Phase transitions in BiFeO

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Ceramics International

journal homepage:www.elsevier.com/locate/ceramint

Phase transitions in BiFeO

nanoislands with enhanced electromechanical response

Fei Sun

, Guo Tian

, Chao Chen

, Xiong Deng

, Peilian Li

, Zoufei Chen

, Wenda Yang

, Xingsen Gao

, Zhen Fan

, Minghui Qin

, Min Zeng

, Xubing Lu

, Guofu Zhou

, Deyang Chen

, Jun-Ming Liu

aInstitute for Advanced Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, China

bGuangdong Provincial Key Laboratory of Optical Information Materials and Technology, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, China

cGuangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, China

dNational Center for International Research on Green Optoelectronics, South China Normal University, Guangzhou 510006, China

eLaboratory of Solid State Microstructures and Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China

A R T I C L E I N F O

Keywords:

Bismuth ferrite Nanoislands Phase transition Electromechanical response

A B S T R A C T

We propose a novel method to drive phase transitions by etching strained BiFeO₃thinfilms to nanoislands.

Atomic force microscopy (AFM) measurements reveal that the amount of rhombohedral-like (R) phase increases as the BiFeO₃thinfilms with tetragonal-like (T) matrix are etched to nanoislands and larger fraction of R phase can be obtained with the size reduction from 1μm to 200 nm, indicating the T to R phase transitions induced by partial release of substrate clamping. Using piezoresponse force microscopy (PFM), it is demonstrated that rhombohedral-like (R) to tetragonal-like (T) phase transitions can be reversibly achieved under DC electricfield in BFO nanoislands. Large electromechanical response has been observed in BFO nanoislands as well. This approach can be extended to other strained oxidefilms and provide guidance for the development of high- performance electromechanical lead-free materials.

1. Introduction

Observations of plenty of emerging phenomena in nanostructured BiFeO3(BFO) are attracting extensive attention due to their great po- tential applications in nanoscale data storage and nanoelectronic de- vices[1–5]. Much effort has been focused on the fascinating function- alities observed in BFO nanostructures. Not only controllable domain wall conduction with topological protection in self-assembled BFO na- noislands has been discovered very recently[1], but also various types of topological domain structures have been observed in BFO nanos- tructures[6–11]. Besides, size effects[12], local ferroelectric properties [13–15], high resistance ratio [16,17]and improved magnetic prop- erties[18]in BFO nanostructures were reported as well.

A series of enhanced performance has been revealed in mixed-phase BFO thin films theoretically and experimentally, including excellent piezoelectricity [19,20], electrically controllable magnetism [21], multiflexo-effect at nanoscale phase boundaries[22], domain evolution

[23], persistent photoconductivity[24], and nanoscale shape-memory effect[25]. Thus, a variety of methods have been developed to induce phase transitions in BFO, such as chemical substitution[26], hydro- static pressure[27], electricfield[19,28], mechanical force[29], and mismatch strain[30–32]. Few reports, however, have focused on the possible phase transitions induced by the formation of BFO nanos- tructures. In this report, we propose a novel approach to drive phase transitions by etching the strained BFO thinfilms to nanoislands. Tet- ragonal-like (T) to rhombohedral-like (R) phase transitions are induced in the high-density ordered ferroelectric nanoisland arrays. Improved piezoelectric properties are observed in the nanoislands as well.

2. Experimental

In this study, the BFO thinfilms (~50 nm) werefirstly grown on LaAlO3(LAO) substrates with Ce0.04Ca0.96MnO3(CCMO) as the bottom electrodes using pulsed laser deposition (PLD). Then, the well-ordered

https://doi.org/10.1016/j.ceramint.2018.08.265

Received 15 August 2018; Received in revised form 19 August 2018; Accepted 22 August 2018

⁎Corresponding authors at: Institute for Advanced Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, China.

E-mail addresses:[email protected](C. Chen),[email protected](X. Gao),[email protected](D. Chen).

Ceramics International 44 (2018) 21725–21729

Available online 23 August 2018

0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

T

polystyrene spheres (PS) arrays acting as the mask were stacked onto these thin films. Afterwards, Ar ion beam was applied to etch the PS masked BFO thinfilms to obtain ordered BiFeO3nanoislands. The de- tails of this universal PS-assisted nano-patterning method can be found in the supplementary materialand our previous work[9]. X-ray dif- fraction (XRD) was used to investigate the structures of BFO thinfilms and nanoislands. The morphologies and ferroelectric properties were measured by atomic force microscopy (AFM) and piezoresponse force microscopy (PFM), respectively.

3. Results and discussion

The topography, out of plane and in-plane PFM phase images of BFO thin films grown on LAO substrates with CCMO as the bottom elec- trodes (BFO/CCMO/LAO), are shown inFig. S1in thesupplementary material. The topography image presents the typical characteristic of the R-T mixture with a small amount of stripe-like R phase embedded in the terraced T phase matrix which indicates the high quality growth of epitaxial BFO thinfilm as well. The out of plane PFM phase shows a uniform contrast, indicating the downward polarization direction. In consistent with previous study[33,34], the in-plane PFM phase shows ferroelectric stripe domain structures in the matrix T phase and the distinct contrast in R phase.

