1. Introduction

The ability to continuously control the upconversion photoluminescence (PL) in luminescent materials, in a reversible and real-time manner, is highly desirable for a wide range of applications, including optical waveguides, optoelectronic devices, and biomedicine [1–5]. The common method for tuning the upconversion PL of lanthanide doped compound is to alter the composition of host materials and/or doping ions via chemical approach [6–8]. However, it is difficult to reveal the kinetic process of luminescence based on the irreversible and ex-situ procedure, and other possible factors such as crystal field, chemical inhomogeneities and defects may contribute to the variance significantly. It would therefore be a significant advancement if one could in situ tune the upconversion PL in a single compound [9–11]. Elastic strain as an alternative strategy has the potential to achieve continuous and reversible tuning of the luminescence. This strategy, namely strain engineering recently has received increasing attention. The strain engineering has been widely used to increase the carrier mobility or enhance the emission efficiency in semiconductors. While as a straightforward concept, its potential in optoelectronics and spintronics remains largely under exploited. On the other hand, thin-film phosphors are of great importance from both technical and scientific respects [12]. Very recently, we reported the tunable near-infrared PL of Ni²⁺ doped SrTiO₃ (STO:Ni) thin film grown on piezoelectric Pb(Mg_1/3Nb_2/3)_0.7Ti_0.3O₃ (PMN-PT) substrate by converse piezoelectric effect [13]. Controllable emission of the STO:Ni thin film, including emitting wavelength and intensity, was demonstrated arising from the biaxial strain produced by piezoelectric PMN-PT. However, up to now, the effect of biaxial strain on the upconversion PL is still unknown.

In this work, we present reversible, continuous and dynamic tuning of upconversion PL in Yb³⁺/Er³⁺ co-doped BaTiO₃ (BTO:Yb/Er)/piezoelectric structure via strain engineering. Herein, the BTO is regarded as a model system for investigating crystal structure change under external stimuli. Yb/Er ions are the mostly used doped lanthanide combination in upconversion phosphors. Hence, the modification of upconversion PL in BTO:Yb/Er thin film can be realized by biaxial strain.

2. Experimental

Single crystal piezoelectric PMN-PT is extensively used as a substrate to produce electrically-reversible lattice strain due to its large piezoelectric coefficients. The Yb/Er co-doped BTO target with a chemical formula BaTi_0.97Yb_0.025Er_0.005O₃ was prepared using standard solid state reaction. The BTO:Yb/Er films with the thickness of 200 nm were epitaxially grown on (001) PMN-PT substrate with the dimension of 5 mm × 5 mm × 0.5 mm by pulsed laser deposition (PLD) with a KrF excimer laser at the wavelength of 248 nm. The laser pulse repetition frequency and energy density were 2 Hz and 1.5 J/cm². The growth temperature and oxygen pressure were fixed at 700 °C and 20 Pa, respectively. After the deposition, the films were in situ post-annealed at the growth temperature in 0.5 atm oxygen pressure for 30 min before they were cooled down to room temperature. Conductive ITO transparent electrode (~200 nm) was grown on the top of the BTO:Yb/Er film at 200 °C under 2 Pa oxygen pressure. Au electrode was coated on the back side of PMN-PT substrate by sputtering at room temperature. Figure 1(a) shows the schematic configuration of the change of the upconversion PL from BTO:Yb/Er/PMN-PT heterostructure via elastic strain. The PMN-PT was polarized in the thickness direction using a Keithley 6487 Source Meter. The PL spectra were recorded using an Edinburgh FLSP920 spectrophotometer under the excitation of a 980 nm diode laser. And the laser beam with a power of 200 mW was focused on the center of the (BTO:Yb/Er)/PMN-PT sample. All measurements were performed at room temperature.

 

Fig. 1 (a) The setup used for measuring the upconversion photoluminescence of (BTO:Yb/Er)/PMN-PT under an external electric field. Structural characterization of the (BTO:Yb/Er)/PMN-PT. (b) XRD θ-2θ patterns. (c) Low magnitude brightfield image. (d) HRTEM image for the interface structures. Inset shows the SAED pattern.

