Electrical and optical properties of Nd3+-doped Na0.5Bi0.5TiO3 ferroelectric single crystal

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2013 J. Phys. D: Appl. Phys. 46 245104

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IOP PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS

J. Phys. D: Appl. Phys. 46 (2013) 245104 (5pp) doi:10.1088/0022-3727/46/24/245104

Electrical and optical properties ofNd3+-doped Na0.5Bi0.5TiO3 ferroelectricsingle crystalChongjun He1, Yungang Zhang2, Liang Sun3,4, Jiming Wang1, Tong Wu1,Feng Xu1, Chaoling Du1, Kongjun Zhu5 and Youwen Liu1

1 College of Science, Nanjing University of Aeronautics and Astronautics, Nanjing 210016,People’s Republic of China2 Department of Physics, Harbin Institute of Technology, Harbin 150001, People’s Republic of China3 School of Physics and Electronic Engineering, Yibin University, Yibin 644000,People’s Republic of China4 School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin 150001,People’s Republic of China5 State Key Laboratory of Mechanics and Control of Mechanical Structures, Nanjing University ofAeronautics and Astronautics, Nanjing 210016, People’s Republic of China

E-mail: hechongjun@nuaa.edu.cn and ywliu@nuaa.edu.cn

Received 23 January 2013, in final form 5 May 2013Published 3 June 2013Online at stacks.iop.org/JPhysD/46/245104

AbstractSodium bismuth titanate Na0.5Bi0.5TiO3 (NBT) single crystal doped with Nd3+ was grown by atop-seeded solution growth method. Powder x-ray diffraction revealed a pure perovskitestructure with the rhombohedral phase. We found that the dielectric and ferroelectric propertieswere enhanced by the Nd3+ dopant. After poling along the [1 1 1] direction, transmittance wasenhanced dramatically. The Sellmeier dispersion equation and energy band gaps wereobtained. The absorption band around 808 nm has high full-width at half-maximum and largeabsorption cross-section, which is suitable for AlGaAs diode-laser pumping. A strongemission transition band of Nd3+ at around 1066 nm was observed; a long radiation lifetime324 µs shows a low quenching effect. These results indicate that Nd3+-doped NBT crystalcould be applied in photonic or integrated optoelectronic devices as a multi-functional crystal.

(Some figures may appear in colour only in the online journal)

1. Introduction

Ferroelectric materials have been widely used in advancedphotonic and microelectronic devices, such as high-speed lightmodulators, parametric oscillators and/or nonlinear frequencyconverters [1–4]. Moreover, extraordinary progress hasbeen made in the micro-engineering of ferroelectric domains,which allows generating frequency conversion processes in abroad spectral range [5, 6]. In addition, when convenientlyactivated with optical ions, ferroelectric crystals have alsodemonstrated laser action and intracavity self-frequencyconversion processes, which substantially increase their multi-functionality in integrated photonics [7–10].

Most of the piezoelectric materials used in industry,such as PZT ceramics and PZNT crystal, are lead-containingcompounds [11]. In recent years, with the growing demand

for global environmental protection, lead-free materials havegained considerable attention. Sodium bismuth titanate,Na0.5Bi0.5TiO3 (NBT), is one of the most important lead-freepiezoelectric materials with a perovskite structure [12]. Itsremanent polarization is Pr = 38 µC cm−2, coercive fieldis Ec = 73 kV mm−1, Curie temperature is Tc = 325 ◦Cand piezoelectric coefficient is d33 = 73 pC N−1. However,the main drawback of pure NBT is its high conductivity thatprevents proper poling. It has been reported that NBT-basedceramics modified with rare-earth elements, such as La, Eu, Smor Er ion, show improved piezoelectric properties and easierpolarization [13–16].

Several works have been devoted to the fluorescencebehaviour of rare-earth doped NBT ceramics or films[17, 18]. In Pr3+-doped NBT ceramics, the maximumphotoluminescence intensity occurs in the 0.4 mol% doped

0022-3727/13/245104+05$33.00 1 © 2013 IOP Publishing Ltd Printed in the UK & the USA

J. Phys. D: Appl. Phys. 46 (2013) 245104 C He et al

sample [19]. Ceramics or thin films are usually opaquesince the grain boundary results in light scattering, whichrestricts their applications in optical devices. Thus far, opticalproperties have seldom been reported in rare-earth-dopedNBT single crystals [20]. Here we show that Nd3+-dopedNBT single crystal has high transmittance, and excellentphotoluminescence properties. This work could promote theapplications of NBT crystal in photonic, microelectronic orintegrated optoelectronic devices.

