Effect of concentration of Nd³⁺ on the photoluminescence and ferroelectric properties of Bi_4-xNd_xTi₃O₁₂ films

Abstract

Lead-free films of Bi_4-xNd_xTi₃O₁₂ 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 Bi_4-xNd_xTi₃O₁₂ and clear interfaces between the films of Bi_4-xNd_xTi₃O₁₂ and substrates when the films are annealed at 700 °C. And the films of Bi_4-xNd_xTi₃O₁₂ 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 Bi_4-xNd_xTi₃O₁₂ possess the minimum of band gap energy (2.67 eV). Moreover, the films of Bi_4-xNd_xTi₃O₁₂ exhibit minimal leakage current density and maximal remanent polarization, which is highly beneficial for the potential applications in multi-functional devices.

1 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 lead-based 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_xTi_1-XO₃ [11].

Bismuth titanate (Bi₄Ti₃O₁₂), 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 Bi³⁺, 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 Bi³⁺ through doping of rare elements in Bi₄Ti₃O₁₂ [16,17,18]. M. Chen [19] reported that the substitution of Pr³⁺ for Bi³⁺ will decrease the space charge density which improves ferroelectric properties of Bi₄Ti₃O₁₂ materials. And C. P. Cheng [20] reported that the doping of Dy³⁺ will enhance the ferroelectricity of Bi₄Ti₃O₁₂. U. Chon [21] and X. Y. Mao [22] pointed that the doping of Nd³⁺ in Bi₄Ti₃O₁₂ can also enhance the ferroelectric properties. The reason is that the substitution of Nd³⁺ for Bi³⁺ causes obvious structural distortion along C axis, thus resulting in a large polarization along C axis [21, 23] besides inhibiting evaporation of Bi³⁺. Surprisingly, Bi₄Ti₃O₁₂ materials doped with rare earth elements possess obvious photoluminescence [24, 25]. R. Bokolia [26] reported that Er³⁺-doped Bi₄Ti₃O₁₂ materials possess photoluminescence besides ferroelectricity. And K. Ruan [27] maintained that Eu³⁺-doped films of Bi₄Ti₃O₁₂ possess good photoluminescence. Although, up to date, ferroelectricity of Nd³⁺-doped films of Bi₄Ti₃O₁₂ has been investigated [28, 29], there are a few reports on investigating photoluminescence of Nd³⁺-doped films of Bi₄Ti₃O₁₂ [30, 31]. Therefore, it is necessary to investigate not only ferroelectricity but also photoluminescence of Nd³⁺-doped films of Bi₄Ti₃O₁₂ systematically.

In this work, the films of Bi_4-xNd_xTi₃O₁₂ were prepared on the substrates (Pt(111)/Ti/SiO₂/Si(100)) via the sol–gel and spin-coating method. And impact of the concentration of Nd³⁺ in Bi_4-xNd_xTi₃O₁₂ films on the ferroelectric properties, dielectric properties, leakage current densities and photoluminescence was investigated. Photoluminescence and ferroelectric properties were studied in detail.

2 Experimental

The films of Bi_4-xNd_xTi₃O₁₂ (when x = 0, 0.25, 0.45, 0.65, 0.85) were deposited on the Pt(111)/Ti/SiO₂/Si(100) substrate. And precursors were fabricated by sol–gel method. Neodymium oxide (Nd₂O₃), bismuth nitrate pentahydrate (Bi(NO₃)₃·5H₂O) and tetrabutyl titanate (Ti(OC₄H₉)₄) were selected as starting materials. 2-methoxyethanol (C₃H₈O₂) and acetic acid (C₂H₄O₂) were used as the solvents. Acetylacetone (C₅H₈O₂) was used to stabilize the solution. Firstly, certain proportion of Nd₂O₃ and Bi(NO₃)₃·5H₂O was dissolved in C₂H₄O₂ according to the stoichiometry of Bi_4-xNd_xTi₃O₁₂. Thus, solution A was obtained. And excessive Bi(NO₃)₃·5H₂O (8 wt%) was used to compensate volatilization of Bi³⁺ during annealing process. Then, Ti(OC₄H₉)₄ was dissolved in C₃H₈O₂, and a small quantity of stabilizer (C₅H₈O₂) was dripped into C₃H₈O₂. Then, solution B was obtained. 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-xNd_xTi₃O₁₂ via spin-coating technology. Subsequently, the films of Bi_4-xNd_xTi₃O₁₂ 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-xNd_xTi₃O₁₂. The flowchart of preparation is displayed in Fig. 1.

