Observation of Exotic Domain Structures in Ferroelectric Nanodot Arrays Fabricated via a Universal Nanopatterning Approach

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Observation of Exotic Domain Structures in Ferroelectric Nanodot Arrays Fabricated via a Universal Nanopatterning Approach

Guo Tian,

^†

Deyang Chen,*

^,†

Hua Fan,

^†

Peilian Li,

^†

Zhen Fan,

^†

Minghui Qin,

^†

Min Zeng,

^†

Jiyan Dai,

^‡

Xingsen Gao,*

^,†

and Jun-Ming Liu

^†,§

†Institute for Advanced Materials and Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, China

‡Department of Applied Physics, Hong Kong Polytechnic University, Hong Kong, China

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

*^S Supporting Information

ABSTRACT: We report a facile and cost-competitive nanopatterning route, using Ar ion beam etching through a monolayer polystyrene sphere (PS) array placed on a ferroelectric epitaxial thin ﬁlm, to fabricate ordered ferroelectric nanodot arrays. Using this method, well-ordered BiFeO₃epitaxial nanodots, with tunable sizes from ∼100 to ∼900 nm in diameter, have been successfully synthesized. Interestingly, a plethora of exotic nanodomain structures, e.g., stripe domains, vortex and antivortex domains, and single domains, are observed in these nanodots. Moreover, this novel technique has been extended to produce Pb(Zr,Ti)O₃ nanodots and multiferroic composite Co/BiFeO₃nanodots. These

observations enable the creation of exotic domain structures and provide a wide range of application potentials for future nanoelectronic devices.

KEYWORDS: nanopatterning, ferroelectric, multiferroic, nanodot array, vortex domain, size eﬀect

F

erroelectric nanostructures have attracted much attention both as a playground of emergent phenomena (such as vortex and center type topological domain states) and for potential applications in high-density nonvolatile memories, electromechanical sensors/actuators, and spintronic devices.¹⁻⁹ Motivated by these fascinating functionalities, substantial eﬀorts have been made in fabricating these nanostructures, especially the nanoscale ordered arrays.¹⁰⁻²⁰ Lithography is the most widely used fabrication technique for metallic and semi- conductor nanostructured materials, but this method may not be eﬀective for fabricating oxide nanostructures.^21,22 Several alternative fabrication techniques have been developed recently,²³ including top-down approaches (e.g., focused ion beam (FIB) milling, electron beam direct writing (EBDW), and anodized alumina (AAO) template-assisted ion beam milling)17−19,24−26and bottom-up methods (e.g., self-assembly and AAO template-assisted deposition methods).10−12,15,16 How- ever, these methods have some disadvantages such as low throughput, uncontrollable uniformity, or destructive structure.

Therefore, there are still enormous challenges to fabricate size- controllable, ultrahigh density, and epitaxial high quality ferroelectric nanostructures to promote the study of related physical properties and the realization of functional nanoelectronic devices. In this work, we demonstrate a universal route to fabricate high-density periodically ordered ferroelectric

nanodot arrays by Ar ion beam etching of high quality epitaxial ﬁlms, using monolayer polystyrene sphere (PS) array as membrane templates. Ferroelectric BiFeO₃ (BFO) ordered nanodots have been successfully fabricated by this method.

This facile route is extended to fabricate ferroelectric Pb(Zr_0.4Ti_0.6)O₃ (PZT) and multiferroic composite nanodots as well, demonstrating the universality of this novel approach.

