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Orthogonally Polarized Luminescence of Single Bismuth Phosphate Microcrystal Doped with Europium

Peng Li, Feng Li,* Xiaoyu Zhang, Yiming Li, Xiaoxuan Luo, Ruimin Wang, Yin Cai, and Yanpeng Zhang

DOI: 10.1002/adom.202000583

structures from 0D quantum dots to 2D atomic thin films, depending on their anisotropic nature.[10–20] Compared with these structures, rare-earth dopants in micro/nanocrystals, ben-efitting from the unique and sheltered 4fN electronic configurations, exhibit some unique properties, such as multiple-wave-length emission, long-lived coherence, and excellent photostability at room tem-perature.[21–24] Although the suitable site symmetries of the crystal can endow the doped elements with the polarized emis-sions,[25] measurements are usually per-formed on bulk or powder states in which a lot of small particles have independently random orientations, thus resulting in unpolarized luminescence. Meanwhile, a remarkable number of works showing polarization properties of lanthanide-doped micro/nanocrystals were reported, including the studies on single particle of handedness structure and the electrically

aligned assembles of particles in microfluids.[5,26–31] Neverthe-less, a systematic investigation on the intrinsic linear polariza-tion feature of a single microcrystal, especially via the precise analysis on polarization degree, is still to be demonstrated.

In this article, we perform polarization-resolved spectroscopy on a single Eu-doped BiPO4 microcrystal, and observe a series of orthogonally polarized luminescence in the visible spectrum. Interestingly, a detailed analysis on the measured polarization feature shows that each emission line is partially polarized for this specific microcrystal, which can be well explained by a phe-nomenological model that proposes the existence of multiple C2 axes due to distortion and defects-induced symmetry breaking of the crystalline structure. Due to the polarization angle closely interrelated with the orientation of the targeted microcrystal, their detection can be used as a unique optical method to identify the single micro/nanocrystals from clusters, without needing high-resolution electron microscopies.

2. Results

X-ray diffraction (XRD) pattern of the as-prepared BiPO4:Eu samples is shown in Figure 1 (red line). The diffraction peaks match well with that of the pure monoclinic structure in the space group P21/n (black bars, JCPDS card no. 15–0767). All peaks are comparatively sharp, meaning that the samples

Rare-earth-doped micro/nanocrystals are significant candidates for nanopho-tonic and quantum information elements. Nevertheless, the polarization of light emission, which serves as one of the most widely adopted optical infor-mation carriers, has been rarely studied in such micro/nanocrystals due to the random crystalline orientation of the ensemble cluster as grown. Herein the polarization-resolved spectroscopy on a single europium-doped bismuth phosphate (BiPO4:Eu) microcrystal is performed, and a series of partially linearly polarized emission in the visible spectrum is observed, which can be grouped into orthogonal pairs. Such emission features are well explained by a phenomenological model of group theory that gives rise to the existence of optical C2 axes induced by local symmetry breaking. Such a polarization detection provides a neat optical method to distinguish single micro/nanocrystals from clusters, and confirms the tunable polarization features by selecting the suitable single microcrystal and engineering its orientation of crystalline axis. The rare-earth-doped single micro/nanocrystals, with precise positioning technique realized, can promisingly serve as controllable polariza-tion devices on integrated photonic circuits.

P. Li, Prof. F. Li, X. Y. Zhang, Y. M. Li, X. X. Luo, Dr. Y. Cai, Prof. Y. P. ZhangKey Laboratory for Physical Electronics and Devices of the Ministry of EducationSchool of Electronic Science and EngineeringFaculty of Electronic and Information EngineeringXi’an Jiaotong UniversityXi’an 710049, P. R. ChinaE-mail: [email protected]. R. M. WangSchool of ScienceXi’an Jiaotong UniversityXi’an 710049, P. R. China

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adom.202000583.