Fig. 1

The flowchart of preparation of Bi_4-xNd_xTi₃O₁₂ films

The phase constitutions of the films of Bi_4-xNd_xTi₃O₁₂ 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).

3 Results and discussion

3.1 Phase constitution

The XRD patterns of the five films of Bi_4-xNd_xTi₃O₁₂ are displayed in Fig. 2. As shown in Fig. 2a, it is obvious that the diffraction peaks of the films of Bi_4-xNd_xTi₃O₁₂ are composed of those from Bi₄Ti₃O₁₂ (JCPDS No.72–1019) and from Pt on the top layer of the substrates without any from impurity phases [32, 33]. It indicates that Nd³⁺ has dissolved into the crystal lattice of Bi₄Ti₃O₁₂ and pure films of Bi_4-xNd_xTi₃O₁₂ 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 Nd³⁺ (1.11 Å) is less than that of Bi³⁺ (1.17 Å) [22, 28]. The replacement of Bi³⁺ with Nd³⁺ 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.

Fig. 2

a The XRD patterns of Bi_4-xNd_xTi₃O₁₂ films; b magnified diffraction peaks at about 30.0°

3.2 Microstructure

The FESEM morphologies of surface of the films are illustrated in Fig. 3. It is seen that, for the films of Bi_4-xNd_xTi₃O₁₂ (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-xNd_xTi₃O₁₂ are listed in Table 1. With the increase in concentration of Nd³⁺, the particle sizes decrease and the relative density of the films of Bi_4-xNd_xTi₃O₁₂ increases.

Fig. 3

The FESEM morphologies of surface of Bi_4-xNd_xTi₃O₁₂ films a x = 0; b x = 0.25; c x = 0.45; d x = 0.65; e x = 0.85

Table 1 The average particle sizes, ferroelectric parameters, leakage current densities and bandgap energy of Bi_4−xNd_xTi₃O₁₂ films

The FESEM morphologies of cross-section of the films of Bi_4-xNd_xTi₃O₁₂ are shown in Fig. 4. All the five films exhibit layered structure. The interfaces between the films of Bi_4-xNd_xTi₃O₁₂ and substrates are flat. And there are no inter-layers between the films and substrates, implying that no diffusion exists between the films of Bi_4-xNd_xTi₃O₁₂ and substrates. It can be observed that the thicknesses of the films of Bi_4-xNd_xTi₃O₁₂ are all around 200 nm, which is greater than that of pure films of Bi_4-xNd_xTi₃O₁₂. The difference in thicknesses for the films of Bi_4-xNd_xTi₃O₁₂ with different concentration of Nd³⁺ is mainly due to the preparation of films by spin-coating technology.

Fig. 4

The FESEM morphologies of cross-section of Bi_4-xNd_xTi₃O₁₂ films: a x = 0; b x = 0.25; c x = 0.45; d x = 0.65; e x = 0.85

3.3 Ferroelectric properties

The hysteresis loops of the films of Bi_4-xNd_xTi₃O₁₂ are displayed in Fig. 5. The results obtained from Fig. 5 are listed in Table 1. The films of Bi_4-xNd_xTi₃O₁₂ seem to exhibit good ferroelectricity. The values of remnant polarization (Pr) and saturated polarization (Ps) of the films of Bi_4-xNd_xTi₃O₁₂ are great than those of un-doped film of Bi₄Ti₃O₁₂ which indicates that doping of Nd³⁺ is beneficial for the improvement of the ferroelectric properties. It is seen that, firstly, the values of Pr increase to 17.25 µc·cm⁻² and then decrease gradually with increasing concentration of Nd³⁺. The films of Bi_3.55Nd_0.45Ti₃O₁₂ possess the maximal value of Pr (17.25 µc·cm⁻²) which is greater than those films of Bi₄Ti₃O₁₂ in Ref. [28] (10.5µc·cm⁻²), Ref. [36] (10.5µc·cm⁻²), Ref. [38] (10 µc·cm⁻²) and Ref. [29] (14.3 µc·cm⁻²). And it is also significantly greater than un-doped film of Bi₄Ti₃O₁₂ in Ref. [38] (4.8 µc·cm⁻²).

