Controllable Ferroelastic Switching in Epitaxial Self-Assembled

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Controllable Ferroelastic Switching in Epitaxial self-assembled Aurivillius Nanobricks Rizwan Ullah, Xiaoxing Ke, Iftikhar Ahmed Malik, Zhenao Gu, Chuanshou Wang, Munir Ahmad, Yuben Yang, Wenkai Zhang, Xiaoqiang An, Xueyun Wang, and Jin-Xing Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22080 • Publication Date (Web): 24 Jan 2019 Downloaded from http://pubs.acs.org on January 25, 2019

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ACS Applied Materials & Interfaces

Controllable Ferroelastic Switching in Epitaxial selfassembled Aurivillius Nanobricks Rizwan Ullah1#, Xiaoxing Ke2#, Iftikhar Ahmed Malik1, Zhenao Gu3, Chuanshou Wang1, Munir Ahmad1, Yuben Yang1, Wenkai Zhang1, Xiaoqiang An3, Xueyun Wang4* & Jinxing Zhang1* 1Department 2Institute

of Physics, Beijing Normal University, 100875 Beijing, China

of Microstructures and Properties of Advanced Materials, Beijing University of

Technology, 100124 Beijing, China 3Key

Laboratory of Drinking Water Science and Technology, Research Center for Eco-

Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China 4School

#These

of Aerospace Engineering. Beijing Institute of Technology, 100081, Beijing, China

authors contribute equally to this work

*Corresponding authors email: [email protected], [email protected]

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ABSTRACT Layered perovskites with Aurivillius phase call tremendous attentions recently, owing to their high ferroelectric Curie temperatures, large spontaneous polarization, fatigue-free and environmental friendly natures. Bi2WO6 is one of the simplest member in Aurivillius family with superior ferroelastic and photo-electrochemical behaviors. The self-assembly fabrication of its nano-architectures and strategically modulating their ferroelastic switching are crucial towards the high efficient nanoscale applications. In this work, Bi2WO6 nanobrick arrays were epitaxially grown along the orthorhombic direction in a selfassembled way. Such a nanoscale topology supports out-of-plane and in-plane vectors of ferroelectric polarizations, enabling a perpendicular voltage manipulation of those emerging ferroelectric/elastic domains. Combining scanning probe technique and transmission electron microscope, we confirmed the in-plane polarization vectors of 78.6° and 101.4° within the crystallographic axes of the nanobricks with respect to (110) plane of the substrate. Thus, this work provides new opportunities of ferroelectric/elastic engineering in Bi2WO6 nanostructures for a wide range of applications, such as sensing, actuating and catalysis. KEYWORDS: Bismuth layered ferroelectrics, Bi2WO6, Ferroelastic Switching, Self-assembled nanostructures, Piezoresponse force microscope.

INTRODUCTION: The strategical control of the spontaneous ferroelectric polarization (Ps) and the switching behaviors of domain structures via electric field are of great importance for developing highperformance piezoelectric nanomaterials for a wide range of applications, typically represented by non-volatile memories 1, data storage devices 2, actuators and sensors 3. In principle, the surface reactivity of piezo-catalysts is strongly dependent on the polarization field 4. Therefore, this physical feature could exhibit significant impact on the efficiency of energy and environmental catalysis 5. Bismuth layered ferroelectrics (BLSFs) are the most extensively studied ferroelectric materials in the Aurivillius family

6

due to their high ferroelectric Curie temperatures 7, large

spontaneous polarization 8, fatigue-free and environmental friendly natures 2 ACS Paragon Plus Environment

9-10.

However, the

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manipulation of polarization switching in BLSFs usually suffers from large leakage current and adverse behaviors, such as domain pining, aging, fatigue, depolarization and imprinting 11. The insufficient control of nanoscale ferroelectric/elastic switching hinders their advancement in ferroelectric-based devices. Therefore, a new nanoscale strategy is highly desired for the precise control of polarization switching without suffering from above stated undesired behaviors to enhance the performance of the ferroelectric devices. The ferroelectric and piezoelectric properties of the BLSFs alter with crystallographic directions and with the number of perovskite layers as well

12-13.

