Self-Assembled Magnetic Metallic Nanopillars in Ceramic Matrix with

Jul 20, 2016 - Ordered arrays of metallic nanopillars embedded in a ceramic matrix have recently attracted considerable interest for their multifuncti...
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Self-Assembled Magnetic Metallic Nanopillars in Ceramic Matrix with Anisotropic Magnetic and Electrical Transport Properties Qing Su,† Wenrui Zhang,† Ping Lu,∥ Shumin Fang,⊥ Fauzia Khatkhatay,‡ Jie Jian,‡ Leigang Li,† Fanglin Chen,⊥ Xinghang Zhang,§,△ Judith L. MacManus-Driscoll,# Aiping Chen,∇ Quanxi Jia,∇ and Haiyan Wang*,†,‡,○ †

Department of Material Science and Engineering, ‡Department of Electrical and Computer Engineering, and §Department of Mechanical Engineering, Texas A&M University, College Station, Texas 77843-3128, United States ∥ Sandia National Laboratories, PO Box 5800, MS 1411, Albuquerque, New Mexico 87185-1411, United States ⊥ Department of Mechanical Engineering, University of South Carolina, Columbia, South Carolina 29208, United States # Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, United Kingdom ∇ Center for Integrated Nanotechnologies (CINT), Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States ○ School of Materials Engineering, Purdue University, West Lafayette, Indiana 47907, United States S Supporting Information *

ABSTRACT: Ordered arrays of metallic nanopillars embedded in a ceramic matrix have recently attracted considerable interest for their multifunctionality in advanced devices. A number of hurdles need to be overcome for achieving practical devices, including selections of metal− ceramic combination, creation of tunable and ordered structure, and control of strain state. In this article, we demonstrate major advances to create such a fine nanoscale structure, i.e., epitaxial self-assembled vertically aligned metal−ceramic composite, in one-step growth using pulsed laser deposition. Tunable diameter and spacing of the nanopillars can be achieved by controlling the growth parameters such as deposition temperature. The magnetic metal−ceramic composite thin films demonstrate uniaxial anisotropic magnetic properties and enhanced coercivity compared to that of bulk metal. The system also presents unique anisotropic electrical transport properties under in-plane and out-of-plane directions. This work paves a new avenue to fabricate epitaxial metal−ceramic nanocomposites, which can simulate broader future explorations in nanocomposites with novel magnetic, optical, electrical, and catalytical properties. KEYWORDS: metal−ceramic composite, self-assembly, magnetic storage, thin film, interface

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to directly pattern magnetic alloy thin films in nanometer scale.8 Despite their initial success, they either encounter resolution limitations, e.g., 80 nm or larger using ArF source (193 nm),9 or require complex and tedious fabrication steps.10 In comparison, a self-assembly approach to fabricate such metal−ceramic nanostructures could be more ideal in terms of cost-effectiveness and scale-up capability. It is, however, very challenging.6,11 Very recently, vertically aligned oxide−oxide nanocomposites have been achieved in a range of oxide−oxide systems where two different oxides grew epitaxially and simultaneously using pulsed laser deposition (PLD).12 This approach can provide a new design paradigm to tune/ manipulate functionalities that cannot be obtained in individual

INTRODUCTION Heterostructures of highly ordered metal nanopillars embedded in an oxide matrix have recently attracted significant research interests owing to their great promise in various advanced technological applications, such as ultrahigh density magnetic data recording,1,2 nanostructured catalysts,3 and plasmonic metamaterials.4 For example, periodic arrays of nanometer-scale ferromagnets with high magnetization and suitable coercivity are ideal for recording media.2 Most previous efforts to grow such nanostructures have been focused on either bottom-up template methods or top-down nanofabrication techniques. For example, ferromagnetic metals including Fe, Co, Ni, and so on, have been successfully electrodeposited into the pores of anodized aluminum oxide templates.5,6 Photolithography, as a standard patterning technique widely used in semiconductor industry, is a top-down approach to fabricate such magnetic media.7 Focused ion beam (FIB) is another top-down approach © XXXX American Chemical Society

