm emitting state acquires some properties of the delocalized state, opening the channels for electron delocalization from the Eu2+ luminescent center.

Figure 3
figure 3

TL excitation spectroscopy measurements of h-EAO.

(a), TL curves of h-EAO monitored at 645 nm emission. The sample was illuminated with 320–540 nm light in 10 nm steps for 10 min at 77 K. The measured temperature varied from 77 to 380 K. (b), TL integrated intensities as a function of excitation wavelengths (red balls). The solid line curve is the PL excitation spectrum of h-EAO monitored at 645 nm emission at 77 K, which is the same as the blue line curve in Fig. 1b.

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Discussion

The autoionization-associated 4f65d emitting state proposed here is different from either the localized 4f65d state of the normal 4f65d→4f7 transition or the delocalized exciton-like state of anomalous luminescence. As a result of electron delocalization in the emitting state, the ligand ions around the luminescent center (i.e., Eu1 site) will undergo a considerable relaxation, which is reflected in the emission spectrum by an extraordinarily large FWHM and an extremely large Stokes shift in h-EAO. Besides the energy threshold at ~500 nm, the TL creation spectrum in Fig. 3b also contains a higher energy threshold at ~420 nm. The strong electron delocalization in the high-energy region (i.e., 320−420 nm) can be attributed to the autoionizing intermediate states, i.e., the autoionization of 4f65d states lying in the host conduction band20,21,22.

We have carried out a detailed study on a new hexagonal EuAl2O4 phosphor whose Eu2+ luminescence exhibits a very large bandwidth and an extremely large Stokes shift. At 77 K, two luminescent sites responsible for 645 nm and 550 nm broadband emissions were identified, while at room temperature, the 550 nm emission band was quenched due to energy transfer. The 645 nm emission exhibits both characteristic and uncharacteristic properties of a normal Eu2+ 4f65d→4f7 transition, evidenced by a typical Eu2+ lifetime of 1.27 μs and an extremely large Stokes shift of 0.92 eV, respectively. Photoionization experiments showed that the emitting state of the 645 nm emission is a mixed state consisting of dominant localized 4f65d states and complementary delocalized conduction band-like states.

Methods

The optical properties of the h-EAO phosphor were studied using an array of spectral methods. The PL spectra were recorded using a FluoroLog-3 spectrofluorometer equipped with a 450-W xenon arc lamp as the excitation source. The diffuse reflectance absorption was measured using a Shimadzu UV-2450 UV-Vis spectrometer. The luminescence lifetime was recorded using a photomultiplier tube and a digital oscilloscope under excitation with the third harmonic (355 nm) of a Nd:YAG pulsed laser. The TL curves were recorded using the Fluorolog-3 spectrofluorometer and a homemade TL setup. Appropriate optical filters were used to avoid stray light in all optical measurements.