Self-assembled Ferroelectric Nano-array - ACS Applied Materials

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Functional Nanostructured Materials (including low-D carbon)

Self-assembled Ferroelectric Nano-array Jie Jiang, Qiong Yang, Yi Zhang, Xiao-Yu Li, Pao-Wen Shao, Ying-Hui Hsieh, HengJui Liu, Qiangxiang Peng, Gaokuo Zhong, Xiaoqing Pan, Ying-Hao Chu, and Yichun Zhou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14775 • Publication Date (Web): 09 Nov 2018 Downloaded from http://pubs.acs.org on November 9, 2018

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ACS Applied Materials & Interfaces

Self-assembled Ferroelectric Nano-array Jie Jiang1, Qiong Yang1, Yi Zhang2, Xiao-Yu Li1, Pao-Wen Shao3, Ying-Hui Hsieh3, Heng-Jui Qiang-Xiang Peng1, Gao-Kuo Zhong1, Xiao-Qing Pan2, Ying-Hao Chu3, 5,*, Yi-Chun Zhou1,*

Liu4,

Affiliations: 1Key

Laboratory of Low Dimensional Materials and Application Technology, Ministry of Education, Xiangtan University, 411105. 2Department of Chemical Engineering and Materials Science, University

of California-Irvine, CA 92697.

3Department

of Materials Science and Engineering, National Chiao Tung University, 30010.

4Department

of Materials Science and Engineering, National Chung Hsing University, 40227.

5Material

and Chemical Research Laboratories, Industrial Technology ResearchInstitute, 31040.

* Corresponding author Email: [email protected](YHC)；[email protected](YCZ).

ABSTRACT: Self-assembled heteroepitaxial nanostructures have played an important role for miniaturization of electronic devices, e.g. the ultrahigh density ferroelectric memories, and cause for great concern. Our first principle calculations predict that the materials with low formation energy of the interface (Ef) tend to form matrix structure in self-assembled heteroepitaxial nanostructures, while those with high Ef form nanopillars. Under the guidance of the theoretical modeling, perovskite BiFeO3 (BFO) nanopillars are swimmingly grown into CeO2 matrix on single-crystal (001)-SrTiO3(STO) substrates by PLD, where CeO2 has a lower formation energy of the interface (Ef) than BFO. This work provides a good paradigm for controlling self-assembled nanostructures as well as the application of self-assembled ferroelectric nanoscale memory. KEYWORDS: formation energy of the interface • self-assembly • CeO2-BFO • ferroelectric array • ferroelectric properties INTRODUCTION Miniaturization and multifunctionality are the tideways for the development of nextgeneration devices. To achieve smaller size and better performance, a large wave of interdisciplinary explorations has been inspired to integrate multiple physical properties into one system. Materials of nanopillars embed in matrix of another material, so called vertically self-assembled heteroepitaxial nanostructures, often show intriguing performance1, such as magnetoelectric (ME) coupling2,3, large magnetoresistance (MR)4, and photoelectrochemical performance5,6. Recently, vertically self-assembled heteroepitaxial nanostructures were developed in ferroelectric materials7-10. An immediate advantage is an increase of storage density for the application of ferroelectric nanoscale memories11-14. For example, pillars of ferroelectric materials formed in the matrix of another material is a paradigm of memory geometry with high density12,14. The ordered nano-pillars formed during synthesis in vertically