Note that the bismuth layered

structures consist of fluorite-like (Bi2O2)+2 sheet and perovskite-like (Am−1BmO3m+1)−2 blocks, the net spontaneous polarization (Ps) majorly aligns along [100] direction, resulting from distortion of Bi2O2 layers and perovskite blocks in their crystal structures 14-16, supporting 90° ferroelastic and 180° ferroelectric domain walls 17-18. Therefore, epitaxial control of various crystalline orientations and their nanoscale topologies may provide a platform for possible new polarization switching (not 180˚, 90˚, 109˚ and 71˚ etc.) paths which does not exist in other ferroelectrics. Such nanoscale polarization switching in BLSF plays a vital role in magnetoelectric heterostrucutre system

19.

Ferroelastic switching in traditional perovskite ferroelectric thin films are sensitively affected by substrate clamping. The precise control of the elastic switching is hard to achieved 20-21. In order to reduce the substrate clamping effect, the self-assembly technique is the most convenient and low cost approach to architect nanostructures up to few ten nanometers in size 22. The epitaxial self-assembled nanostructure has been a topic of research interest for the ultra-high density ferroelectric memories

23

and magnetoelectric (ME) devices

24-25.

Such a nanostructures have

shown higher conduction at domain wall in epitaxial ferroelectric 26 and significant ME coupling in two phases multiferroic thin film 24, 27. The nanostructures such as nanoislands, nanopillars etc., in the ferroelectric BFO, and two phase multiferroic thin film systems (BiFeO3-CoFe2O4, BaTiO3CoFe2O4 and PbTiO3- CoFe2O4) form due to the strain relaxation induce by defects, film thickness and growth kinetics26, 28-30. The substrate orientations change the morphology of the nanostructures and morphology dependent ME coupling of the self-assembled multiferroic nanostructures on different oriented substrate can be controlled by adjusting the stress state, 30-31 and different growth modes in the epitaxial thin film of the two phases system 28. The growth kinetics such as growth rate and growth temperature play a vital role to control the size of the self-assembled BiFeO3CoFe2O4 and BaTiO3- CoFe2O4 nanostructures 28-29. 3 ACS Paragon Plus Environment


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Bi2WO6 (BWO), the simplest candidate of the Aurivillius group members, exhibits ferroelectric property with rather large remanent polarization water splitting

33-34

and degradation of the organic pollutants

32,

photocatalytic activity toward

35

due to high ionic conductivity

owing to its fast oxygen ion transport 36 and as a piezoelectric material with high Curie temperature (Tc) and high piezoelectric coefficients 37. The epitaxial thin film growth on single crystal substrate is one of the effective approach to discover new catalyst systems, leading to potential application in the efficient water splitting and degradation of the organic pollutants38-41. The BWO is also potential materials for electrical and optoelectronic applications due to good electrical and broad photo response behavior 42. The ferroelectric polarization in BWO derives from the displacement of W and Bi opposite to its surrounding oxygen atoms in 3:1 respectively 12. Recently, for the first time, Wang, C. et al., reported the controllable in-plane ferroelastic switching in c-oriented BWO thin film with non-volatile strain of 0.4%

18.

Ferroelastic switching in BWO thin film

heterostrucutre has negligible epitaxial constraints. Moreover, growth of the film with tilted axis (non-c-oriented) is the prerequisite to attain a large out-of-plane (perpendicular to the film) polarization 43-44, such large out-of-plane polarization is very important for the BLSFs to enhance the device density 17. Under different growth conditions and substrate orientations, such epitaxial growth of c-oriented BWO can be controlled into non-c-oriented growth, which supplies an inclined polarization with both parallel and perpendicular components to the film plane

17, 45-47.

Therefore, such non-c-oriented BWO nanostructures with polarization normal to the film plane and control of such polarization via out-of-plane electric field over large area might result in unprecedented properties for practical applications. In this work, we report the successful fabrication of self-assembled (113)-oriented BWO nanobricks, which supports out-of-plane ferroelectric polarization. TEM and X-ray diffraction confirm the epitaxial relationship between the nanobricks and substrate. Piezoresponse force microscopy (PFM) and scanning transmission electron microscopy (STEM) studies reveal a new type of ferroelastic domains walls and their controllable switching for the first time. Such reversible control of out-of-plane polarization is critical for the potential non-volatile memories and nanoscale catalysis applications.