Download Full Size | PDF

3. Results and discussion

The X-ray diffraction (XRD) patterns of the BTO:Yb/Er thin film on the PMN-PT (001) substrate are shown in Fig. 1(b). Due to the small lattice mismatch between BTO and PMN-PT, there are strong overlaps of reflection (00l) peaks for BTO:Yb/Er film and PMNPT substrate. Only the (00l) peak of BTO are presented along with those of the PMN-PT substrate. This result indicates that the BTO:Yb/Er thin film are grown with its c-axis normal to the PMNPT substrate. Cross-sectional transmission electron microscopy (TEM) investigations were carried out to further explore the interface microstructure of the BTO:Yb/Er films on PMN-PT substrates. Figure 1(c) presents a low-magnification bright-field TEM image of the sample. The thickness of BTO:Yb/Er film and ITO transparent electrode are both around 200 nm. The BTO:Yb/Er film has a uniform thickness, and clearly defined interface with the PMN-PT substrate. The crystallographically textured film is in the form of columnar crystallites, with their long axis perpendicular to the interface, indicating a c axis growth. The high-resolution transmission electron microscopy (HRTEM) of the BTO:Yb/Er film on PMN-PT reveals that the lattice misfit strain between the BTO:Yb/Er and PMN-PT is relaxed via the formation of dislocation near the interface, which is extensively observed and studied in oxide based heterostructures [14]. Selected area electron diffraction (SAED) contains two sets of reflections and indicates epitaxial cube-on-cube film growth in nature (inset of Fig. 1(d)). Hence, the epitaxial relationships between the grown film and substrate are BTO (001) || PMN-PT (001) and BTO [100] || PMN-PT [100]. From the diffraction pattern, we calculated the BTO:Yb/Er film with an in-plane parameter of a = 4.010 Å and an out-of plane parameter of c = 3.991 Å. While, the lattice parameters measured for PMN-PT substrate are a = 4.024 Å and c = 4.019 Å. Thus, the as-grown BTO:Yb/Er thin film should be subjected to an in-plane tensile strain due to the lattice mismatch.

Figure 2(a) shows the effects of biaxial strain on the upconversion spectra of BTO:Yb/Er under different electric field across PMN-PT substrate. Green emission bands centered at 523/548 nm correspond to ²H_11/2/⁴S_2/3→⁴I_15/2, while red emission centered at 656 nm originates from ⁴F_9/2→⁴I_15/2 transitions of Er³⁺ ion. Herein, taking advantage of their large absorption and emission cross-section, Yb³⁺ ions were used as the sensitizer to enhance the upconversion efficiency of Er³⁺ ions. The inset of Fig. 2(a) presents the PL spectra of (BTO:Yb/Er)/PMN-PT heterostructure under unpolarized and polarized states, respectively. After poling the PMN-PT substrate in the thickness direction, we applied different electric field onto PMN-PT along the polarized direction. PMN-PT is capable of providing in-plane strain arising from converse piezoelectric effect. It is noticeable that the electric controlled strain generated from PMN-PT has been utilized to tune electric, magnetic and optical properties of various materials in previous studies [15–18]. As shown in Fig. 2(a), with increasing electric field from 0 to 10 kV/cm, the PL intensity gradually decreases. The change ratios I₁₀/I₀ for the green band and red band are ~25% and 21%, respectively (Fig. 2(b)). Note that the modification in intensities is almost linearly dependent on the electric field applied to the PMN-PT substrate, indicating that the PL intensity can be finely controlled by the applied voltage.

 

Fig. 2 (a) The upconversion emission spectra of the (BTO:Yb/Er)/PMN-PT heterostructure under dc bias voltage ranging from 0 to 10 kV/cm. Inset shows the PL spectra of (BTO:Yb/Er)/PMN-PT heterostructure under unpolarized and polarized states. (b) The change ratios I₁₀/I₀ for green band and red emission as a function of the applied dc voltage.