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self-assembled heteroepitaxial nanostructures can be directly used as functional element, which can avoid the potential damage from the process photolithography. BiFeO3 (BFO) is a good candidate for the application of ferroelectric nanoscale memories due to its large spontaneous polarization. BFO related vertically self-assembled heteroepitaxial nanostructures has been intensively studied: the reported (001)-BFO in the vertically selfassembled heteroepitaxial nanostructures formed all nearly as matrix and (111)-BFO was triangular-shaped pillar15-18. Unfortunately, these wouldn’t accelerate the development of BFObased nanoscale memories due to the fact that the flat BFO hinders the high density when it forms as matrix, and triangular-shaped BFO pillars prevent the integration for nanoscale memories. Therefore, forming BFO pillars with flat surface spontaneously in nanostructures is vital for BFO-based ferroelectric nanoscale memories. Nevertheless, it seems that flat BFO pillars are scarcely reported. In terms of this issue, we considered the growth rule of vertically self-assembled heteroepitaxial nanostructures. It is obviously that the material, which is easier to grow on a given substrate, tends to form matrix on the substrate. This stimulated us to consider the role of formation energy of the interface (Ef) in the growth process of the nanostructures19,20. Ef, which is defined as the energy released in the forming of an interface between two parent materials, represents the formation possibility of a specific interface and may be the essential factor in controlling the growth of matrix and pillars21. This opinion was confirmed by the comparison between following density functional theory calculation and the already existing experiment results. Therefore, Ef is an effective parameter in the designing of self-assembled nanostructures, and we believe that flat BFO pillars can be grown by a carefully control of the Ef. In this work, first principle calculations were carried out on both (001) BFO-CFO and (111) BFO-CFO self-assembled system. The results show that the Ef of (001)-BFO/SRO interface is lower than (001)-CFO/SRO interface, on the other hand, (111)-BFO/SRO interface is higher than (111)-CFO/SRO interface, which predicts (001)-BFO forms as matrix and (111)-BFO is to be pillars. The results are in good agreement with the experimental results reported in the literature18 and validates the importance of the Ef for self-assembled growth of thin films. Where after, we used the same calculations to predict that the (001)-BFO nanopillars can be realized by controlling formation energy of the interface (Ef) in the process of growth. Experimentally, we verified (001) perovskite BFO nanopillars can grow into the low interface formation energy CeO2 matrix on single-crystal (001)-STO substrates, besides, the (001)-BFO pillars exhibit fairish ferroelectric properties. The material types in this paper are abbreviated as shown in Part SI in Supporting Information. CALCULATED AND EXPERIMENTAL SECTION Theory calculation and prediction. We did the theory calculation and prediction under the premise of that two phases are chemically immiscible during growth. In order to clarify the growth mechanism of the self-assembled thin films, the typical above interface


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structures were studied by the density functional theory (DFT) calculation with projectoraugmented-wave (PAW) method (VASP code)22. It was reported that SRO would be selfterminated with SrO surface for volatile properties of RuO2-layer23-26. Additionally, it was also confirmed in this work by the calculation as shown in Figure S1 and the following experiment, so we assumed that SRO bottom electrode is with the SrO-terminated in the calculations. The (111)-BFO/SRO, (111)-CFO/SRO, (001)-CFO/SRO and (001)-BFO/SRO interface systems were constructed and their most stable interface structures were found out by calculation in order to verify the effectiveness of interface energy in explaining the growth rule of reported nanostructures. And then the (001)-CeO2/SRO interface system was examined in order to compare its interface energy with that of (001)-BFO/SRO. In our calculation, the formation energy of the interface was defined as: Ef = [E (film/substrate)-E (film)-E (substrate)]/S for each interface system27-29. Where, E (film/substrate) is the total energy of the most stable and fully relaxed film/substrate interface system based on the DFT calculation, E (film) is the total energy of the free standing and strain free film, E (substrate) is the total energy of substrate and S is the area of the interface in this supercell. Thus, Ef actually contains the strain energy, which also plays an important role in determining the film structure. For more details about the calculation method, please see the Part II in Supporting Information. Thin Film Fabrication. In order to verify the proposed working principle for the selfassembled nanostructures above, we grew BFO pillars embedded into the CeO2 matrix on the single crystalline (001)-STO substrate with SRO buffer layer by using dual-target pulsed laser deposition (PLD) (Figure 1a and b). 20nm SrRuO3 thin film was grown at 700ºC in O2 (80 mTorr) on a (001)-oriented SrTiO3 as the bottom electrode. A dual-target system of bulk CeO2 and bulk BiFeO3 discs mounted on a computer-controlled exchanging stage was used to fabricate epitaxial self-assembled CeO2-BFO nanostructures using the pulsed laser (KrF excimer). Samples were grown at 680°C and in a dynamic oxygen pressure of 150 mTorr. After the films growth，the samples were cooled at 300 Torr oxygen pressure in order to reduce oxygen vacancies. Characterization. A combination of X-ray diffraction (XRD, Bruker D2, λ = 1.5406 Å), scanning transmission electron microscopy (STEM, JEOL ARM-300CF). Local ferroelectricity of CeO2-BFO heterostructure was performed using piezoresponse force microscopy (PFM, Asylum Research MFP-3D-Infinity) under dual AC resonance tracking (DART) mode.