RESULTS AND DISCUSSION


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BWO (100 nm thickness) was deposited on SrTiO3 (STO) (110) buffered with SrRuO3 (SRO) via the pulsed laser deposition (PLD) technique. X-ray diffraction (XRD) analysis confirms the preferred (113) growth direction with a set of (001) oriented BWO peaks (represented by asterisk in Fig. 1(a)). An interfacial layer of 20-30 nm SrWO4 (SWO) with (112)-oriented peak are also indicated, which is clearly visible in the cross-sectional TEM image as shown in Fig. 1(b). The atomic force microscope (AFM) scanned topographic image in Fig. 1(c) shows the self-assembled nanobricks with nano- to micron-meter size with certain orientation. The average size of the selfassembled nanobricks can be controlled by varying the growth temperature, as shown in Figs. S1. The nanobrick at lower temperature are randomly oriented than those at higher temperature. The domain configurations and domain walls (yellow dashed lines) within BWO nanobricks are shown in Fig. 1(d). The epitaxial self-assembled ferroelectric BWO nanobricks may possible provides a model system for the ultra-high density ferroelectric memories, to study the exotic domain structure and new domain switching paths. Note that the valley in dark color is (001)-oriented BWO, which is confirmed by performing the local piezoresponse hysteresis. It’s also worthy to emphasize that the formation of interfacial layer SWO is mainly due to chemical reaction, which stabilizes the BWO layer during the epitaxial growth. Similar process is also found in other Bismuth/Tungsten-containing epitaxial growth procedure 48-49. (See supplementary information Section 1 for more discussion of SWO layer). The orientation of self-assembled BWO nanobricks with respect to the STO substrate are thoroughly analyzed with TEM. TEM Images for zone axes [001] and [1-10] of STO substrate are obtained as shown in Figs. 2(a) and (d) (See Fig. S2(a) for zone axis [1-11]) with BWO, SWO and SRO layers. Magnified images in yellow and blue-boxed regions are displayed in Fig. 2(b), (c), (e) and (f), respectively, by performing high-angle annular dark-field (HAADF) scanning. Insets display the corresponding diffraction patterns (DP’s). By analyzing the TEM images, the relationship of crystallographic orientations between BWO film and substrates were summarized as following: (113) [-110]BWO || (110) [001]STO

(1)

(113) [33-2]BWO || (110) [1-10]STO

(2)

(113) [03-1]BWO || (110) [1-11]STO

(3)



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Here, we define [-110] of (113)BWO as the [-110] direction of the (113) plane of BWO, same definition for STO. After comparing the DPs of Fig. 2(b) and (c), we can conclude that the [001] growth direction of BWO film deviates ~9° from [010] direction of STO. The DPs in Figs. 2(e) and (f) were obtained with respect to the [1-10] direction of STO, confirm the epitaxial relation of film growth plane on (110)STO substrate. Similarly, the DPs shown in Fig. S2(c) and (d) confirm that (113)BWO is parallel to the (110)STO, when viewing along the [1-11] direction of STO. Based on the above summarized orientations, the in-plane angle between the crystallographic axes (polarization vectors) and possible in-plane ferroelastic switching mechanisms (which will be discussed later) are schematically drawn in Figs. 3(a) and 3(b), respectively. The 90° angle between a and b axes in BWO has an in-plane (IP) projection on (110)STO surface with IP angle of 101.4°. The IP projection of the a and b axes are represented by black dot arrow a' and b'. Note that the IP projection of c axis is along [-110] of STO substrate, as labeled by black dot arrow c', as shown in Fig. 3(a). The ferroelastic switching mechanism for c-orientated BWO was well established

