Effect of concentration of Nd3+ on the photoluminescence and ferroelectric properties of Bi4-xNdxTi3O12 films

Lead-free films of Bi4-xNdxTi3O12 were deposited on Pt(111)/Ti/SiO2/Si(100) substrate via spin-coating methods. It is shown that there are no secondary phases in the films of Bi4-xNdxTi3O12 and clear interfaces between the films of Bi4-xNdxTi3O12 and substrates when the films are annealed at 700 °C. And the films of Bi4-xNdxTi3O12 also exhibit a blue light emission peak at 437 nm and a yellow light emission peak at 580 nm. There are narrower band gaps, greater values of dielectric constant and lower values of dielectric loss when the concentration of Nd3+ varies from 0 to 0.85. And the films of Bi4-xNdxTi3O12 possess the minimum of band gap energy (2.67 eV). Moreover, the films of Bi4-xNdxTi3O12 exhibit minimal leakage current density and maximal remanent polarization, which is highly beneficial for the potential applications in multi-functional devices.


Introduction
Since the ferroelectricity in Rochelle salt crystals was discovered by French Valasek in 1920, ferroelectric materials have become a research hotspot [1] and been widely used in transducers, filters, resonators, ferroelectric memories and other fields [2][3][4][5][6]. Up to date, the widely used ferroelectric materials are leadbased because of their stable and excellent ferroelectric properties [7][8][9][10]. However, lead-based ferroelectric materials undesirably bring about harm to human beings and pollution to environment during the preparation and application processes. As a result, researchers have been looking for alternative lead-free ferroelectric materials with good ferroelectricity to rival the lead-based ferroelectrics such as PbZr x Ti 1-X O 3 [11].
Bismuth titanate (Bi 4 Ti 3 O 12 ), a typical lead-free multiferroics, has drawn widespread attention owing to its high Curie temperature and excellent fatigue resistance [12][13][14]. Unfortunately, its remnant polarization is rather low because of the evaporation of Bi 3? , which gives rise to the increase in concentration of oxygen vacancies and the deterioration of ferroelectricity [15]. Thus, more and more researchers have been trying to inhibit the evaporation of Bi 3? through doping of rare elements in Bi 4 Ti 3 O 12 [16][17][18]. M. Chen [19] reported that the substitution of Pr 3? for Bi 3? will decrease the space charge density which improves ferroelectric properties of Bi 4 Ti 3 O 12 materials. And C. P. Cheng [20] reported that the doping of Dy 3? will enhance the ferroelectricity of Bi 4 Ti 3 O 12 . U. Chon [21] and X. Y. Mao [22] pointed that the doping of Nd 3? in Bi 4 Ti 3 O 12 can also enhance the ferroelectric properties. The reason is that the substitution of Nd 3? for Bi 3? causes obvious structural distortion along C axis, thus resulting in a large polarization along C axis [21,23] besides inhibiting evaporation of Bi 3? . Surprisingly, Bi 4 Ti 3 O 12 materials doped with rare earth elements possess obvious photoluminescence [24,25]. R. Bokolia [26] reported that Er 3? -doped Bi 4 Ti 3 O 12 materials possess photoluminescence besides ferroelectricity. And K. Ruan [27] [30,31]. Therefore, it is necessary to investigate not only ferroelectricity but also photoluminescence of Nd 3? -doped films of Bi 4 Ti 3 O 12 systematically.
In this work, the films of Bi 4-x Nd x Ti 3 O 12 were prepared on the substrates (Pt(111)/Ti/SiO 2 /Si(100)) via the sol-gel and spin-coating method. And impact of the concentration of Nd 3? in Bi 4-x Nd x Ti 3 O 12 films on the ferroelectric properties, dielectric properties, leakage current densities and photoluminescence was investigated. Photoluminescence and ferroelectric properties were studied in detail.

Experimental
The films of Bi 4-x  Then, the above two solutions were mixed to get a sol. And the sol was deposited on the substrates to get monolayered films of Bi 4-x Nd x Ti 3 O 12 via spincoating technology. Subsequently, the films of Bi 4-x Nd x Ti 3 O 12 were pre-annealed and annealed in tube furnace. At last, the above-mentioned process needs to be repeated three times to acquire four-layered films of Bi 4-x Nd x Ti 3 O 12 . The flowchart of preparation is displayed in Fig. 1.
The phase constitutions of the films of Bi 4-x Nd x Ti 3 O 12 were carried out by X-ray diffraction (X 'Pert Pro MPD, PANalytical, Holland). The morphologies of the surface and cross-section were investigated though FESEM (SU8020, Hitachi, Japan). Ag dot electrodes with thickness of 100 nm and diameter of 1 mm were deposited on the surface of the films through vacuum evaporation coater (HFJS-DZ300, JieShuo, P. C. China). P-E hysteresis loops of the films were determined by Precision LC Unit (Radiant Precision LC, Radiant Technologies, USA). The leakage current densities were determined by semiconductor tester (4200-SCS, Keithley, USA). Absorption spectrum of films was obtained by the UV spectrophotometer (Cary-5000, Agilent, USA). Photoluminescent spectrum of the films was measured via fluorescence spectrophotometer (F4500, Hitachi, Japan).