Figure 1. BFO-CeO2 nanostructures design. (a) Illustration of the dual-target system PLD setup. (b) A typical depiction of the vertical heteroepitaxial nanocomposites, in which the BFO nanopillars are embedded into CeO2 matrix.

RESULTS AND DISCUSSION Theoretical calculation results. Based on DFT calculation, the correlation between the growth rule and the interface energy (Ef) for the reported nanostructures of BFO and CFO on the (111)- and (001)-orientated SRO/STO substrate were studied. The atomic structures and Ef for these film/substrate interface systems are shown in Figure 2. For the BFO and CFO nanostructures on (111)-orientated SRO substrate, the Ef of (111)-BFO/SRO and (111)CFO/SRO are calculated to be -1.59 and -4.38 J/m2, respectively, as plotted in Figure 2a. Therefore, CFO is more likely to grow on (111)-SRO/STO substrate because of the lower Ef. This indicates that CFO would forms as matrix and BFO would be pillars. Using the same method, we got the Ef of CFO/SRO and BFO/SRO interfaces on a (001)-STO substrate as shown in Figure 2b, which are -1.02 J/m2 and -1.812 J/m2, respectively. From this result, CFO would be pillars and BFO forms as matrix. Thus, the interface energy, which indicates the interface compatibility between the film material and the substrate, may be a reasonable explanation for the growth rule for the self-assembled nanostructures. From this point of view, in order to fabricate BFO nanopillars embedded in another insulating matrix with flat surface, one material having lower Ef on (001)-SRO compared with BFO should be selected. Here, CeO2 was selected as a matrix material due to its good insulating properties and lower Ef of CeO2/SRO interface from DFT calculation. The atomic structure of the most possible interface configuration was obtained as shown in Figures 2c. The most stable (001)-CeO2/SRO interface from the calculation is exactly proved by the TEM measurement in the following experiments. From calculation, the formation energy (Ef) of CeO2/SRO interface was calculated to be -3.33 J/m2, which is lower than that of BFO/SRO interface (-1.812 J/m2). Therefore, it shows CeO2 is much easier to be combined with (001)-SRO than the BFO. Based on the calculation results, (001)-BFO can naturally form as pillars embedded into the (001)-CeO2 matrix in the selfassembled epitaxial growth. More detailed results are given in Part II of Supporting Information.


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Figure 2. The atomic configurations and formation energies (Ef) for the 5 types of interface structures. (a) Bi-SrO and FeCo-SrO of BFO-CFO self-assembled nanostructure on a (111)-STO substrate. (b) CoO-SrO and FeO-SrO of BFO-CFO self-assembled nanostructure on a (001)-STO substrate. (c) FeO-SrO and CeO-SrO of BFO-CeO2 self-assembled nanostructure on a (001)-STO substrate. The black line with five-pointed star represents Ef of the five types of interface structures.

Figure 3. X-Ray structural information. (a) Typical θ-2θ scan of the self-assembled nanostructure. (b) Φ scans at BFO (111), STO (111), and CeO2 (111) diffraction peaks. a.u., arbitrary units. (c) The reciprocal space mapping of the nanostructure. r.l.u., reciprocal lattice unit. (d) Illustration of the relationship among STO, CeO2 and BFO.