1. Introduction

The polarized features of photons are of great importance for variety of lighting applications. Especially, the linearly and/or orthogonally polarized light is desirable for use in laser, display, bioimaging, precision sensing and metrology, quantum com-munication, and computing,[1–9] and numerous applications are related to the fact that single micro/nanosized emitters can generate light with independently predesigned polariza-tion features. To date, the polarized luminescence behaviors have been widely explored in low-dimensional semiconductor

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exhibit good crystallinity. Compared with the hexagonal struc-ture, the formation of the monoclinic one needs the crystal-lographic system in higher free energy, which was confirmed by chemical potential theory as well as the experiments that involve phase transition by varying ion concentration or heating temperature.[32,33] The samples consist of polyhedral microcrys-tals with lengths ranging from 1 to 5 µm (Figure S1, Supporting Information). The long axis of the microcrystal is parallel to its crystallographic c-axis. The polyhedral morphologies are con-sidered as the geometrical evolutions of the standard mono-

clinic structure, in which the most exposed crystallographic faces are low-index (100) and (010) ones due to lower surface energies/activities.[32,34] By monitoring the direct emission wavelength of 593 nm of Eu3+ ion, the excitation spectrum of the samples (Figure S2, Supporting Information) shows the 1S0→3P1 transition of Bi3+ ions and Eu−O charge transfer band as well as a group of shape f−f transitions ranging from 300 to 450 nm within the Eu3+ 4f6 electronic configuration.[35,36] The most intense peak is located at 395 nm originating from the 7F0→5L6 transition which is chosen to excite the single BiPO4:Eu microcrystals.Figure 2a shows the optical microscope image of a single

microcrystal whose axis of length (crystalline c-axis) is oriented parallel to the surface of the quartz substrate, showing a length of ≈2 µm. With the 395 nm excitation beam focused on the microcrystal, it exhibits two distinguishable colors of blue and red under dark field imaging (Figure 2b), corresponding to the scattered excitation beam from the microcrystal and the emis-sion from 570 to 700 nm of the doped Eu3+ ions (Figure 2c), respectively. In the experiment, we measure the in-plane (i.e., parallel to the substrate surface) polarization angle, as the light collection is from the top of (i.e., perpendicular to) the substrate surface by an objective lens. We define the in-plane direction that is imaged parallel to the spectrometer entrance slit as the angle of 0°, and the microcrystal in Figure 2b is therefore oriented 30°. For the 395 nm excitation, the doped Eu3+ ions are first populated to the 5L6 state from the 7F0 one.


Figure 1. XRD patterns of the as-prepared BiPO4:Eu samples (red line) and the reference monoclinic structure of the undoped BiPO4 crystals (black bars, JCPDS card no. 15–0767).

Figure 2. a) Optical microscope image and b) the corresponding PL image and c) the spectrum of the single BiPO4:Eu microcrystal excited at 395 nm and recorded at 0° polarization. The dashed line in (b) indicates the 0° of the linear polarization angle defined by the experimental setup.




Then, they relax to the 5D0 singlet by nonradiative transitions of multiphonon,[37,38] finally to the 7FJ multiplet yielding the emis-sion bands consisting of the spiky sublevels. The quantities and intervals of these sublevels are determined by the surrounding crystal filed of the Eu3+ ions, which has been discussed much in literature.[39–41] For J = 0 with respect to the 5D0→7F0 tran-sition, it should not be observed in single crystals because of theoretical forbiddance. However, its intensity appears in our BiPO4:Eu microcrystal, most probably arising from the J–J mixing between J = 0 and J = 2 at the 7FJ states induced by local lattice distortion or defect.[42] On the basis of the J number, the spectrum in Figure 2c can be classified into five groups relative to the 5D0→7FJ (J = 0, 1, 2, 3, and 4) transitions in which some most intense sublevels are marked as No. 1 to 7 in Figure 3 and 4 for polarization analysis.