Fig. 5

The P-E hysteresis loops of Bi_4−xNd_xTi₃O₁₂ films; inset: the values of Pr of Bi_4−xNd_xTi₃O₁₂ films

In addition, it is worth noting that volatilization of Bi³⁺ 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 Nd³⁺ will inhibit the volatilization of Bi³⁺ 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 Nd³⁺, particle size decreases and the values of Pr reduce correspondingly owing to the pinning effect [33, 37]. Therefore, the film of Bi_3.55Nd_0.45Ti₃O₁₂ possesses the maximal value of Pr.

3.4 Leakage current densities

The leakage current densities of the films of Bi_4−xNd_xTi₃O₁₂ are displayed in Fig. 6. Obviously, when the values of applied electric field (E) are below 100 kV·cm⁻¹, the leakage current densities increase rapidly with increasing E. However, when the values of E are greater than 100 kV·cm⁻¹, the leakage current densities increase slowly and remain constant finally. Compared with the film of Bi₄Ti₃O₁₂, Nd³⁺-doped films possess lower leakage current densities, indicating that doping of Nd³⁺ is beneficial for the reduction in leakage current densities. The reason is that volatilization of Bi³⁺ during the annealing process induces higher concentration of oxygen vacancies [39]. And the substitution of Nd³⁺ for Bi³⁺ inhibits the volatilization of Bi³⁺ 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_3.55Nd_0.45Ti₃O₁₂ possesses the minimum of leakage current density (1.02 × 10^–6 A·cm⁻²), which is close to the data from Ref. [28] (2 × 10^–6 A·cm⁻²) and lower than those from Ref. [40] (1.15 × 10^–5 A·cm⁻²) and Ref. [29] (2.0 × 10^–5 A·cm⁻²). It should be noted that leakage current density of the films of Bi_3.55Nd_0.45Ti₃O₁₂ is about one order lower than that of the film of Bi₄Ti₃O₁₂ (1.85 × 10^–5 A·cm⁻²).

Fig. 6

The leakage current densities of Bi_4−xNd_xTi₃O₁₂ films under different electric field; inset: the leakage current densities of Bi_4−xNd_xTi₃O₁₂ films at 300 kV·cm⁻¹

3.5 Dielectric properties

Variation of dielectric constant (ε) and dielectric loss (tanδ) with frequency is shown in Fig. 7. As shown in Fig. 7a, at lower frequency (< 10,000 Hz), the values of ε decrease rapidly with increasing frequency. Nevertheless, at higher frequency (> 10,000 Hz), the values of ε 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 tanδ 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 ε increase and the values of tanδ decrease with the increasing concentration of Nd³⁺. The structural inhomogeneity arising from Nd³⁺ 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 Nd³⁺. The replacement of Bi³⁺ with Nd³⁺ inhibits the formation of oxygen vacancies [31]. And decrease in the values of dielectric loss with the increase in doping concentration of Nd³⁺ may be attributed to the reductions of concentrations of oxygen vacancy [42]. This indicates that doping of Nd³⁺ enhances the dielectric properties of the films effectively.

Fig. 7

a Variation of dielectric constant with frequency for Bi_4−xNd_xTi₃O₁₂ films; b variation of dielectric loss with frequency for Bi_4−xNd_xTi₃O₁₂ films

3.6 Photoluminescence

Excitation spectra of the films of Bi_4−xNd_xTi₃O₁₂ 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 ⁴I_9/2 → ²H_11/2 [43, 44]. The emission spectra of the films of Bi_4−xNd_xTi₃O₁₂ 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 ⁴G_9/2 → ⁴F_9/2 and ⁴G_7/2 → ⁴F_9/2, respectively [43, 44]. The values of wavelength of the emission peaks remain almost constant under different doping concentration of Nd³⁺. With increasing concentration of Nd³⁺, the emission intensities increase firstly and then decrease. The emission intensity reaches the maximum when the concentration of Nd³⁺ 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].