Structure characterization and analysis. The microstructure and epitaxy of the CeO2-BFO self-assembled film were examined by X-ray diffraction (XRD). The θ-2θ scan (Figure 3a) shows no other peak can be seen except CeO2 (00Ɩ), BFO (00Ɩ), and STO (00Ɩ), indicating both CeO2 and BFO were epitaxially grown on the SRO / (001)-STO substrate. From the results of θ-2θ scan, the c-axis lattice constants of CeO2 phase and BFO phase in this nanostructured thin film are calculated to be 5.442 Å and 3.944 Å, respectively. In order to further examine the inplane crystallography for the nanostructure, reciprocal space mapping (RSM, Figure 3b) and Φ-scan (Figure 3c) were conducted. RSM was performed around STO (012) diffraction peak, showing that pure CeO2 (113), BFO (012) and STO (012) diffraction peaks are strictly arranged along in-plane STO direction (CeO2(113) // BFO (012) // STO (012)). Based on the RSM, we also learned in-plane orientation relationship between STO substrate and the two phases as (113) [ 110] CeO2 // (012) [100] BFO // (012) [100] STO by means of reciprocal spaces of BFO and CeO2 in Figure S3. Again, we can see that both CeO2 phase and BFO phase were grown on (001)-STO with highly epitaxial quality. The in-plane lattice constants of CeO2 phase and BFO phase are a=b=5.398Å and a=b=3.952Å, respectively. Accordingly, the c/a ratios of 1.008 for the CeO2 and 0.998 for BFO can be determined in this nanostructured thin film, and BFO is compressive strained, while CeO2 is tensile strained along the c- direction. According to the Φscan, 4-fold symmetry is observed in (111)-STO, (111)-CeO2 as well as (111)-BFO. Moreover, the angles of these four peaks for STO and BFO coincide completely, suggesting BFO phase epitaxially grows on STO substrate with cube-on-cube structure. However, the four peaks of CeO2 phase are offset by accurate 45° compared with the ones of STO substrate, which proves that the in-plane of CeO2 phase rotates 45° relative to STO substrate. From above XRD results, the orientation relationships of BFO、STO and CeO2 /STO are schematically shown in Figure 3d. Figures 4a, 4b and 4c show cross-section scanning transmission electron microscopy (STEM) images of this nanostructured CeO2-BFO thin film. The low magnification image (Figure 4a) reveals that BFO and CeO2 are vertically aligned structure. The high-resolution image (Figure 2b) shows that BFO and CeO2 are phase separated, which indicates that the CeO2-BFO nanostructure is self-assembled and BFO pillars are embedded in CeO2 matrix as what we expected. We also carried out high-resolution high angle annular dark field (HAADF) image at the SRO/CeO2 interface, as shown in Figure 4c. We can see that the SRO/CeO2 interface is atomically sharp, showing the white dot is Ce and green one is Sr, which is in accordance with the calculation (CeO-SrO) as shown in Figure 1c and further suggesting the interface structure of CeO2/SRO is correct. The atomic force microcopy (AFM) image displays the surface morphology of CeO2-BFO nanostructured thin film in Figure 4d, demonstrating the rectangular-shaped BFO pillars in continuous CeO2 matrix and the Rms of the BFO pillars is about 0.8 nm. Based on the analysis of the XRD, TEM and AFM results, the CeO2-BFO assembled heterostructure can be schematically shown in Figure 4e, which are agreement with the proposed model. Therefore, we believe that the interface formation energy (Ef) mainly


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control the structure of self-assembled thin films. In addition, we can control the lateral size of BFO pillars by adjusting growth oxygen pressure and ratio of pulse numbers for CeO2 and BFO, which is described in Figure S4.

Figure 4. TEM and AFM structural information. (a) Cross–sectional TEM image of BFO-CeO2 heterostructures on a STO substrate. (b) High-resolution TEM image showing the interface between BFO and CeO2. (c) High-resolution TEM image showing the interfaces between SRO and CeO2. (d) Top view - AFM topography image. (e) Schematic self-assembled BFO-CeO2 vertical heterostructures in which perovskite-BFO pillars are embedded in the CeO2 matrix.