Phase constitution
The XRD patterns of the five films of Bi 4-x Nd x Ti 3 O 12 are displayed in Fig. 2. As shown in Fig. 2a, it is obvious that the diffraction peaks of the films of Bi 4-x Nd x Ti 3 O 12 are composed of those from Bi 4 Ti 3 O 12 (JCPDS No.72-1019) and from Pt on the top layer of the substrates without any from impurity phases [32,33]. It indicates that Nd 3? has dissolved into the crystal lattice of Bi 4 Ti 3 O 12 and pure films of Bi 4-x Nd x Ti 3 O 12 have been prepared [34,35]. And the sharp peaks imply good crystallinity [26]. As shown in Fig. 2b, for all the five films, the diffraction peaks at about 30.0°shift slightly to a greater angle. This is mainly due to the fact that the radius of Nd 3? (1.11 Å ) is less than that of Bi 3? (1.17 Å ) [22,28]. The replacement of Bi 3? with Nd 3? will lead to reduction in lattice parameters and crystal plane spacing, which give rise to shift of the diffraction peaks to a greater angle according to Bragg equation.

Microstructure
The FESEM morphologies of surface of the films are illustrated in Fig. 3. It is seen that, for the films of Bi 4- x Nd x Ti 3 O 12 (x being 0 and 0.25, respectively), there are obvious cracks and pores on the surfaces of these samples. However, for the other films, a few pores can be found. For five films, grain boundaries are not be found. And there are polygonal particles without plate-like particles. And the average particle sizes of five films of Bi 4-x Nd x Ti 3 O 12 are listed in Table 1. With the increase in concentration of Nd 3? , the particle sizes decrease and the relative density of the films of

Ferroelectric properties
The hysteresis loops of the films of Bi 4-x Nd x Ti 3 O 12 are displayed in Fig. 5 In addition, it is worth noting that volatilization of Bi 3? during annealing process inevitably brings about increased oxygen vacancies and degraded ferroelectric properties of the films [32,34]. The doping of a small amount of Nd 3? will inhibit the volatilization of Bi 3? and reduce the concentration of oxygen vacancies. As shown in Fig. 5, the values of Pr increase. However, with the further increase in the concentration of Nd 3? , particle size decreases and the values of Pr reduce correspondingly owing to the pinning effect [33,37]. Therefore, the film of Bi 3.55 Nd 0.45 Ti 3 O 12 possesses the maximal value of Pr.

Leakage current densities
The leakage current densities of the films of Bi 4-x Nd x Ti 3 O 12 are displayed in Fig. 6. Obviously, when the values of applied electric field (E) are below 100 kVÁcm -1 , the leakage current densities increase rapidly with increasing E. However, when the values of E are greater than 100 kVÁcm -1 , the leakage current densities increase slowly and remain constant finally. Compared with the film of Bi 4 Ti 3 O 12 , Nd 3?doped films possess lower leakage current densities, indicating that doping of Nd 3? is beneficial for the reduction in leakage current densities. The reason is that volatilization of Bi 3? during the annealing process induces higher concentration of oxygen vacancies [39]. And the substitution of Nd 3? for Bi 3? inhibits the volatilization of Bi 3? effectively, thus reducing oxygen vacancies and leakage current densities. The data summarized from Fig. 6 are shown in Table 1. Among the five films, the film of Bi 3.55 Nd 0.45 Ti 3 O 12 possesses the minimum of leakage current density (1.02 9 10 -6 AÁcm -2 ), which is close to the data from Ref. [28] (2 9 10 -6 AÁcm -2 ) and lower than those from Ref. [40] (1.15 9 10 -5 AÁcm -2 ) and Ref. [29] (2.0 9 10 -5 AÁcm -2 ). It should be noted