The CCMO bottom electrode in the BFO/CCMO/LAO hetero- structure enables the study of T-R phase transitions and polarization

switching behaviors of BFO under electricfield. The topography images of the BFO thinfilm before and after applying electric field are dis- played inFig. 1a-d.Fig. 1a shows the as-grownfilm with mixed T- and R- phases. After applying a−9 V electricfield in the green dashed-line square (Fig. 1b), the stripe-like features transform toflat terraces, re- vealing the electricfiled induced R- to T- phase transitions. Then, a positive voltage was applied to the blue dashed-line region (Fig. 1c), demonstrating the phase transitions from pure T phase to R-T mixed phases. Subsequently, another−9 V electric field re-induced pure T phase in the small green box (Fig. 1d). These results indicate the electric field reversibly driven T to R phase transitions in the BFO thinfilm. In addition, the corresponding out of plane PFM phase images shown in Fig. 1e-h reveal the reversible 180° polarization reversal of the BFO thin film under electric field. These results indicate that the out-of-plane 180° polarization reversal is accompanied with structural deformations, which is consistent with previous report revealing the strong coupling between polarization and phase transitions[25].

InFig. 1, two methods have been presented to induce the R-T phase transitions, using mismatch strain (between BFOfilms and LAO sub- strates,Fig. 1a) and electric field (Fig. 1b–d), respectively. Here, we propose another approach to induce phase transitions by etching the BFO thinfilm into nanoisland structures. The T and R mixed-phase BFO films with only a small amount of R phase, as shown inFig. 1, were used to fabricate different sizes of BFO nanoislands by the PS-assistant nano- patterning method (the experimental details can be found in the Fig. 1.(a) The AFM topography images of the as-grown BFO thinfilm, and (b)–(d) after applying a series of electricfield (−9 V, 7 V and−9V) in the dashed regions, revealing the electricfield reversible control of T-R phase transitions. (e)-(h) Out of plane PFM phase images correspond to (a)–(d) indicate the polarization reversal. Scale bar of (a)–(h), 500 nm.

Fig. 2.AFM topography images of BFO nanoislands with the lateral diameters of (a) 1 µm, (b) 400 nm and (c) 200 nm. Scale bar of (a)–(c), 1 µm.

supplementary material). By choosing appropriate sizes of PS as the etching mask and adjusting the oxygen plasma etching time, various BFO nanoislands with different lateral diameters of 1 µm, 400 nm and 200 nm were obtained (Fig. 2). The vertical height of all the BFO na- noislands is ~ 30 nm. It is revealed that high-density ordered BFO na- noislands are successfully prepared.

Next, we look into the morphologies of BFO nanoislands with dia- meters of 1 µm, 400 nm and 200 nm using high-resolution AFM, re- spectively. As presented inFig. 3a–c, the topography images show that R and T mixed phases exist in every single nanoisland, which is very different from the as-grown BFO thinfilm with matrix T phase and a

small fraction of R phase (Fig. 1). The corresponding out of plane and in plane PFM phase images are shown inFig. 3d–i. It is also found that the amount of R phase increases as the diameters of nanoislands decrease from 1 µm to 200 nm. It is worth to mention that the mixed T and R phases in the 200 nm nanoislands (Fig. 3c) cannot be identified clearly.

We assume that the small size of PS gives rise to the increase of surface energy which leads to the aggregation of PS that is difficult to remove, resulting in the unclear AFM scanning. However, the PFM images dis- played inFig. 3i present the stripe contrasts aroused from the stripe R phase, revealing the coexistence of T and R phases. These results in- dicate the T to R phase transitions induced by partially releasing the Fig. 3.Morphologies of BFO nanoislands with the lateral diameters of (a) 1 µm, (b) 400 nm and (c) 200 nm. (d)–(f) Out of plane and (g)-(i) in-plane PFM phase images correspond to (a)–(c) indicate the coexistence of T and R phases. (002) Reciprocal space mappings (RSMs) of (j) the as grownfilm and (k) the nanoislands with the size of 200 nm.

F. Sun et al. Ceramics International 44 (2018) 21725–21729

clamping effect between BFO and substrates due to the formation of BFO nanoislands. Previous studies have discovered topological domains in BFO nanoislands with R phase[1,6]. However, no topological do- mains have been observed in this study which might result from the T phase matrix of the nanoislands.

To further study the strain relaxation in the BFO nanoislands, re- ciprocal space mappings (RSMs) of (002) reflections were carried out as shown in Fig. 3(j) and (k). It is revealed that the amount of R phase increases in the 200 nm size nanoislands (Fig. 3(k)) comparing to the as-grown thinfilm (Fig. 3(j)). For further confirmation, XRD measure- ments were carried out. As shown in Fig. S2 in the supplementary material, the XRD data of the as-grown BFOfilm and various lateral sizes (1 µm, 400 nm and 200 nm) of BFO nanoislands demonstrate the epitaxial growth and the matrix T phase on LAO substrate. Although R phase is observed in all the samples by AFM (Fig. 1andFig. 3), the XRD peaks of R phase are invisible in the as-grown film (due to a small amount ofRphase in the as-grownfilm) and very weak in different diameters of nanoislands. With the decrease of the diameters from 1 µm to 200 nm, the AFM images show an increase of surface area fraction of the R phase, but the R phase is tapered towards the substrate [21]

which results in the weak peak intensity inFig. S2. Nevertheless, the T

phase peaks shift to the high angle with the reduction of the diameters of nanoislands, demonstrating the strain relaxation (the decrease of clamping effect). Therefore, the AFM measurements and XRD data, as shown in Fig. 3, demonstrate the T-R phase transitions induced by partial release of substrate clamping in BFO nanoislands.