Ferroelectric properties by PFM. We also characterized the ferroelectricity of the CeO2BFO heterostructured thin film via piezoresponse force microscopy (PFM), which is an ideal tool for both probing and switching the local ferroelectric polarization at nanoscale30. Using PFM, the -9V bias was written on an entire BFO pillar via a conducting tip on this thin film. Figure 5a and 5b shows the as - grow BFO phase and the switched out-of-plane (OP) polarization signal. The BFO pillar nearly all switched from the dark and bright contrast, suggesting the domains of BFO pillars are controllable and the polarization of as-grow BFO is downward. We also researched ferroelectric properties of BFO pillars with different lateral sizes but with same height using a conductive tip of PFM, which are marked in Figure 5c. In order to analyze the diversification in ferroelectric properties of the BFO pillars with different lateral sizes, Figure 5d shows the variation in right coercive voltage (Vc,R), left coercive voltage (Vc,L)and Vmid (Vmid = (Vc,R + Vc,L)/2) of local phase hysteresis with lateral size of BFO pillars, which are depicted by the schematic in the illustration, respectively. It demonstrates the ferroelectricity doesn’t disappear even though the lateral size of BFO pillar is less than 50nm. It is normal the Vc,R and Vc,L become a little larger when BFO pillar less than 50nm because it causes large strain when the pillar is very small which leads to BFO pillars harder to be switched. Figure 5e shows the representative local PFM amplitude and phase hysteresis loop of the BFO pillar with lateral size of 40 nm. The square loop demonstrating a 180° change in PFM phase and a clear butterfly loop in PFM amplitude demonstrates good ferroelectric switching nature



of the 40 nm BFO pillar. The local coercive voltages are ~ 2.3V and -3.2 V, respectively, which are indicated by the minima of the amplitude loop, suggesting an asymmetric loop with a weak ferroelectric imprint. Based on above results, the ferroelectric properties of the BFO pillars are outstanding though the pillars are very small.

Figure 5. Piezoresponse force microscopy. (a) Out-of-plane phase of the BFO pillars. (b) Out-ofplane phase under -9V. (c) Top view - AFM topography on different top surface area of the BFO pillars marked by 1-9. (d) The local coercive voltage variation as a function of top surface area of the BFO pillars in corresponding with (c). (e) Representative local PFM amplitude and phase hysteresis loops of BFO pillar.

CONCLUSIONS We studied the effect of interface compatibility on the self-assembled growth of vertical heteroepitaxial nanostructures. The results show that Ef is the dominant factor in the control of the self-assembled heteroepitaxial nanostructures. The material possessing lower Ef with substrates is much easier to form as matrix in self-assembled nanostructures and the higher ones forms as pillars. Following the prediction of calculation results, a nanostructure of CeO2-BFO self-assembled thin films, composing of higher Ef BFO nanopillars and lower Ef CeO2 matrix, was designed. Moreover, the CeO2-BFO epitaxial nanostructures on (001)-STO substrate were fabricated by controlling the Ef by PLD. The ferroelectric and piezoelectric properties of CeO2BFOnanostructure suggest that this self-assembled ferroelectric is of great potential application in information storage nanotechnology and microelectronic mechanical system. This work supplied a feasible strategy for the designing of self-assembled vertical heteroepitaxial nanostructures and promoted the study of emergent materials for high density memory application. ASSOCIATED CONTENT Supporting Information


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The interface structures and Ef of SrO-terminal and RuO2-terminal SRO, data about the lattices of the materials and the calculated results, relaxed interface structures and Ef of different interfaces, the reciprocal spaces of BFO and CeO2, the top view - AFM topography image with different ratio of pulse number for CeO2 : BFO and growth oxygen pressure. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Y.-C.Z.). *E-mail: [email protected] (Y.-H.C.). ORCID Yi-Chun Zhou: 0000-0002-0924-3792 Ying-Hao Chu: 0000-0002-3435-9084 Jie Jiang: 0000-0002-1540-4281 Qiong Yang: 0000-0002-3235-1986 Qiang-Xiang Peng: 0000-0003-2959-6648 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Grant Nos. 11402221 and 11502224), the fund of the State Key Laboratory of Intense Pulsed Radiation Simulation and Effect (SKLIPR1513) and the Hunan Provincial Key Research and Development Plan (No. 2016WK2014). REFERENCES (1) Liu, H. J.; Liang, W. I.; Chu, Y. H.; Zheng, H.; Ramesh, R. Self-assembled Vertical Heteroepitaxial Nanostructures: From Growth to Functionalities. MRS Commun. 2014, 4, 31-44. (2) Zheng, H.; Wang, J.; Lofland, S.; Ma, Z.; Mohaddes-Ardabili, L.; Zhao, T.; SalamancaRiba, L.; Shinde, S.; Ogale, S.; Bai, F. Multiferroic BaTiO3-CoFe2O4 Nanostructures. Science 2004, 303, 661-663. (3) Chu, Z.; Shi, H.; Shi, W.; Liu, G.; Wu, J.; Yang, J.; Dong, S. Enhanced Resonance Magnetoelectric Coupling in (1-1) Connectivity Composites. Adv. Mater. 2017, 29, 1606022 (1-9). (4) Liu, H. J.; Tra, V. T.; Chen, Y. J.; Huang, R.; Duan, C. G.; Hsieh, Y. H.; Lin, H. J.; Lin, J. Y.; Chen, C. T.; Ikuhara, Y. Large Magnetoresistance in Magnetically Coupled SrRuO3-CoFe2O4 Self-Assembled Nanostructures. Adv. Mater. 2013, 25, 4753-4759. (5) Quynh, L. T.; Van, C. N.; Bitla, Y.; Chen, J. W.; Do, T. H.; Tzeng, W. Y.; Liao, S. C.; Tsai, K. A.; Chen, Y. C.; Wu, C. L. Self‐Assembled BiFeO3-ε-Fe2O3 Vertical Heteroepitaxy for Visible Light Photoelectrochemistry. Adv. Energy Mater. 2016, 6, 1600686 (1-8).