Not only are the fabrication techniques of ferroelectric nanostructures gaining intense interest, but also the exotic domain structures in such materials have recently become an active subject due to a wealth of intriguing physical phenomena and potential applications.^27,28 Domain wall functionalities including domain wall conductivity, photovoltaic effects, and magnetoelectric coupling have been observed at stripe domain walls in BFO, providing great opportunities for future electronic, photonic and spintronic devices.²⁹⁻³³ Beyond the regular domain walls, the study of ferroelectric vortex structures is a new fascinating area as well.³⁴⁻⁴³Ferroelectric vortex state was initially predicted in ferroelectric nanostructures,⁴⁴ but it was first discovered in PbTiO₃/SrTiO₃ multilayer films and superlattices very recently.^45,46 Although previous studies

Received: August 22, 2017 Accepted: September 29, 2017 Published: September 29, 2017

www.acsami.org Cite This: ACS Appl. Mater. Interfaces 2017, 9, 37219-37226

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showed that ferroelectric vortices could be formed during electrical switching in nanodots,^34,35,38 it is still a main challenge to produce static ferroelectric vortices.⁴⁷ In this study, the coexistence of stripe domains, vortex domains, and antivortex domains is observed in BiFeO₃nanodots.

The ordered ferroelectric nanodots in this work are fabricated by a modified nanopatterning method. In this universal approach, the well-ordered PS arrays acting as the scarified mask are stacked onto an epitaxial oxide thin film which was grown by the pulsed laser deposition (PLD) system, following Ar ion beam etching onto the stacked structure to synthesize nanodot arrays. Sketches of the fabrication process and corresponding SEM images are shown inFigure 1. First,

the dispersed monolayer PS laying in a mixture of ethanol and water are transferred onto a BFO epitaxial ﬁlm (grown via PLD) to form a well-packed monolayer (Figure 1-I and a).

Then, the PS beads are etched to a desired size by oxygen plasma to make up a discrete ordered array (Figure 1-II and b).

Subsequently, Ar ion beam is applied to etch the stacked structure with appropriate etching time. During this process, the monolayer PS are taken as the etch mask, and thus, the regions of BFO blocked by PS are protected while the rest parts are etched. Thereby, the nanodots are produced as illustrated in Figure 1-III. Finally, the PS layer is removed by chloroformic solution to obtain the periodically ordered BFO nanodot arrays (Figure 1-IV and c).

By easily choosing varied sizes of PS as the etch mask and adjusting the oxygen plasma etching time, periodically ordered nanodots with tunable sizes (diameters∼100−900 nm) can be fabricated, as shown inFigure 2. Various sizes of PS including 1 μm, 500 nm, and 250 nm were used to fabricate 900, 400, and 200 nm sizes of BFO nanodots, as shown inFigure 2a−c. BFO nanodots with diameter of 120 nm (Figure 2d) were obtained by changing the plasma time of the PS with the size of 250 nm.

This method is also promising to produce nanodots with smaller sizes less than 100 nm by selecting corresponding sizes

of the PS. However, it is diﬃcult to form the monolayer PS as shown in Figure 1a because smaller size of PS increases the surface energy which leads to the aggregation of PS. The study of the addition of surfactant is in process to solve this problem.

It is worth to note that here we propose an efficient solution to avoid the crosstalk effects among different memory units (which is a critical issue for high density ferroelectric nanodot memory devices) by the pretreatment of PS with oxygen plasma etching to increase the distance among the neighboring dots.

Therefore, size tunable BFO nanodot arrays can be successfully fabricated via this facile route. Next, we focus on the structure of these samples.Figure 3a shows the XRDθ−2θ scan of BFO nanodot arrays on (001) Nb:SrTiO₃(Nb:STO) substrates, revealing the epitaxy of the nanodots. The epitaxial structure is further conﬁrmed by (113) reciprocal space map (RSM) as shown in Figure 3b. Derived from the XRD and RSM data, we can obtain the in-plane and out-of-plane lattice constants of BFO nanodots,a∼b∼0.391 nm andc∼0.406 nm, respectively. Thus, it is revealed that BFO nanodots are fully strained (compressive strain) on Nb:STO substrate. The typical cross-sectional TEM images, as shown in Figure 3c, demonstrate the sharp BFO/Nb:STO interface and the high quality of the nanodots. These results conﬁrm that high quality epitaxial BFO nanodots are produced.