18,

however, in (113)-orientated BWO, the possible polarization directions and their related switching mechanism are unexplored. In Fig. 3(b), the schematic for possible polarization switching mechanism are illustrated. Note that the orthorhombic crystal structure of the BWO is in hexagonal geometry (Fig. 3(b)) when viewing along the body diagonal direction, as shown in the right side of Fig. 3(b). The four possible polarization states of (113) BWO, ±Pa and ±Pb (labeled by black arrows) have IP projections on the surface of STO substrate, which are represented by ±Pa', ±Pb' (dashed arrows with different colors). For example, the IP projection vector of the Pa is along [112]STO, represent by Pa'. For the sake of simplicity, the out-of-plane (OP) polarization (projection) vectors, normal to the plane are considered along [00±1]BWO represented by ±Pc' respectively. It’s obvious that the polarization Pa changes to ±Pb after 90° rotation, and changes to –Pa after 180° rotation. Similarly, the corresponding in-plane polarization vector Pa' rotate to ±Pb' through an angle of 78.6° or 101.4° , and rotate to -Pa' with 180° rotation with respect to the substrate during ferroelastic and ferroelectric switching respectively, as shown in Fig. 3(b). Experimentally, piezoresponse force microscopy (PFM) scanning has been performed to understand the in-plane ferroelectric domain configurations. Figs. 4(a) and (d) are the topographic images on the identical region, obtained before and after rotating sample 90°. Two random 6 ACS Paragon Plus Environment

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nanobricks are selected, as yellow and red dashed lines isolated, shown in Figs. 4 (b) and (e). Different IP domain configurations were acquired within these nanobricks at 0° and 90° rotation of the sample with respect to the cantilever direction. As discussed above about the possible inplane polarization directions, the schematic of the in-plane polarization for these labeled nanobricks are illustrated as shown in Figs. 4 (c) and (f). Note that due to the in-plane 78.6° between possible ferroelectric polarizations, we performed more comprehensive study of the three dimensional IP domain mapping using angle-resolved PFM as shown in Fig. S3(a)-(e), detail is provided in supplementary information section 2. Each scanning was performed with 60° rotation of the sample with respect to the cantilever direction. Moreover, the ferroelectric property has also been studied by performing local piezoresponse hysteresis loop within nanobrick, which shows weak negative polarization offset (imprint effect) as shown in fig. S6(e). The offset are consistent with the as grown downward polarization state, most probably due to the asymmetric top/bottom electrodes, leading to Schottky barriers 50 or presence of the non-switching (001) layer in heteroepitaxial ferroelectric thin film

51.

No hysteresis loop is acquired between the nanobricks (the

valley in the topographic image), which is consistent with the property of (00l) plane of BWO film. SWITCHING MECHANISM: The switching experiment was performed in the presence of different vertical biases as shown in Fig. S4. Two random nanobricks are chose to reveal the detailed switching mechanism, the yellow dashed isolated nanobrick is shown in Fig. 5. The topographic, corresponding IP and OP PFM phase images are shown in Figs. 5(a-c), 5(d-f) and 5(g-i) respectively, which are acquired at 0 V (as grown domain state), +12 V and -12 V consecutively. The OP PFM as shown in Figs. 5(g-i) indicate mono-domain can be completely switched in the presence of vertical electric field. However, the IP PFM images reveal the different ferroelastic switching mechanism i.e. ferroelastic (78.6° or 101.4°) and ferroelectric (180°) in regions 1 and 2, respectively. The IP phase image of the as grown domain state in Figs. 5(d) shows medium and dark contrast for region 1 and 2 respectively, with schematics as shown in figures 5(j) and (k). In the presence of +12 V applied electric field, the IP phase contrast for region 1 changes from medium to bright, and the switching path schematically shown from Fig. 5(j) to (l) illustrate that the IP polarization rotates from -Pa' to Pb' as a consequence of dominant 101.4° ferroelastic switching. In region 2, bright and medium 7 ACS Paragon Plus Environment