Received: May 20, 2016 Accepted: July 20, 2016

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DOI: 10.1021/acsami.6b05999 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. Structural demonstration of metal−ceramic Ni-BZY composite films. (a) 3D-viewed diagram of Ni-BZY thin films consisted of plan-view and cross-sectional transmission electron microscopy (TEM) images. (b) Typical plan-view high-resolution scanning transmission electron microscopy (STEM) image of metal−ceramic composite thin film exhibiting epitaxial nanopillars in oxide matrix. (c) Magnified plan-view STEM image of metal pillars in oxide matrix showing two preferred in-plane orientations. One is hexagon-shaped, and the other is round-shaped with inplane rotation. A typical Ni nanopillar has facets with Ni (011)//BZY (200) and Ni (110)//BZY (110). The inset is the corresponding diffraction pattern. Hexagon diffraction pattern (marked in red) is observed, which further confirms the Ni phase. The square set of diffraction pattern (marked in blue) corresponds to oxide matrix. (d) High-resolution cross-sectional STEM image of Ni-BZY interface exhibiting two-phase contrasts and perfect out-of-plane lattice matching. (e) Illustration of Ni-BZY out-of-plane lattice matching where oxide is in (00l) direction and Ni is in (111) direction. There is a slight in-plane rotation for Ni pillar. (f) Typical plan-view STEM HAADF image of metal−ceramic composite thin film exhibiting epitaxial metal nanopillars in oxide matrix. Atomic-scale EDS elemental maps of (g) Ni (Kα), (h) Ba (Lα + Lβ), (i) Y (Lα), (j) Zr (Lα + Lβ), (k) O (Kα), and (l) overall EDS color map, confirming presence of pure metal Ni within the oxide matrix. Scale bar, 5 nm.

constituents.13−16 For example, interfacial coupling of the two oxide materials in the nanocomposites gives rise to artificial multiferroics,13 enhanced curie temperature of BaTiO3,17 superior oxide−ion conductivity for composite electrolyte in solid oxide fuel cells,14,18 and many others. Despite the fact that bulk metal−ceramic composites (called cermet) have been developed previously and applied in several fields,19−21 such self-assembled metal−ceramic nanocomposites are very difficult to achieve in epitaxial thin film form, largely due to the vastly different materials chemistry and growth conditions required by the two constituents. For example, surface wetting (surface energy), oxygen diffusion, and growth kinetics are considerably different, which poses great challenges in the cogrowth of the two. In this work, we overcome the above growth challenges by the demonstration of self-assembly of metal−ceramic vertically aligned nanocomposites (VAN). For this study, Ni-BaZr0.8Y0.2O3 (BZY) nanocomposite system has been selected for the demonstration. One unique property, anisotropic magnetic properties with potential applications in magnetic data storage, is highlighted and illustrated in the schematic diagram (Figure 1a). Such ordered nanostructure is achieved by a careful selection of the metal and matrix materials and special cares in

controlling growth conditions. This approach creates nearly perfect vertical metal nanopillar arrays with an average diameter of 10 nm or less in the ceramic matrix. Magnetic and electrical transport properties in-plane and out-of-plane were measured and compared to demonstrate the novel properties in these metal/ceramic nanocomposite systems for potential applications in novel magnetic, optical, electrical and catalytical devices, and beyond. Nickel (Ni) with a face-centered cubic (fcc) structure and a lattice parameter of a = 0.352 nm has been chosen as the representative metal for this demonstration. BaZr0.8Y0.2O3 (BZY), a stable ceramic with a cubic structure, a lattice parameter of a = 0.421−0.423 nm, and good compatibility with Ni22,23 is selected as the representative ceramic matrix. The metal−ceramic composite target (Ni-BZY) is used for the growth of metal−ceramic thin films. To illustrate the architecture of the composites, a 3D diagram of Ni-BZY thin films with both plan-view and cross-sectional TEM images is shown as Figure 1a. It is clearly seen that the two phases coexist in the thin films with a fairly uniform distribution of nanopillars embedded in the matrix. The plan-view scanning transmission electron microscopy (STEM) image (Figure 1b) exhibits highly epitaxial growth of two phases where matrix phase is cubic and B