2. Experimental

Nd3+-doped NBT single crystal was grown using a top-seeded solution growth (TSSG) method [20]. Enlightenedby the results in Pr3+-doped NBT ceramics, the Nd3+ ionconcentration was selected as 0.4 mol%, which was confirmedby inductive coupled plasma atomic emission spectrometry(ICP-AES) in the as-grown crystal. The crystal structurewas determined using an x-ray diffraction (XRD) apparatus(Almelo, The Netherlands) with CuKα (0.1541 nm) radiation.The grown crystals were oriented along the pseudo-cubic[0 0 1], [0 1 1] and [1 1 1] directions with 15′ accuracy using anx-ray orientation device. Silver paste was painted on the squareslices surfaces, and sintered at 650 ◦C to make electrodes.The polarization–electric field (P –E) hysteresis loop wasmeasured at 1 Hz frequency, using a ferroelectric analyser (TF1000, aixACCT, Germany). Temperature-dependent dielectricconstant was measured using an impedance analyser (HP4294A type).

For optical measurements, specimens were poled at 80 ◦Cunder 5 kV mm−1 for 30 min in a silicon oil bath. In orderto achieve a final optical polish with specular reflection, thesquare surfaces for light transmission were polished usingalumina and diamond polishing compounds (with decreasingaverage grit size down to 0.5 µm). Refractive indiceswere obtained by the Brewster’s angles (θB = tan−1n) atdifferent wavelengths. For polarized light with polarizationdirection parallel to incident plane, Brewster’s angle (θB)

is the incident angle at which the reflection intensity iszero. Transmittance spectra were measured as a function ofwavelength using a Perkin Elmer Lambda-900 UV-Visible-NIR spectrophotometer. The Infrared emission spectrum wasmeasured using a ZOLIX SBP300 spectrophotometer with theInGaAs detector under the excitation of 808 nm laser. Tomeasure fluorescence lifetime, the continuous wave 808 nmlaser was modulated by an electro-optic modulator (LeysopEM200K) with square wave modulation. The induced time-resolved curve was recorded by a digital phosphor oscilloscope(Tektronix TDS 5052).

3. Results and discussion

3.1. Structure and electrical investigation

Figure 1 shows the XRD pattern of Nd3+-doped NBT crystalin the 2θ range 20◦–80◦. The crystal shows a rhombohedralphase with unit-cell parameters a = b = c = 0.3883 nm,α = β = γ = 89.21◦. When Nd3+ ion is doped in the

Figure 1. Powder XRD of Nd3+-doped NBT crystal, the diffractionpeaks are marked with pseudo-cubic structure.

Figure 2. Temperature dependence of dielectric constant εr at 1 kHzfor 〈0 0 1〉-oriented Nd3+-doped NBT crystal.

NBT crystal, Nd3+ can occupy A-site (Na+ and Bi3+) sincethe ionic radius of Nd3+ (0.098 nm) closely matches that ofNa+ (0.102 nm) or Bi3+ (0.103 nm). It can be seen that theNd3+-doped NBT crystal displays typical ABO3 perovskitediffraction peaks at room temperature [17, 20]. Meanwhile,no second phase can be detected. Therefore, it can be deducedthat the Nd3+ has entered into crystalline lattice structure ofthe NBT crystal to form a homologous solid solution. Theaddition of Nd3+ ion does not lead to an obvious change in thephase structure.

Figure 2 shows temperature dependence of dielectricconstant εr for 〈0 0 1〉-oriented Nd3+-doped NBT crystal. Amaximum can be observed around 350 ◦C, which may beattributed to the tetragonal anti-ferroelectric to paraelectricphase transition. The broadness of temperature-dependent εr

indicates a diffuse transformation somewhat analogous to thatof relaxor ferroelectrics. Figure 3 shows the P –E hysteresisloop of 〈0 0 1〉-oriented crystal at room temperature. A nearlysaturated hysteresis loop was found under E = 7 kV cm−1.The coercive field (Ec) and remanent polarization (Pr) were

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J. Phys. D: Appl. Phys. 46 (2013) 245104 C He et al

Figure 3. Polarization hysteresis loop of 〈0 0 1〉-orientedNd3+-doped NBT crystal at room temperature.

Figure 4. Transmittance spectra of Nd3+-doped NBT single crystalpoled along the [0 0 1] direction (dashed line), the [0 1 1] direction(dotted line), and the [1 1 1] direction (solid line). All the specimensare with 1.0 mm thickness.

about 3.7 kV mm−1 and 30 µC cm−2, respectively. In thecrystal, the nonvolatile Nd3+ ion tends to replace the volatileBi3+ ion due to their same valence. Therefore, oxygenvacancies created by Bi-vacancy are reduced. The addition ofNd3+ ion reduces leakage of NBT crystal which in turn leadsto the ferroelectric property improvement.