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(23) Han, J. H.; Lee, W.; Jeon, W.; Lee, S. W.; Hwang, C. S.; Ko, C.; Gatineau, J. Growth of Conductive SrRuO3 Films by Combining Atomic Layer Deposited SrO and Chemical Vapor Deposited RuO2 Layers. Chem. Mater. 2012, 24, 4686-4692. (24) Rijnders, G.; Blank, D. H.; Choi, J.; Eom, C.-B. Enhanced Surface Diffusion Through Termination Conversion During Epitaxial SrRuO3 Growth. Appl. Phys. Lett. 2004, 84, 505-507. (25) Koster, G.; Klein, L.; Siemons, W.; Rijnders, G.; Dodge, J. S.; Eom, C.-B.; Blank, D. H.; Beasley, M. R. Structure, Physical Properties, and Applications of SrRuO3 Thin Films. Rev. Mod. Phys. 2012, 84, 253-298. (26) Yu, P.; Luo, W.; Yi, D.; Zhang, J.; Rossell, M.; Yang, C H.; You, L.; Singh-Bhalla, G.; Yang, S.; He, Q. Interface Control of Bulk Ferroelectric Polarization. Proc. Natl. Acad. Sci. USA 2012, 109, 9710-9715. (27) Velev, J. P.; Duan, C. G.; Burton, J.; Smogunov, A.; Niranjan, M. K.; Tosatti, E.; Jaswal, S.; Tsymbal, E. Y. Magnetic Tunnel Junctions with Ferroelectric Barriers: Prediction of Four Resistance States from First Principles. Nano Lett. 2008, 9, 427-432. (28) Ramesh, R.; Spaldin, N. A. Multiferroics: Progress and Prospects in Thin Films. Nat. Mater. 2007, 6, 21-29. (29) Aguado-Puente, P.; Junquera, J. Ferromagneticlike Closure Domains in Ferroelectric Ultrathin Films: First-principles Simulations. Phys. Rev. Lett. 2008, 100, 177601 (1-4). (30) Strelcov, E.; Kim, Y.; Yang, J. C.; Chu, Y. H.; Yu, P.; Lu, X.; Jesse, S.; Kalinin, S. V. Role of Measurement Voltage on Hysteresis Loop Shape in Piezoresponse Force Microscopy. Appl. Phys. Lett. 2012, 101, 192902 (1-4).

AUTHOR CONTRIBUTIONS J. J., Q. Y., Y.-H. C. and Y.-C. Z. conceived and designed the experiments. J. J., X.-Y. L. and Q. Y. performed the experiments and calculations. J. J., Y. Z., P.-W. S., Y.-H. X., H.-J. L., G.-K. Z. and X.-Q. P. contributed measurements and analysis. J. J., Y. Z., Q. Y., Y.-H. C. and Q.-X. P. co-wrote the paper.



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Self-assembled Ferroelectric Nano-array - ACS Applied Materials

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