DOI: 10.1021/acsami.6b05999 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. Phase identification of metal−ceramic Ni-BZY nanocomposite films. (a) Typical X-ray diffraction pattern of the Ni-BZY composite films grown at 700 °C under vacuum. It shows distinct Ni and BZY peaks. (b) Φ-scans of Ni (110), BZY (202), and STO (101) revealing a 4-fold symmetry of both Ni and BZY phase. (c) XRD plot of Ni-BZY composite thin films deposited at different temperatures. (d) Magnified XRD pattern for Ni-BZY film deposited at different temperatures exhibiting peak shift. (e) Calculated out-of-plane strain state for both Ni and BZY phase. It demonstrates that the growth temperature results in different strength of strains for BZY and Ni phases.

clearly demonstrate the presence of separated pure Ni and BZY phases and abrupt interfaces between the two phases. X-ray diffraction (XRD) is used to examine the crystal structure and strain state of Ni-BZY nanocomposite thin films. The as-grown Ni-BZY composite films (Figure 2a) displays distinct peaks from Ni (111) and BZY (002). The results suggest that both metal Ni and ceramic BZY phases grow highly textured without noticeable interdiffusion. Ni (111) can have a good matching relationship (3:4 domain matching) with STO (001). Similar to MgO, BZY can have a cubic-on-cubic domain matching that of STO (001). In addition, the Φ-scans of BZY (202) and STO (101) (as seen in Figure 2b) reveal a typical 4-fold symmetry of BZY phase on STO, confirming a cubic-on-cubic epitaxial growth. While Φ-scans of Ni (110) shows 4-fold symmetry indicating good crystalline of Ni nanopillars. Figure 2c presents the XRD patterns of the Ni-BZY films grown at different temperatures where distinct Ni and BZY peaks start to present for these films deposited at temperatures over 500 °C. The enlarged XRD patterns (Figure 2d) in the range of 41−49° show the peak shifting as a function of deposition temperature. The calculated out-of-plane lattice parameter of Ni and BZY deposited at different temperatures is summarized in Figure 2e. The out-of-plane lattice parameter for BZY phase is larger compared with that of bulk phase, indicating tensile strain out-of-plane, whereas the Ni strain state is compressive in the out-of-plane direction. The results clearly demonstrate the strong strain coupling between the metal and ceramic phases and possibility of strain tuning along the vertical interfaces. Further strain control is possible by selecting different ceramic phases, growth directions, or energy density of the laser beam. Similar to the growth of self-assembled heteroepitaxial oxide nanocomposite systems,24 the growth of metal−ceramic

the metallic pillars are uniformly distributed with obvious faceted structure. The epitaxial metal−ceramic composite thin film is further proved by the corresponding selected area electron diffraction (SAED) pattern shown as inset in Figure 1c. It displays the distinguished hexagonal pattern of Ni (resulted from (111) Ni) and the cubic pattern of BZY. There are two dominating types of Ni nanopillars with an average pillar width of ∼4 nm, and the detailed in-plane matching between Ni nanopillars and BZY matrix is seen in Figure 1c. A typical Ni nanopillar has facets with the BZY matrix with the lattice-matching relationships of Ni (011)//BZY (200) and Ni (110)//BZY (110). To confirm that both Ni and BZY grow as composite phases without intermixing, high-resolution STEM coupled with energy-dispersive X-ray spectra (EDS) mapping was conducted. Clear self-assembled nanopillars in a matrix have been observed by high-resolution cross-sectional STEM images (along the [100] zone axis) of the film as seen in Figure 1d. It is very interesting to note that the pillars are straight from substrate all the way to the top film surface, indicating the growth of highly oriented pillars. Figure 1e shows an enlarged area from Figure 1d. A close matching relationship between Ni and BZY out-ofplane, shown as illustration, has been observed where two of Ni (111) lattices match with one BZY (200) lattice. The slightly blurred STEM image at the Ni nanopillar side is due to the minor in-plane rotation for the nanopillar, which is also seen in plan-view TEM images (Figure 1c). EDS mapping was conducted to determine the chemical composition of the nanopillars, and the matrix from a typical area is shown in Figure 1f. EDS elemental maps for Ni, Ba, Zr, Y, and O are shown in Figure 1g−k, respectively. A colored composite map from all the elements is also shown in Figure 1l. The findings C