3.2. Transmittance

Figure 4 shows transmittance spectra of different orientedNd3+-doped NBT crystals. For all specimens, thetransmittance begins to increase abruptly at around 400 nm,which corresponds to optical absorption edges in the ultravioletregion. The curves rise near 450 nm, the crystals aretransparent in the visible and near IR spectral region.These characteristics are similar to what was observed inmost crystals with the oxygen-octahedral perovskite structure[21, 22]. As compared with [0 0 1] and [0 1 1]-poled crystals,[1 1 1]-poled crystal has higher transmittance and sharper

Figure 5. Refractive indices of [1 1 1]-poled Nd3+-doped NBTsingle crystal as a function of wavelength. The symbols aremeasured values and the solid line is fitting result of Sellmeierdispersion equation.

UV absorption edge. The transmittance of [1 1 1]-poledNd3+-doped NBT single crystal is more than 50% above0.45 µm. In general, the optical transmittance is reduced byreflection and scattering losses. By the Fresnel expression, thereflection loss at two faces of crystal plate can be calculatedby R = (n − 1)2/(n2 + 1). As shown in figure 4, reflectionloss is about 30%, the scattering and absorption losses are verysmall. Therefore, the [1 1 1]-poled Nd3+-doped NBT crystalcan be used as an optical crystal in a wide wavelength region.

3.3. Dispersion behaviour

Figure 5 shows measured refractive indices of [1 1 1]-poledNd3+-doped NBT crystal. By least square fitting, typicalSellmeier dispersion equation can be obtained, that is

n2 = 5.358 +0.254

λ2 − 0.049+ 0.016λ2, (1)

where λ is wavelength in micrometres [23, 24]. The solid linein figure 5 depicts the fitting result of the Sellmeier dispersionequation. It can be seen that the refractive index decreasesdramatically when the wavelength increases.

Similar to most ABO3 type perovskite structurecompounds, Nd3+-doped NBT single crystal has largerefractive indices and obvious dispersion relation. Its refractiveindices decrease fast with increasing wavelength. Becauseof the similar basic BO6 octahedron building block, theyhave similar energy band structure determining the refractiveindices. B-cation d orbital and O-anion 2p orbital are themajor contributors to the refractive indices. The commonstructural unit in oxygen-octahedral ferroelectrics leads tosimilar behaviour of the refractive indices [21–24].

3.4. Absorption and band gap

Absorption coefficient α can be calculated from the relation

α = − 1

Lln

−(1 − R)2 +√

(1 − R)4 + 4R2T 2

2R2T, (2)

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J. Phys. D: Appl. Phys. 46 (2013) 245104 C He et al

Figure 6. Absorption spectrum of [1 1 1]-poled Nd3+-doped NBTsingle crystal.

where L is the thickness of the specimen, T is the transmittanceandR is the surface reflectivity [R = (n−1)2/(n+1)2] [21, 25].Figure 6 shows the absorption coefficient of [1 1 1]-poledNd3+-doped NBT crystal. The spectrum reveals absorptionpeaks due to transitions from ground 4I9/2 state to differentexcited states. Strong Nd3+ absorption lines occur near 585(2G7/2 + 4G5/2), 748 (4F7/2 + 4S3/2), 805 (4F5/2 + 2H9/2) and878 nm (4F3/2) [17]. The absorption band at 808 nm has a15 nm full-width at half-maximum (FWHM), which is suitablefor AlGaAs diode-laser pumping. Absorption cross-sectionσab = α/Nc, where α is the absorption coefficient and Nc isthe Nd3+ ion concentration (6.79 × 1019 cm−3) in the crystal.Thus, the absorption cross-section is σab = 1.47 × 10−20 cm2

at 808 nm.The optical band gap, which is responsible for the sharp

absorption edge of [1 1 1]-poled Nd3+-doped NBT crystal,can be estimated by the predominant mechanism of interbandtransitions [22, 25]. In allowed direct transition, the electronsin the valence band transit vertically to the conduction bandunder photon excitation. Absorption coefficient as a functionof photon energy in the allowed direct transition can beexpressed as

(αhν)2 = A(hν − Egd), (3)

where A is a constant, Egd is the direct energy band gapand hv is the energy of incident light. In indirect transition,the electrons transit from top of the valence band to bottomof the conduction band with participation of photons andsuitable phonons. Indirect band gap can be deduced from theabsorption coefficient α through

(αhν)1/2 = B(hν − Egi ± Ep), (4)

where B is a constant, Egi is the allowed indirect energy bandgap and Ep is the energy of the absorbed (+) or emitted (−)phonons. Figure 7 shows the curves of (αhv)2 and (αhv)1/2

versus hv. The direct energy band gap was determined asEgd = 3.05 eV by extrapolating the linear portion of the curveto zero. For indirect transition, the values of Eg1 = Egi + Ep

and Eg2 = Egi − Ep were deduced as Eg1 = 2.93 eV and

Figure 7. Photon energy hv dependence of (αhv)2 and (αhv)1/2 in[1 1 1]-poled Nd3+-doped NBT single crystal.