Fig. 8

a The excitation spectra of Bi_4−xNd_xTi₃O₁₂ films with different doped concentration of Nd³⁺; b the emission spectra of Bi_4−xNd_xTi₃O₁₂ films with different doped concentration of Nd³⁺; c partial energy level diagram of Nd³⁺

Next, we give an intuitive description on the luminescence mechanism of the films of Bi_4−xNd_xTi₃O₁₂ based on the possible partial energy level diagram of Nd³⁺, as shown in Fig. 8c. It can be clearly seen that Nd³⁺ was excited from ground state (⁴I_9/2) to excited state (²H_11/2) when excited at a deep ultraviolet light (DUV, 294 nm). Then, Nd³⁺ was relaxed from excited state of ²H_11/2 to the energy level of ⁴G_9/2 and ⁴G_7/2 via non-radiative-relaxation [44, 46]. Finally, the electronic transitions from energy level of ⁴G_9/2 and ⁴G_7/2 to the energy level of ⁴F_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−xNd_xTi₃O₁₂ are shown in Fig. 9. It can be found that CIE chromaticity coordinates of Bi_3.75Nd_0.25Ti₃O₁₂, Bi_3.55Nd_0.45Ti₃O₁₂, Bi_3.35Nd_0.65Ti₃O₁₂ and Bi_3.15Nd_0.85Ti₃O₁₂ are (0.2568, 0.2545), (0.2402, 0.2552), (0.2509, 0.2610), (0.2499, 0.2688), respectively. The CIE coordinates are located in the blue region of CIE diagram, as a contrast, CIE coordinates of nanometer powder prepared in Ref. [30] are in the green-yellow region.

Fig. 9

The diagrams of CIE chromaticity coordinate of Bi_4-xNd_xTi₃O₁₂ films

The ultraviolet (UV) -visible (Vis) -near infrared (NIR) absorption spectra of the films of Bi_4−xNd_xTi₃O₁₂ 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]:

$$ \alpha h\nu^{1/2} = A(h\nu - Eg) $$

(1)

Fig. 10

a UV–Vis-NIR absorption spectra of Bi_4−xNd_xTi₃O₁₂ films; b variation of (αhν)^1/2 with hν

where α is absorption coefficient from the absorption spectrum, A is constant and hν is energy of a photon, respectively. Variations of (αhν)^1/2 with hν 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 Nd³⁺, which is beneficial for enhancing visible light catalytic activity [47, 49]. It is reported that the doping of Nd³⁺ gives rise to the distortions of TiO₆ octahedra [42], which is a dominating reason for reducing band gap energy [31]. Therefore, the value of Eg decreases with the increase in Nd³⁺ concentration. The value of Eg of the film of Bi_3.15Nd_0.85Ti₃O₁₂ 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].

4 Conclusions

The films of Bi_4−xNd_xTi₃O₁₂ doped with different concentrations of Nd³⁺ were deposited on the substrate via spin-coating technology. The as-prepared films of Bi_4−xNd_xTi₃O₁₂ are pure and have a few pores and cracks on the surfaces of the films. The interfaces between the films of Bi_4−xNd_xTi₃O₁₂ and the substrates are clear and flat. The film of Bi_3.55Nd_0.45Ti₃O₁₂ possess the maximum of Pr (17.25 µc·cm⁻²) and the minimum of leakage current density (1.02 × 10^–6 A·cm⁻²). With the increasing concentration of Nd³⁺, the values of ε of the films increase and the values of tanδ decrease. The films of Bi_4−xNd_xTi₃O₁₂ exhibit a blue light emission peak at 437 nm and a yellow light emission peak at 580 nm. With increasing concentration of Nd³⁺, the values of Eg decrease gradually. For the film of Bi_3.15Nd_0.85Ti₃O₁₂, the value of band gap energy is 2.67 eV. The photoluminescence and ferroelectric properties of the film of Bi_3.55Nd_0.45Ti₃O₁₂ are, on the whole, good and helpful for the applications in multi-functional devices.