We next turn to study the ferroelectric properties of BFO nanodots by piezoresponse force microscopy (PFM). The sample of BFO nanodot arrays with diameter of ∼400 nm is used here. PFM images of the as-grown BFO thinﬁlms with the 71°stripe domain structure are shown inFigure S1. The out-of- plane phase and amplitude (V-Pha. and V-Amp.) PFM images of BFO nanodots are shown inFigure 4a and b, respectively.

The region within dashed line squares are poled with a voltage of +9 V while the rest regions are poled with−9 V. As shown in Figure 4a, the dark and bright contrasts of the out-of-plane phase (V-Pha.) PFM image demonstrate the polarization switching of BFO nanodots, indicating their strong ferroelectricity. In the meantime, it is found that the amplitude for the downward polarization is smaller than that of upward (Figure 4b), revealing extent of preferred polarization orientation. From Figure 1. Fabrication of BFO nanodot arrays: (I−IV) Schematic

diagram for the PS layer assisted ion beam etching of epitaxial BFO thinﬁlms on Nb:STO substrate. The corresponding SEM images: (a) close-packed monolayer PS array placed on a BFO thin ﬁlm; (b) discrete ordered monolayer PS array after oxygen plasma etching; (c) ordered BFO nanodot arrays after Ar⁺ion beam etching and PS layer cleaning. Scale bar, 500 nm.

Figure 2.AFM images of the BFO nanodots array with various sizes.

The average diameters of the dots are (a)∼900 nm, (b) 400 nm, (c) 200 nm, and (d) 120 nm. Scale bar, 500 nm.

ACS Applied Materials & Interfaces Research Article

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the initial state, it is found that all the nanodots are upward self- polarized, supporting the occurrence prefer orientated. The local piezoresponse phase-voltage hysteresis and butterﬂy-like amplitude-voltage loops for a randomly selected nanodot are presented in Figure 4c and d, indicating a well-established polarization reversal. The switching behavior of BFO nanodots with diameter of ∼120 nm has been studied as well, demonstrating the electric ﬁeld reversible switching of polarization, as presented inFigure S2. These results all point to the

excellent ferroelectricity and electrically switchable polarization in the BFO nanodot arrays fabricated by this PS-assisted nanopatterning method.

To gain further insight into the 3D domain structures in BFO nanodots, vector PFM was employed to simultaneously capture the vertical (out of plane) and lateral (in plane) phases (V-Pha. and L-Pha.) and amplitudes (V-Amp. and L-Amp.) of piezo-response signals. As shown inFigure 5, the topography and vector PFM images were captured at 0° and 90° Figure 3.Microstructures for the BFO nanodot array. (a) XRD diﬀraction pattern and (b) reciprocal space mapping (RSM) adjacent to the STO (113) plane. (c) High resolution TEM image for a randomly selected BFO nanodot. Inset is the corresponding TEM cross-sectional image.

Figure 4.Vertical PFM (a) phase and (b) amplitude images for the polarization reversal process in the BFO nanodot arrays, in which the dashed line square regions are poled downward (with a bias voltage of +9 V) while the rest of the parts are poled upward with−9 V. Scale bar, 500 nm. (c) PFM phase−voltage hysteresis loop. (d) Butterﬂy like amplitude−voltage loop signals.

ACS Applied Materials & Interfaces

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orientations, respectively (the details of the scanning directions are illustrated in the top-right sketches inFigure 5b and c). At angle of 0° (Figure 5b), the lateral phase of most nanodots shows dark/bright contrast, indicating the positive/negative polarization orientation components along thex-axis (⟨100⟩), developing antiparallel stripe domain structures (details are shown is Figure 5d). And likewise, the y-axis components (⟨010⟩) can be determined according to lateral PFM image in Figure 5c. It turns out that some nanodots (such as the dot marked with a red square) show uniform bright contrast, revealing the lateral downward polarization component along

⟨010⟩, while other nanodots possess antiparallel stripe domain structures (such as the dots marked with blue and black squares). The corresponding vertical PFM images with a uniform bright contrast are also shown inFigure S3, indicating the uniform vertical polarization component along the z-axis.