phase contrasts (Fig. 5(e)) were observed in lower and upper area (reference to the white line). The schematic for the region 2 (Fig. 5(h)) illustrate two (represented by the red dots in the corresponding schematic) possible 78.6° or 101.4° ferroelastic switching in upper part above white line (Fig. 5(e)). In case of 78.6° (101.4° ) ferroelastic switching, IP polarization vector rotate from -Pb' to -Pa' ( -Pb' to Pa'). Ferroelectric (180°) switching was observed in the lower part (below white line) in region 2, as shown in Fig. 5(e). The schematic study for the region 2 as shown in Fig. 5(m) reveals the complete 180° rotation for the IP polarization (-Pb' to Pb') and OP polarization (Pc' to Pc') vectors. During back switching process with applying -12 V bias to the scanning probe tip, both IP and OP polarization are switched back in both region to the as grown domain state as shown in Fig. 5(f) and (i) respectively. The schematic (Fig. 5(n)) for region 1 shows that the IP polarization vector rotation from Pb' to -Pa', which certifies 101.4° ferroelastic switching. Fig. 5(f) shows IP phase change from bright to dark in lower part of region 2, illustrating the ferroelectric switching as a result of IP polarization rotation from Pb' to -Pb'. The corresponding schematic illustrates ferroelastic (101.4° or 78.6° switching) as a result of IP polarization vector rotation from ±Pa' to Pb' respectively as shown in Fig. 5(o). Therefore, we assume that for the upper area in region 2, ferroelastic (101.4° or 78.6° switching) is favorable in order to restore the as grown domain state within nanobrick. Similar ferroelastic switching behaviors were observed for white dashed nanobrick as shown in Fig. S5, and the detailed switching mechanisms are provided in supplementary information section 3. The polarization switching study reveals that observed ferroelectric switching are very rare as compared to ferroelastic switching in different nanobricks. Apparently 101.4° ferroelastic switching is dominant in our work. Moreover, the detailed IP and OP polarization switching study in the presence of various bias were shown in Fig. S4(a)-(t). Further increasing the switching bias to -13 V, the motion of ferroelastic domain wall to the edges of the nanobricks was observed, as shown in Figs. S6(a) and (c). With increasing the bias, the domain can be completely manipulated into mono-domain. CONCLUSIONS: In summary, we have shown the successful fabrication of epitaxial self-assembled Bi2WO4 nanobricks with a non-c orientated feature, enabling the perpendicular manipulation of 8 ACS Paragon Plus Environment

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ferroelectric polarization through vertical electric field. Such characteristic in large area is deterministic for the precise control of polarization switching to increase the density and enhance the performance of the ferroelectric devices. Moreover, the principle of ferroelastic switching were exemplified by performing comprehensive switching experiment, which reveals multiple polarization switching paths (78.6°, 101.4° and 180°) via out-of-plane electric field. Reversible control of such polarization is the key physical phenomena towards the non-volatile memories and nanoscale catalysis. Therefore manipulation of the domain via domain/strain engineering and growth mechanism lead layered perovskite as a potential candidate for the future nano-electronic applications.



MATERIALS AND METHODS Materials synthesis and characterization. 100 nm thickness (113)-oriented orthorhombic Bi2WO6 (BWO) film was grown on (110)-oriented SrTiO3 (STO) substrate buffered with 10-15 nm thick SrRuO3 (SRO) conductive electrode via pulse laser deposition (PLD). Both stoichiometric targets (SRO and BWO) were ablated at a laser energy density of 1 J cm-2 and a repetition rate of 1 Hz for the growth of (10-15 nm) SRO and (100 nm) BWO thickness respectively. The substrate temperature was kept at 700 °C at 100 mTorr oxygen environment for SRO growth. For the growth of BWO, the substrate temperature was maintain at 730 °C at an oxygen pressure of 100 mTorr. The TEM lamellae were prepared along the [001], [1-10] and [1-11] direction of STO substrate with thickness of approximately 50–100 nm. High-resolution HAADF-STEM images shown in this work were obtained from a FEI Titan G2 microscope fitted with an aberration corrector for probe-forming lens, operated at 300 kV with a semi-convergence angle of approximately 21.4 mrad. Diffraction patterns were acquired using the same microscope. The PFM measurements were carried out using a Bruker Multimode 8 AFM with commercially available platinum coated Silicon tips (Mikro Masch). The cantilever was kept along [-112]STO direction in order to get strong in-plane domain contrast. The typical scanning velocity was 2 µms-1 for imaging. An ac input of amplitude 1Vpp and frequency of 25 kHz were used during measurement. The maximum 12 V bias were used during area poling. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website, which includes: Atomic force microscopy (AFM) topographic images of BWO nanobricks under different growth temperatures. The cross sectional HR-STEM image of the heterostructure and epitaxial relation study between film and substrate from the HAADF-STEM and diffraction pattern (DP) images of film and substrate. The 3–dimensional angle resolved Piezoresponse force microscopy (3D ARPFM) images of the domain configurations within BWO nanobricks by rotating sample through 10 ACS Paragon Plus Environment