The photoluminescence (PL) intensity of each emission peaks changes dramatically when anticlockwise rotating the linear polarizer, as shown in Figure 3a, suggesting the emis-sion from the single microcrystal exhibits a relatively high linear polarization degree. The relative intensities of the marked seven peaks are plotted in Figure 3b, respectively, and

their oscillations can be fitted by cosine-squared functions of angle. The group of peaks marked as 1, 3, 4, 5, 6 (named as group I hereafter) exhibits intensity maxima at the same polarization angles, ≈π/6 and 7π/6 corresponding to the axial direction of the microcrystal (parallel to the crystal c-axis, see Figure 2b), while the peaks 2 and 7 (named as group II here-after) exhibit a π/2 rotation in polarization with respect to Group I peaks, corresponding to the radial/azimuthal direction of the microcrystal (perpendicular to the crystalline c-axis). We further confirm that the rotation of the c-axis of microcrystal on the substrate surface is accompanied by the same amount of rotation of the polarization angle of the luminescence, as will be shown later. The degree of polarization (DOP) for each peak is calculated by (Imax − Imin)/(Imax + Imin), and the results are listed in Table 1 (sample 1), showing that they are not purely linearly polarized (DOP < 100%).Figure 4a,b displays the optical microscope and PL images

of another single BiPO4:Eu microcrystal, exhibiting a rotation of the crystal c-axis on the substrate surface with respect to the one studied in Figure 3. The intersection angle is about 130° between its c-axis and the 0° polarization. As we anticlockwise


Figure 3. Changes of the emission intensities of a) the broad wavelength range and b) the marked seven peaks of the single BiPO4:Eu microcrystal with the polarizer angle, respectively. Dotted data in (b) are fitted with cosine-squared functions of angle (solid lines).

Table 1. The J number and the corresponding transition as well as the location, polarization, and degree of the polarization (DOP) of the marked seven intense sublevels.

J number Transition Peak Wavelength [nm] Polarizationa) DOP [%]

Sample 1 Sample 2

0 5D0→7F0 1 578.7 π 89 47

1 5D0→7F1 2 588.1 σ 40 43

3 593.5 π 42 40

2 5D0→7F2 4 612.9 π 31 37

5 621.8 π 33 5

4 5D0→7F4 6 686.1 π 56 57

7 699.0 σ 47 40

a)The polarization characteristics of the light associated with the electric vector: π, the dominant electric vector parallel to the c-axis of the microcrystal; σ, electric vector perpendicular to it.




rotate the polarizer, the collected luminescence intensities that cover the broad wavelength range are shown in Figure 4c. Also plotted in Figure 4d are the intensities of the selected seven peaks as the cosine-squared functions of angle. The corre-sponding DOPs are listed in Table 1 (sample 2). From Figure 4d, one can also see groups of peaks with intensity maxima at the same polarization angles, ≈130° and 310° which are parallel and perpendicular to the microcrystal c-axis, belonging to the so-called π and σ polarizations,[39] respectively. This observation, together with that in Figure 3, further confirms the direct rela-tion between the luminescence polarization and the crystalline orientation. These results are in contrast with the measurement on powders and colloidal solutions which usually exhibit unpo-larized luminescence due to randomly oriented crystalline axes.

3. Discussion

The polarization of the emitting lanthanide ion doped into host crystals is determined by its site symmetry.[25] The group theory considers the ion as a dipole oscillator or a magnetic rotor when it is in a symmetry site with one of the C2 sym-metry axis.[43–45] The light emitted through the dipole oscillator or magnetic rotor is polarized either linearly along the C2 axis (π polarization) or in the plane normal to this axis (σ polari-zation), as shown in Figure 5a. The site symmetry of Bi3+ ion