To reconstruct the lateral domain conﬁgurations, the amplitude and phase images are converted to the PFM piezoresponse signal contours ofx-axis component,y-axis component, andz- axis component, according to function Rcos(θ) (R is the amplitude, and θ is the phase angle).⁴⁸ Finally, the 3D polarization vector mapping can be obtained using the MATLAB programing, following the method by Rodriguez and Kalinin.⁴⁸

Then, we focus on the study of the typical domain structures in the representative nanodots marked with red, blue and black squares as shown in Figure 5. The topography, vertical phase image and two lateral directions upon sample clockwise 0°and 90° are summarized in Figure 5(d-f). Interestingly, stripe domains (Figure 5d), antivortex core (Figure 5e) and vortex and antivortex pair (Figure 5f) are observed in these nanodots.

Previous studies have demonstrated the formation of halfflux- closure quadrants in BFO thinfilms and the creation of vortex and antivortex domains induced by external electricfield.^38,39 However, it is difficult to observe the static ferroelectric vortices. Very recently, Tang et al.⁴⁵ and Yadav et al.⁴⁶ discovered the full flux-closure quadrants and ferroelectric vortices in PbTiO₃/SrTiO₃ multilayer films and superlattices, respectively. Here we observe the coexistence of isolated static ferroelectric vortices and stripe domains in BFO nanodots. As shown in Figure 5, many nanodots remind the stripe domain structure consistent with the original domain state in the as grown BFO thin film before etching into nanodots.

Interestingly, the domain structure in some other nanodots evolves toflux-closure vortex or antivortex topologic domains, probably due to the large variation in depolarizationfield and strain conditions after the BFO thinfilm is etched to nanodots.

The coexistence of stripe-domain patterns and vortex domains in nanodots has also been predicted in BaTiO₃by using Monte Carlo simulation,^49,50 which can be interpreted by the competition of elastic and electrostatic interactions. Exper- imentally, the occurrence of spontaneousflux closure vortex in this study is also similar to what have been observed in PbTiO₃/SrTiO₃multilayerfilms and superlattices,^45,46in which the ferroelectric layers were covered by insulating layer leading to large unscreened depolarization field that favors the formation of flux-closure vortex domains. The occurrence of spontaneous vortex domains in our nanodots indicates the existence of large depolarization as well. Nevertheless, the BFO nanodots in our study were mainly exposed in air, thus the depolarization field in nanodots can be partially screened by absorbed charges from ambient gas or internal mobile charge Figure 5.(a)Topology, Lateral phase (L-Pha) PFM images with the sample rotation for two different angles of (b) 0°and (c) 90°of nanodots with the diameter of∼400 nm. (d−f) Details of the typical domain structures of three nanodots in (a−c) marked with red, blue, and black squares. From left to right: topography, vertical PFM phase (V-Pha.), lateral PFM phase (L-Pha.) with sample rotation at two different angles of 0°and 90°, phase angle map converted from the two different L-PFM phase images, and a schematic of the typical topological domains in the selected nanodots: (d) stripe domains (circled in red); (e) antivortex cores (circled in blue); (f) vortex−antivortex pairs (circled in black). Shematic diagrams inset in the bottom left to (d−f) present the four possible polarization orientations in the nanodots. Scale bar, 500 nm.

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carriers, allowing the existence of the antivortex and other domain states. These observations provide the possibility to study the functionalities of these novel domain states and enhance magnetic properties of multiferroic BFO from Dzyaloshinski−Moriya interactions.