Table 1. Energy band gap Eg of NBT single crystals at roomtemperature.

Single crystals Nd : NBT Mn : NBT NBT NBT

Eg (eV) 3.05 2.92 ∼3 3.03Reference This work [20] [26] [27]Method Experimental measurements Calculation

Eg2 = 2.78 eV, respectively. Therefore, the indirect energyband gap and the phonon energy are Egi = 2.86 eV andEp = 0.07 eV, respectively.

As shown in table 1, our experimental results areconsistent with other groups. Ge et al observed absorptionpeak of Mn-doped NBT crystal at 425 nm, indicating 2.92 eVband gap [20]. Andriyevsky et al measured optical spectra ofNBT crystal by spectroscopic ellipsometry, and believed thatenergy E = 3 eV is close to the band gap energy Eg [26].Chan et al studied optical properties of NBT crystal by first-principles calculation, and estimated that the optical band gapis about 3.03 eV [27].

3.5. NIR fluorescence

Figure 8 shows the infrared emission spectrum of Nd3+-dopedNBT crystal excited by 808 nm laser (associated withNd3+ : 4I9/2 → 4F5/2 + 2H9/2) at room temperature. Thespectrum consists of broad bands at 860–940, 1040–1120 and1310–1420 nm. Each band composes of several sharp peaks.These emission bands are consistent with the Nd3+ emissionobserved in NBT ceramics [17]. The 860–940 nm emissionwith two peaks at 883 and 911 nm results from the transition4F3/2 → 4I9/2, the band at 1040–1120 nm with highest peak at1066 nm is due to the electronic transition 4F3/2 → 4I11/2, andthe band at 1310–1420 nm with the highest peak at 1344 nm isowing to the transition 4F3/2 → 4I13/2 [28, 29]. Fluorescencelifetime was determined to investigate fluorescence quenchingeffect of Nd3+-doped NBT crystal. The inset of figure 8depicts fluorescence decay curve of the emission transition(4F3/2 → 4I11/2) at 1066 nm, which demonstrates a singleexponential decay. The excited state lifetime is estimated as

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J. Phys. D: Appl. Phys. 46 (2013) 245104 C He et al

Figure 8. Fluorescence spectrum of [1 1 1]-poled Nd3+-doped NBTsingle crystal pumped by 808 nm continuous wave laser. The insetshows the single exponential decay curve of the emission transition(4F3/2 → 4I11/2) at 1066 nm pumped by 808 nm pulsed laser.

τs = 324 µs [30]. This result signifies that the Nd3+-dopedNBT crystal has very low fluorescence quenching effect.

4. Conclusion

We have demonstrated the incorporation of Nd3+ ion intothe relaxor ferroelectric NBT single crystal. A purerhombohedral perovskite phase is proved by powder XRD.The dielectric and ferroelectric properties are improved by0.4 mol% Nd3+ dopant. For the application in optical devices,the optical properties of Nd3+-doped NBT crystal were studiedsystematically. After poled along the [1 1 1] direction, thetransmittance is more than 50% above 0.45 µm, which ismuch higher than the crystals poled along the [0 0 1] or [0 1 1]direction. In practical application, 30% reflection loss can beeliminated by antireflection coating. The Sellmeier dispersionequation was obtained. Energy band gap of direct and indirecttransitions are Egd = 3.05 eV and Egi = 2.86 eV, phononenergy is Ep = 0.07 eV. These results are in agreement withthe data reported by other researchers. The absorption bandaround 808 nm has 15 nm FWHM, absorption cross-sectionσab = 1.47 × 10−20 cm2, which are suitable for diode-laserpumping. Excited by 808 nm laser, strong emission band wasobserved around 1066 nm. Lifetime 324 µs of metastable state4F3/2 shows a very low fluorescence quenching effect.

Acknowledgments

This work has been financially supported by the FundamentalResearch Funds for the Central Universities (Grant NoNS2012120), the National Natural Science Foundation ofChina (Grant Nos 11104144, 11174147, 51172108, 61205201,50902027, 11004103), the Natural Science Foundation of

Jiangsu Province of China (Grant No BK2011721), theSpecialized Research Fund for Doctoral Program of HigherEducation, China (Grant No 20093218120030), the Programfor New Century Excellent Talents in University, China (GrantNo NCET-10–0070) and the Cheung Kong Scholars InnovativeResearch Team Project (Grant No IRT0968).

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