in the monoclinic phase BiPO4 is C1.[46,47] The optically active Eu3+ ions enter the BiPO4 lattice by substitution in Bi3+ sites, thus possessing the same site symmetries. The C1 site sym-metry, which is the lowest one in the 32 point groups, in prin-ciple does not have an optically polarized C2 axis.[48,49] However, π and σ polarizations occur, arising from lattice distortion and oxygen vacancies/defect formed during the doping and growth. Such cases cause symmetry breaking of the C1 sites and yield local C2 symmetry axes in the crystals. The clear observation of the 5D0→7F0 transition (peak 1) in the emission spectrum (Figure 2c), which would have been otherwise forbidden by C1 symmetry, is a good indicator for the existence of such defects. In addition, the reported investigation of Eu3+ in the ultraphos-phate lattice has also confirmed that slight lattice distortion can induce local properties of C2v site even though the overall site symmetry is extremely low (C1).[50] For π polarization, if all the induced local C2 axes are oriented in the crystallographic c-axis, purely linearly polarized emission (DOP = 100%) should be detected whatever the observation direction is. Interestingly, only partially π polarizations (DOP < 100%) was observed in our case of BiPO4:Eu microcrystals, revealing that multiple ori-entations of C2 axes might exist.

The simplified structure of monoclinic BiPO4 can be described as an assembly of monoclinic unit cells (Figure 5b), where the oxygen atoms occupy all the vertices of these little unit cells, each phosphorus atom is linked to four oxygen atoms to form


Figure 4. a) Optical microscope image and b) the corresponding PL image of the single BiPO4:Eu microcrystal excited at 395 nm laser and recorded at 0° polarization. Changes of the emission intensities of c) the broad wavelength range and d) the marked seven peaks with the polarizer angle, respectively. Dotted data in (d) are fitted with cosine-squared functions of angle (solid lines).




the discrete PO4 tetrahedral,[51] and the bismuth atoms occupy the centers of the two of every eight unit cells due to 1:4 of Bi:O in BiPO4. Due to randomness of the oxygen vacancies/defect, it is impossible that all the induced local C2 axes are oriented only along the c-axis of the microcrystal. However, due to the anisot-ropy of the crystal, it is very likely that most of them are oriented along the c-axis. The whole crystal could be phenomenologically modeled as having two overall C2 axes parallel and perpendicular to the c-axis of the microcrystal, respectively. These two orthog-onally orientated C2 axes thereby result in two orthogonally ori-ented transition dipole moments for π light: the major one μ par-allel and the minor one μ′ perpendicular to the microcrystal c-axis.

In order to understand the detected partial polarization fea-tures of emission, we draw an x–y–z coordinate in Figure 5, where the substrate surface is in the y–z plane, and the light detection is from the top of the substrate, i.e., along the x-axis (X view). If the two orthogonal C2 axes are orientated as illustrated in Figure 5b, a partially polarized π fluorescence should be observed through the X view, with DOP equal to µ µ θµ µ θ

−+

′′

′′

| | | | sin ( )

| | | | sin ( )

2 2 2

2 2 2, where θ is the angle between μ′ and the

x-axis direction. It is clear that the DOP varies with the relative values of μ, μ′, and θ, while a fully analytical prediction of the values of μ and μ′ from the crystalline structure is nevertheless hard to achieve due to the randomness of the asymmetry dis-tribution. This can well contribute to the fact that the DOPs of each peak in Figures 3 and 4 are different, as the orientations and values of the two dipole moments in the two measured microcrystals are not likely to be coincidently the same, due to the slight difference of the lattice distortion and defect distribu-tion between the two microcrystals. Especially, the difference in DOP of peak 1, which corresponds to a transition solely induced by the symmetry breaking (5D0→7F0), is extremely large. Com-pared to the π light, the case of σ light is more complicated, which involves a certain amount of circularly polarized emis-sion from the μ′ dipole. Nevertheless, it also essentially leads to partial linear polarization when detecting with X view. Finally,

it should be noted that the behavior of peak 5 in both Figures 3 and 4 show exclusive features, the reason of which is not unam-biguously clear. Most possibly, peak 5 may consist of a super-position of a π and a σ transition, which are too close to each other in spectrum to be resolved from the emission linewidth.