We then focus on the electricfield control of phase transitions in BFO nanoislands with the diameter of 400 nm. The topography (Fig. 4a) and the out of plane PFM image (Fig. 4b) of the as-prepared nanoislands show the coexistence of the R/T mixture and the downward polariza- tion direction, respectively. After applying an electricfield of−7 V in the green dashed box, the stripe-like features of R phase vanish (Fig. 4c), indicating the electricfield driven R- to T- phase transitions in BFO nanoislands. The polarization reversal is demonstrated inFig. 4d as well.

To further study the phase transition behaviors in BFO nanoislands under the electricfield, a single nanoisland was randomly selected in the ordered BFO nanoisland (diameter ~ 400 nm) arrays. As shown in Fig. 4e, a series of DC electricfield (-7 V ~ 7 V) has been applied in the nanoisland. The AFM topography image indicates the coexistence of R- and T- phases in the initial state. These R and T mixed phases are in- duced to pure T phase by applying a−7 V electricfield. Then a positive voltage (+ 4 V) drives the pure T phase to R and T mixed phase again.

Further increase of the positive electricfield to + 6 V enables the for- mation of more stripe-like R phase. Moreover, a higher electricfield (+ 7 V) leads to pure T phase, which can be induced to R and T mixed phases again by applying a negative voltage (-4 V). These results de- monstrate the electricfield reversibly driven R- to T- phase transitions in BFO nanoislands.

Large electromechanical response in the R/T morphotropic phase boundary (MPB) of BFO has been reported previously[19,25]. We next turn to investigate the electromechanical response in the as-grown T and R mixed-phase BFO thinfilm and the nanoislands with different lateral sizes (1 µm, 400 nm and 200 nm) by measuring piezoresponse hysteresis loops using PFM. To obtain the comparable piezoresponse amplitude data, all the experiments were carried out under identical conditions. As shown inFig. 5, no obvious piezoresponse amplitude differences are observed in the as-grown thinfilm and the 1 µm-size nanoislands. However, significant improvement of piezoresponse am- plitude is achieved in the 400 nm-size nanoislands and further en- hancement is obtained with the size of nanoislands reduced to 200 nm.

Fig. 4.Electricfield control of phase transitions in BFO nanoislands with the diameter of 400 nm. (a) Topography and (b) out of plane PFM images of the as-prepared BFO nanoislands. (c) Corresponding topography and (d) out of plane PFM phase images after applying of a−7 V voltage (in the green dashed box). (e) The AFM topography images of a single BFO nanoisland under a series of DC electricfield (−7 V ~ 7 V), showing the reversible T-R phase transitions.

Fig. 5.Piezoresponse hysteresis loops of the as-grown BFO thinfilm and the nanoislands with lateral sizes of 1 µm, 400 nm and 200 nm.

The phase transitions induced in BFO nanoislands indicate that the enhancement of electromechanical response is not only aroused by the increase of T/R phase boundaries, but also attributed by the decrease of substrate clamping effect with the decrease of lateral sizes.

4. Conclusions

In summary, we demonstrate the T to R phase transitions in BFO nanoislands induced by the strain relaxation as the highly strained BFO thinfilms are etched to nanoislands. Reversible T-R phase transitions driven by electric field are achieved in BFO nanoislands as well.

Compared to the as grown thinfilms, larger electromechanical response has been observed in BFO nanoislands (especially the dots with lateral sizes of 400 nm and 200 nm). Thesefindings provide a novel approach to induce phase transitions in nanostructured BFO and are potentially attractive for applications of high-performance electromechanical lead- free materials.

Acknowledgements

The work was supported by the National Key Research and Development Program of China (No. 2016YFA0201002) and the National Natural Science Foundation of China (Grant Nos. 11704130, U1832104, 11674108, 51332006). Authors also acknowledge the fi- nancial support of the Natural Science Foundation of Guangdong Province (Nos. 2017A30310169, 2016A030308019). X. Gao and X. Lu acknowledge the Project for Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme 2014 and 2016, respec- tively.

Supporting information

The topography, out of plane and in-plane PFM phase images of the as-grown BFO thin film are shown in Fig. S1and X-ray diffraction (XRD)θ-2θscans of the as-grown BFO thinfilm and BFO nanoislands with lateral sizes of 1 µm, 400 nm and 200 nm are presented inFig. S2.

Appendix A. Supplementary material

Supplementary data associated with this article can be found in the online version atdoi:10.1016/j.ceramint.2018.08.265.

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