DOI: 10.1021/acsami.6b05999 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 3. Summary of structure−magnetic properties correlation of metal−ceramic Ni-BZY composite films. (a) Average size of metal nanopillar as a function of deposition temperature. The insets are typical plan-view TEM images for metal−ceramic thin films deposited at different temperatures. Scale bar, 20 nm. It clearly shows that the average metal nanopillars size increases as deposition temperature increases. (b) In-plane (field applied perpendicular to the pillars axis) room-temperature magnetization data of the films grown in vacuum at different temperatures. The inset is magnified M−H plot from −250 to 250 Oe. Obvious magnetic hysteresis with a coercivity of was observed demonstrating room temperature ferromagnetism. (c) Typical out-of-plane and in-plane magnetization loop for Ni-BZY film with 700 nm deposited at 700 °C. (d) Coercivity of Ni-BZY thin film as a function of aspect ratio, which reveals little change for coercivity field by increasing aspect ratio. The inset shows similar value of coercivity for NiBZY film when the metal nanopillar size is below 10 nm and film thickness is around 80 nm. (e) In-plane magnetization loop for Ni-BZY film with different thickness. (f) High-resolution plan-view STEM image of Ni-BZY film, which clearly presents Ni and Ba domain matching. Typical 12 Ni/7 Ba and 9 Ni/5 Ba matchings are highlighted. These findings strongly support strain coupling between metal Ni and ceramic BZY phase.

nanocomposite films mainly involves two stages: cluster nucleation and film growth. At the very early stage of composite film growth, metal and ceramic adatoms are covaporized from composite target by PLD, and different adatoms arrive at the substrate surface in parallel. Because the films were deposited under high vacuum that can be treated as a reducing atmosphere, the oxygen species evaporated by laser prefers to bond with oxide phase to maintain its stoichiometry rather than to react with Ni. In addition, same-phase molecules accumulate and nucleate to minimize the total system energy. Because of the high surface energy of metal (∼1.9 J/m2 for Ni

(111) phase),25 Ni clusters nucleate and grow more strongly bound to each other than to the substrate and ceramic phase, resulting in a Volmer−Weber growth mode (3D island growth). The surface energy of ceramic is lower (estimated ∼1 J/m2 for BZY (002) phase), so it therefore tends to have either a Frank-van der Merwe (2D layer) or a StranskiKrastanov (2D + 3D) growth mode to form the matrix. To further examine the growth mechanism of metal−ceramic composite thin film, the Ni-BZY films were deposited at various temperatures ranging from 300 to 800 °C. Cross-sectional STEM images of the samples deposited at different temperD

DOI: 10.1021/acsami.6b05999 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

The self-assembled high-quality Ni nanopillars in this study have large anisotropy energy, owing to high film crystallinity and large nanowire aspect ratio. Figure 3c shows the out-ofplane and in-plane magnetic hysteresis loops of a 700 nm thick nanocomposite film deposited at 700 °C. The film exhibits a preferred anisotropy in the (111) plane. The effective anisotropy energy (Keff) is estimated by

atures reveal the formation of different metal−ceramic composite nanostructures (Figure S2). When the deposition temperature is below 500 °C, composite films exhibit metal nanoparticles randomly distributed in the matrix due to limited diffusion of adatoms. The structure of metal nanopillars embedded in ceramic matrix starts to appear in the films deposited at temperatures higher than 500 °C. The formation of metal−ceramic composite structure in this work results from simultaneous vaporization of both metallic and ceramic phases. It is thus different from the spontaneous phase decomposition method and the method of codepositing separate targets.26,27 The former method is hard to control the saturated metallic phases, whereas the latter method could only yield discontinuous metallic pillars. Figure 3a shows the average diameter of metallic nanopillars as a function of deposition temperature. It is interesting to note that as the growth temperature increases the average diameter of the epitaxial nanowires increases from ∼2.0 nm at 500 °C to ∼4.1 nm at 700 °C and finally to ∼5.5 nm at a deposition temperature of 800 °C seen as insets of Figure 3a. The temperature-dependent nanopillar size suggests primarily a diffusion-controlled growth mechanism. Assuming that the size of the pillars (d) is approximately equal to the average diffusion distance, the size of the pillars (ln(d)) is proportional to 1/T at a constant growth rate. It can be expressed by

ln(d) = A − B

1 T

Keff =

1 [μ Ms(HOP − HIP)] 2 0

(2)