Dielectric properties
Variation of dielectric constant (e) and dielectric loss (tand) with frequency is shown in Fig. 7. As shown in Fig. 7a, at lower frequency (\ 10,000 Hz), the values of e decrease rapidly with increasing frequency. Nevertheless, at higher frequency ([ 10,000 Hz), the values of e decrease slowly and remain nearly constant. The high values of dielectric constant at low frequency are due to the fact that the charges were trapped at interface states, which follows the alternating current variations [30]. And the space charge effect is suppressed at a higher frequency [30], thus the values of dielectric constant are greater at low frequency than those at high frequency. It is clearly seen from Fig. 7b that, with increasing frequency, the values of tand remain nearly unchanged when frequency is smaller than 10,000 Hz and increase rapidly when frequency is above 10,000 Hz. The significant increase in dielectric loss at high frequency may be ascribed to Maxwell-Wagner interfacial polarization or space charge polarization [31]. At the same frequency, the values of e increase and the values of tand decrease with the increasing concentration of Nd 3? . The structural inhomogeneity arising from Nd 3? doping breaks the translational symmetry which is not conducive to the coupling of electric dipoles. Thus, the concentration and the coupling of the dipoles decrease [41]. And dielectric constant increases due to the doping of Nd 3? . The replacement of Bi 3? with Nd 3? inhibits the formation of oxygen vacancies [31]. And decrease in the values of dielectric loss with the increase in doping concentration of Nd 3? may be attributed to the reductions of concentrations of oxygen vacancy [42]. This indicates that doping of Nd 3? enhances the dielectric properties of the films effectively.

Photoluminescence
Excitation spectra of the films of Bi 4-x Nd x Ti 3 O 12 are shown in Fig. 8a. Obviously, there is a distinct excitation peak at about 294 nm which is close to the value of wavelength in Ref. [43] (292 nm). This position of wavelength corresponds to the energy level transition of 4 I 9/2 ? 2 H 11/2 [43,44]. The emission spectra of the films of Bi 4-x Nd x Ti 3 O 12 are illustrated in Fig. 8b, showing that there is a blue light emission peak at 437 nm and one yellow light emission peak at 580 nm which are close to the values of wavelength in Ref. [43] (434 nm and 582 nm). These may be due to the energy level transitions of 4 G 9/ 2 ? 4 F 9/2 and 4 G 7/2 ? 4 F 9/2 , respectively [43,44]. The values of wavelength of the emission peaks remain almost constant under different doping concentration of Nd 3? . With increasing concentration of Nd 3? , the emission intensities increase firstly and then decrease. The emission intensity reaches the maximum when the concentration of Nd 3? is equal to 0.45, which indicates quenching concentration is about 0.45. Concentration quenching mechanism is highly relevant to the critical interaction distance between neighboring activators in the host lattice [45,46].
Next, we give an intuitive description on the luminescence mechanism of the films of Bi 4-x Nd x Ti 3 O 12 based on the possible partial energy level diagram of Nd 3? , as shown in Fig. 8c. It can be clearly seen that Nd 3? was excited from ground state ( 4 I 9/2 ) to excited state ( 2 H 11/2 ) when excited at a deep ultraviolet light (DUV, 294 nm). Then, Nd 3? was relaxed from excited state of 2 H 11/2 to the energy level of 4 G 9/2 and 4 G 7/2 via non-radiative-relaxation [44,46]. Finally, the electronic transitions from energy level of 4 G 9/2 and 4 G 7/2 to the energy level of 4 F 9/2 give rise to a blue light emission peak at 437 nm and a yellow light emission peak at 580 nm, respectively.
The diagrams of Commission Internationale de L'Eclairage (CIE) chromaticity coordinate of the films of Bi 4-x Nd x Ti 3 O 12 are shown in Fig. 9 [30] are in the green-yellow region.
The ultraviolet (UV) -visible (Vis) -near infrared (NIR) absorption spectra of the films of Bi 4-x Nd x Ti 3 O 12 are shown in Fig. 10a. It can be seen that the films exhibit obvious absorptions from 350 to 500 nm. In general, for the indirect transitions, the band gap energy (Eg) can be obtained via following formula [47,48]: where a is absorption coefficient from the absorption spectrum, A is constant and hm is energy of a photon, respectively. Variations of (ahm) 1/2 with hm of the films are shown in Fig. 10b. The values of Eg are determined from the intersections of the curve tangent and the abscissa [49,50], as shown in Table 1. Apparently, the values of Eg decrease gradually with the increasing concentration of Nd 3? , which is beneficial for enhancing visible light catalytic activity [47,49]. It is reported that the doping of Nd 3? gives rise to the distortions of TiO 6 octahedra [42], which is a dominating reason for reducing band gap energy [31]. Therefore, the value of Eg decreases with the increase in Nd 3? concentration. The value of Eg of the film of Bi 3.15 Nd 0.85 Ti 3 O 12 is 2.67 eV, which is smaller than that reported in Ref. [47] (2.82 eV). The narrowed band gaps imply the potential application in light catalytic fields as smaller Eg is beneficial for enhancing visible light catalytic activity [47,49].

Conclusions
The