To further study the size effect on the domain structure evolutions of BFO nanodots, the PFM data of the samples with diameters of 900 and 100 nm were conducted as well. As shown in Figure 6a−c, most of the BFO nanodots with the diameter of∼900 nm maintain the typical 71° stripe domain structure which is the original domain state in the as grown BFO thin film. As described above (Figure 5), the mixed domain structures including 71° stripe domains, antivortex domains and vortex and antivortex pairs have been demonstrated in the∼400 nm nanodots, which is in agreement with previous theoretical work.^49,50Further decrease of the diameter of BFO nanodots to 100 nm leads to the formation of single domains along with a small amount of stripe domains, as shown in Figure 6d−f. This may be interpreted by the variation in long-range electrostatic interactions and elastic interactions, which can be greatly affected by the geometric size of the nanodots. As the small nanodots were exposed in air, the screening charges from the absorbed electrons on the surfaces of nanodots can greatly weaken the depolarization field, favoring the formation of single domains rather than flux- closure vortex. In addition, the small lateral size of nanodots (comparable to the width of the stripe pattern in their parent film) can lower the elastic energy of single domain. As a result, the single domain structure becomes more energetic favorable in small nanodots than stripe or vortex domains, and such single domain state has been demonstrated in ferroelectric nanoparticles, nanodots and nanowires using phase field simulations.^51,52

After successfully fabricating the well-ordered BFO ferroelectric nanodot arrays, we next try to extend this novel approach to other oxides. PZT nanodot arrays have been produced by this route and the related vertical PFM phase (V- Pha.) and amplitude (V-Amp.) images are shown in Figure

7a,b. Bubble domain structures are observed in these PZT nanodots, which might be resulted from the competition

between depolarizationﬁled and perpendicular anisotropy from substrate clamping.⁵³This method is also employed to fabricate 0−0 type multiferroic composite Co/BFO nanodots. The topography and magnetic domain structures of these nanodots, measured by magnetic force microscopy (MFM) are presented inFigure 7c and d, respectively. The dark and bright contrasts represent diﬀerent direction of the magnetization (Figure 7d).

Our previous study has demonstrated the strong magneto- Figure 6.(a) Topography, (b) vertical phase (V-Pha.) PFM, and (c) lateral phase (L-Pha.) PFM of nanodots with the diameter of∼900 nm, maintaining the typical 71°stripe domain structure of the as grown BFO thinﬁlm. (d) Topography, (e) vertical phase (V-Pha.) PFM, and (f) lateral phase (L-Pha.) PFM of nanodots with the diameter of∼100 nm, mainly consisting of the single domain structure.

Figure 7.(a) Vertical PFM phase (V-Pha.) and (b) vertical PFM amplitude (V-Amp.) images of PZT nanodot arrays grown on STO substrate with a SRO bottom electrode. (c) Topography and (d) corresponding MFM images of multiferroic composited Co/BFO nanodots. Scale bar, 500 nm.

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electric coupling eﬀects in these 0−0 type multiferroic composite nanodots.¹¹Thus, the fabrication of these nanodots provides a pathway to study the electric ﬁeld control of magnetism. In this work, well-ordered BFO, PZT and Co/BFO nanodot arrays are produced via this nanopatterning method, demonstrating its universality for fabricating oxide nanodot arrays.

In summary, an efficient and universal nanopatterning approach, using Ar ion beam to etch a monolayer PS array (as the template) which placed on a high quality epitaxial thin film, to synthesize well-ordered ferroelectric nanodot arrays has been proposed. The BFO nanodots synthesized using this method maintain the epitaxial structure and strong ferroelectric properties. Moreover, the coexistence of stripe domains, vortex and antivortex domains (1D topological domains), and single domains is observed in these nanodots with different sizes. To demonstrate its universality, this approach is also extended to prepare PZT and Co/BFO nanodot arrays. These findings open a route to study the fundamental physics of the exotic domain structures and might promote the realization of high- density ferroelectric nanodot devices.