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different angles with respect to the cantilever direction and its schematics illustration. The controlled (in-plane and out-of-plane) polarization switching study under different switching bias via PFM on large area of the film. The detailed polarization switching mechanism within a BWO nanobrick and its schematics illustration. The phase vs. bias loop at single point inside nanobricks. AUTHOR INFORMATION Corresponding Authors *Email: [email protected] *Email: [email protected] Funding This work was supported by the National Key Research and Development Program of China, The Fundamental Research Funds for the Central Universities, National Natural Science Foundation of China, Ministry of Education of China, and Beijing Nova Program. ORCID Xiaoxing Ke: 0000-0003-2004-6906 Xiaoqiang An: 0000-0003-3681-6418 Xueyun Wang: 0000-0001-5264-9539 Jinxing Zhang: 0000-000108977-5678 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS The work at Beijing Normal University is supported by the National Key Research and Development Program of China through Contract 2016YFA0302300. J.Z. also acknowledges the support from “The Fundamental Research Funds for the Central Universities” under Contracts 2017EYT26 and 2017STUD25. X.W. acknowledges the National Natural Science Foundation of China with Grant No. 11604011. X. K. acknowledges Beijing Nova Program Z161100004916153.



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List of figures Legends Figure 1. (a) XRD of the BWO/SRO heterostructure on (110)-oriented STO. (b) Cross-sectional TEM image of the BWO/SRO/STO. (c) Topography of BWO thin film. (d) IP-PFM image of the BWO thin film. The yellow dashed lines indicate the domain walls within nanobricks. Figure 2. (a) Cross-sectional HR-STEM image of the BWO/SRO/STO heterostructure. HAADFSTEM images of (b) BWO thin film and (c) STO along [001] zone axis of substrate. (d) Crosssectional HR-STEM image of the BWO/SRO/STO heterostructure, HAADF-STEM images of (e) BWO thin film and (f) STO along [1-10] zone axis of substrate. Inset in (b), (c), (e) and (f) show corresponding diffraction patterns. Red and white arrows in (b) indicate WO4 and Bi2O2 layers. Figure 3. (a) Schematic representation of BWO/STO heterostructure with a, b and c (solid black arrows) crystallographic axes of BWO film, and in-plane projections a', b' and c' (black dashed arrows) on (110)-oriented STO. The purple rectangle represents (113) plane of BWO thin film. (b) Schematic representation of possible polarization switching in (113)-oriented BWO thin film with respect to STO substrate. The ±Pa and ±Pb (solid black arrows) are four in-plane polarization in BWO. ±Pa', Pb' and -Pb' (light blue, orange and red dashed arrows respectively) represent their inplane polarization components on (110)-STO with respect to the cantilever direction along [112]STO. Black and white arrows represent the out-of-plane polarization components with respect to the STO plane. Dark blue and dark red double side arrows indicate 101.4° and 78.6° switching step respectively. The schematic at the right side shows viewing angle of the (113)-oriented BWO crystal corresponds to (b). Figure 4. IP-PFM images shown in (a) is scanning along 0° (scanning direction parallel to [110]STO). (d) is after 90° rotation of the sample. (b) and (e) are magnified images of the dashed nanobricks in IP-PFM images (a) and (d), respectively. Schematics representation of the polarization state within nanobrick for (b) and (e) are displayed in (c) and (f), respectively. Figure 5. (a-c) topographic images (d-f) IP-PFM images and (g-i) OP-PFM images of yellow dashed labeled nanobrick for as grown domain state, at +12 V and at -12 V. Different colored arrows in regions 1 and 2 (d-f) represent IP polarization directions with respect to scanning direction. Green dashed line represents domain wall between regions 1 and 2. (j-o) are schematics



illustration of polarization in regions 1 and 2 for as grown state, at +12 V and at -12 V. Scale bar in (a-f) is 450 nm.


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Figure 1



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Figure 2


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Figure 3



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Figure 4


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Figure 5



Table of Contents (TOC) Graphic


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Controllable Ferroelastic Switching in Epitaxial Self-Assembled

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