Download Full Size | PDF

Figure 2 demonstrates the feasibility that the utilization of elastic strain to tune the upconversion PL. The observed results might be ascribed to the lattice deformation of BTO host under strain. As shown in the Fig. 1(d), the obtained BTO:Yb/Er thin film has a cubic structure with an in-plane parameter of a = 4.010 Å, which is nearly 0.35% smaller than that of PMN-PT. Therefore, the BTO:Yb/Er film is subjected to lateral restraint from the PMN-PT substrates. The up left inset of Fig. 3 presents the XRD patterns of BTO:Yb/Er thin film on PMN-PT substrate under different dc bias voltage. The systematic shift of PMN-PT (002) and BTO:Yb/Er (002) reflection peaks indicates that the electric field steadily modified lattice distortion of PMN-PT as well as BTO:Yb/Er thin film. The calculated out-of-plane lattice constant c of PMN-PT increases from 4.019 to 4.027 Å. Figure 3 shows the linear dependence of relative changes Δc/c of PMN-PT with increasing electric field, indicating the piezoelectric nature in PMN-PT. The maximum values of Δc/c for PMN-PT are obtained as 0.21% . It implies that the c lattice constant of PMN-PT is increased up to 0.21%. According to approximate volume preserving and Poisson effect, it is evident that an increase of the out-of-plane lattice constant would be accompanied by a decrease of the in-plane lattice constant. Therefore, we can readily conclude that the decrease of the in-plane lattice constant of PMN-PT will result in a release of lateral restraint on BTO:Yb/Er film (bottom right inset of Fig. 3). It is widely accepted that the crystal symmetry of the host materials plays an essential role in the PL of dopant ions. In our case, the Ti⁴⁺ ions in BTO are replaced by the substitution Er³⁺ ions, Er³⁺ ions locate the center of the [ErO]₆ octahedron [11,19]. When the PMN-PT substrate is unpolarized or under lower electric field, the large lateral strain originated from lattice mismatch induces lower symmetry around Er³⁺ ions, leading to the higher PL emission. When the electric field is increased, the lateral tension between BTO:Yb/Er film and PMN-PT releases. The decreased in-plane tensile strain produced by PMN-PT substrate could be transferred to the BTO:Yb/Er film, resulting in higher symmetrical structure of BTO host. Thus, it is suggested that enhanced upconversion PL could be achieved in BTO:Yb/Er film by increasing in-plane tensile strain.

 

Fig. 3 The relative changes in the lattice constantΔc/c of PMN-PT as a function of electric field. Up left inset present the in situ XRD patterns in the vicinity of reflections for (BTO:Yb/Er)/PMN-PT under different electric fields. Bottom right inset shows the schematic of the compressive BTO:Yb/Er thin film biaxially strained to match the substrate PMN-PT.

Download Full Size | PDF

Moreover, of particular interest is that PMN-PT can readily generate dynamic strain, implying that reversible and dynamic tuning of PL in BTO:Yb/Er/PMN-PT heterostructure could be realized. Figure 4 shows the kinetic PL response at the wavelength of 523 nm and 656 nm of BTO:Yb/Er film as a function of time under a sinusoidal ac bias. It is found that the PL intensity of the film at 523 and 656 nm can be tuned with almost same frequency as that of the driving electric voltage. The phase difference between the applied electric field and the PL intensity is π, which means that applying a positive electric field along the polarization direction of PMN-PT causes a decrease in PL intensity of the BTO:Yb/Er film. Obviously, strain-engineered modulation of upconversion PL enables dynamic response, which cannot be realized by conventional chemical methods.

 

Fig. 4 The kinetic PL response at the wavelength of 523 nm and 656 nm of BTO:Yb/Er film as a function of time under a sinusoidal ac bias.

Download Full Size | PDF

4. Conclusions

In summary, we have demonstrated a strain engineered tuning of upconversion PL of BTO:Yb/Er film grown on PMN-PT substrate. The PL intensity can be in situ and reversible modulated by an applied electric field. These effects could be caused by the strain-mediated lattice deformation, resulting from the converse piezoelectric effect of PMN-PT substrate. Moreover, the PL intensity can be dynamically tuned under an ac external electric field. Our work should be significant for developing electric-field-controllable luminescent materials and devices.