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Fabrication of Nanodot Arrays.^METHODS The details of the fabrication procedure for the nanodot arrays have been illustrated in the schematicflowchart inFigure 1a. The BiFeO₃ thinfilm is deposited on a (100)-oriented Nb:SrTiO₃ substrate by pulsed laser deposition (PLD) using a KrF excimer laser (wavelengthλ= 248 nm) at 660°C in an oxygen ambient of 15 Pa. The laser pulse energy is 300 mJ with a repetition rate of 8 Hz. The polystyrene spheres (PS) dispersed in a mixture of ethanol and water are transferred onto the as grown BFO film to form a close-packed monolayer. The beads are then etched to the desired size by oxygen plasma to form a discrete ordered array.

This is followed by Ar ion beam etching with appropriate etching time.

Finally, the PS layer is removed by chloroformic solution and the periodically ordered BFO nanodot arrays are obtained.

Ion Beam Etching Process. The samples are etched by Ar ion beam etching in a vacuum pressure of 7 × 10⁻⁴ Pa at room temperature. During the etching, the incident ion beam is perpendicular to the sample surface. The etching parameters have been carefully optimized, using a cathode current of 15 A, an anode voltage of 50 V, a plate voltage of 300 V, an ion accelerating voltage of 250 V, a neutralization current of 11.7 A, and a bias current of 1.2 A.

Microstructural Characterizations.The structure of nanodots is characterized by X-ray diﬀraction (PANalytical X′Pert PRO), including θ−2θ scanning and reciprocal space mapping (RSM) along the (113) diﬀraction spot. The cross-sectional TEM images are taken by high resolution transmission electron microscopy (HRTEM, JEOL-2011), the top view surface images are obtained by scanning electron microscopy (SEM, Zeise Ultra 55), and the topography images are taken by atomic force microscopy (Asylum Cypher AFM).

PFM and MFM Characterizations. The ferroelectric domain structures of these nanodots were characterized by piezoresponse force microscopy (PFM) with a scanning probe mode (Asylum Research) using conductive PFM probes (Nanoworld). The local piezoresponse loop measurements are carried out byﬁxing the PFM probe on a selected nanodot and then applying a triangle square waveform accompany with ac driven voltage, via the conductive PFM probe. The vector PFM function of our AFM unit allows simultaneous mapping of the vertical (out-of-plane) and lateral (in-plane) amplitude and phase signals from the nanodots one by one. To determine the 3D domain structures, both the vertical and lateral PFM images for two diﬀerent in-plane sample rotation angles are conducted. We mark the sample before the rotations, so that the sample scanned area can be tracked.

The magnetic domain structures are characterized by magnetic force microscopy (MFM) by Asylum Research using MFM probes (Multi75M-C, Budget Sensors).

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ASSOCIATED CONTENT

*^S Supporting Information

The Supporting Information is available free of charge on the ACS Publications websiteat DOI:10.1021/acsami.7b12605.

Topography and PFM images of the as-grown BFO thin ﬁlm, switching behavior of BFO nanodots with diameter of∼120 nm, and topography and PFM images of BiFeO₃ nanodot array (PDF)

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AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected].

ORCID

Deyang Chen:0000-0002-8370-6409

Peilian Li:0000-0003-0902-9689

Zhen Fan:0000-0002-1756-641X

Xingsen Gao:0000-0002-2725-0785

Jun-Ming Liu:0000-0001-8988-8429 Notes

The authors declare no competingﬁnancial interest.

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ACKNOWLEDGMENTS

The authors would like to thank the National Key Research P r o g r a m o f C h i n a ( N o s . 2 0 1 6 Y F A 0 3 0 0 1 0 1 , 2016YFA0201002), the State Key Program for Basic Researches of China (No. 2015CB921202), the Natural Science Foundation of China (Nos. 11674108, 11704130, 36651272078), the Project for Guangdong Province Uni- versities and Colleges Pearl River Scholar Funded Scheme (2014), the Natural Science Foundation of Guangdong Province (No. 2016A030308019), and the Science and Technology Planning Project of Guangdong Province (No.

2015B090927006).

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