4. Conclusion

In conclusion, we observed orthogonally partially polarized emission from an individual monoclinic BiPO4:Eu micro-crystals, which was very different from the as-grown cluster or powder states and could be well explained by a phenom-enological model involving distortion and defects-induced symmetry breaking. Such a mechanism provides an onsite optical tool for distinguishing single micro/nanocrystals from polycrystalline pieces and clusters, without needing high-resolution electron microscopy. The method is even applicable for distinguishing very small clusters, whose DOP would still be much lower than a single microcrystal due to the randomness of the microcrystal orientation. The single microcrystal can be used to generate predesigned polarization states on photonic chips by selecting or engineering the suit-able microcrystal and its axis orientation, and could be poten-tially used for on-chip polarization devices and environment micro/nanosensors.

5. Experimental SectionSynthesis of BiPO4 Microcrystals Doped with 5 Atom% Eu Ions (written

as BiPO4:Eu): Dissolving Eu2O3 and Bi(NO3)3·5H2O in HNO3 solution and NH4H2PO4 in deionized water, Bi0.95Eu0.05(NO3)3 and NH4H2PO4 aqueous solutions were obtained, respectively. Then, these two were mixed with the same amount of substance to form BiPO4:Eu precursor. Its pH value was adjusted to be about 1 by adding aqueous ammonia. Under 1 h of vigorous stirring, the precursor was poured into the Teflon-lined stainless steel autoclave and heated at 180 °C for 9 h. After cooled down to room temperature naturally, the white precipitates were washed with absolute ethanol and deionized water, and collected by centrifugation, and then dried at 70 °C for 12 h. The as-prepared BiPO4:Eu powders were suspended in ethanol solution and dispersed onto quartz substrates, forming a distribution containing single rod-like microcrystals and clusters of microcrystals with random orientation of each microcrystal. In this work, the interest was in the optical properties of the single microcrystals.

Characterization: The crystalline structure of the as-prepared BiPO4:Eu powders was characterized by XRD (Rigaku D/max-2200) with Cu Kα (λ = 1.5406 Å) radiation at 40 kV, 50 mA. Scanning rate was 8° min−1 in the 2θ ranging from 15° to 50°. Scanning electron microscope image was recorded by a Hitachi S-4800 microscope at an accelerating voltage of 3 kV. Excitation spectrum was performed on a Hitachi F-4600 fluorescence spectrophotometer equipped with a 150 W xenon lamp source. For polarized PL measurements, a tunable femtosecond Ti:Sapphire laser (wavelength ≈800 nm) and a frequency doubler with BBO nonlinear crystal constituted a 395 nm excitation source. The images and spectra of the single microcrystals were recorded on a Leica DM2700 M microscope combined with a MS60 digital microscope camera and a SR-500i-D2 spectrometer equipped with an iXon Ultra EMCCD camera from Oxford Instrument. In this setup, a linear polarizer with working wavelength ranging from 400 to 700 nm was placed in front of the spectrometer entrance slit to characterize the linear polarization angle of the PL from the microcrystals. All spectra were taken with same integration times at room temperature.


Figure 5. a) Mechanisms of π and σ polarized light, respectively. b) Partially π polarized luminescence from single microcrystal. The little monoclinic unit cells are treated as the building blocks for the micro-crystal whose crystalline c-axis is oriented parallel to the z-axis. The two orthogonally orientated C2 axes result in two orthogonally oriented transi-tion dipole moments μ and μ' at the red Eu3+ sites. The major μ is parallel to the c-axis, and the minor μ' has an angle θ with the x-axis direction. The y–z plane is in the substrate surface, and the detection direction is then parallel to the x-axis (X view).





Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsThis research was supported by the National Natural Science Foundation of China (11804267, 11904279), the Natural Science Foundation of Shaanxi Province (2018JQ6041), and the National Science Foundation of Jiangsu Province (BK20180322).

Conflict of InterestThe authors declare no conflict of interest.

Keywordslinearly polarized emission, micro/nanosized emitters, rare-earth ions, single micro/nanocrystals

Received: April 6, 2020Published online:

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