where μ0Ms is the saturation magnetization, HOP and HIP are the applied saturation magnetic field along the out-of-plane and in-plane directions, respectively.34 Keff of 3.24 × 104 J/m3 was obtained for the as-grown Ni nanopillars, which is much larger than the ones fabricated by combinatorial deposition27 or anodized alumina template methods.34 This explains the higher transition temperature and the room-temperature ferromagnetism observed in the self-assembled epitaxial Ni nanopillars in matrix in this work. The magnetic anisotropy was further engineered by tuning the nanopillar aspect ratio. Figure 3d shows the obtained HIP as a function of the aspect ratio, which was controlled by tuning the nanopillar length (equivalent to film thickness) with a fixed nanopillar diameter of 4 nm. It is obvious that HIP increases gradually with enhanced aspect ratio and saturates when the ratio is over 100. The magnetic anisotropy is also enhanced when film thickness increases from 80 to 700 nm, as seen from Figure 3e. The room-temperature magnetic properties in ultrafine Ni nanopillars are attributed to several sources. First, the strain coupling at the vertical Ni-BZY interfaces stabilizes the magnetism of metal nanopillars. The plan-view high-resolution STEM image in Figure 3f shows two typical domain-matching relationships between metal Ni and ceramic BZY. Compressive strain is induced in Ni nanopillars through the interface coupling,35 which leads to a magnetoelastic anisotropy (Kme) for perpendicular magnetic anisotropy. Kme is given by

(1)

where A and B are constants, B = Ea/2kT. The average pillar size as the function of reciprocal temperature is fitted to an Arrhenius-type plot yielding an activation energy of 0.18 eV. The calculated activation energy corresponds to the diffusion barrier for the formation of nanopillars for this particular system.28 To demonstrate the power of these novel epitaxial vertically aligned metal−ceramic nanocomposite systems, we measured the magnetic and electrical transport properties in both in-plane and out-of-plane directions. The in-plane magnetic hysteresis loops of the nanocomposite films deposited under different temperatures are shown in Figure 3b. The magnetization results were normalized with the Ni volume density estimated from plan-view STEM images. Overall, the film saturation magnetization (Ms, 420 emu/cm3) is comparable to that of bulk nickel (420 emu/cm3).29 The unsaturated magnetization at high fields is more likely from the interface disorder between Ni nanopillars and the BaZr0.8Y0.2O3 matrix, which results in a spin glass state at broken bonds or phase boundaries.30 The general trend is that Ms increases with increased growth temperature and saturates above 700 °C, which is attributed to increased growth crystallinity. The inset of Figure 3b shows the enhanced room-temperature coercivity (Hc) of all the films compared to that in bulk Ni (∼0.7 Oe).31,32 The observation of room-temperature Hc in the array of these ultrathin Ni nanopillars is interesting. Typically, the superparamagnetic limit occurs in Ni nanopillars with a sub 10 nm diameter due to the insufficient anisotropy energy to overcome the thermal fluctuation.33 These vertically aligned Ni nanopillars take advantage of their anisotropic morphology, i.e., ∼5 nm in diameter and about a few hundreds of nanometers in length, and thus overcome the superparamagnetic limit to achieve high-density pillar arrays in lateral direction. More detailed discussions on their anisotropic magnetic properties as well as their tunability follow.

K me =

3 λsεEeff 2

(3)

where λs is saturation magnetostriction coefficient (−33 × 106), ε is the strain obtained from XRD results, and Keff is effective Young’s modulus of Ni (taken to be the bulk value of 2.2 × 1011 Pa).34 It yields a Kme of 1.09 × 104 J/m3 for Ni nanopillars deposited at 700 °C. Second, in the high-density array of ultrathin Ni nanopillars, the strong dipolar coupling between neighboring nanopillars, as well as the wire shape anisotropy, contributes to the overall magnetic properties. Combing the above contributions, the net magnetostatic energy (Km) is calculated by Km =

1 (μ Ms2)(1 − 3P) 4 0

(4)

where P is the volume fraction of Ni nanopiallrs in the composite film.36 For the Ni-BZY film deposited at 700 °C (P = 0.21), Km of 2.12 × 104 J/m3 is obtained. Third, other possible factors, including the magnetic coupling between Ni and BZO or the formation of antiferromagnetic NiO, may also introduce extra anisotropy energy. However, they are unlikely to be the major source because the XRD and EDX results confirm the high purity of both the matrix and pure Ni phases, and principal component analysis (PCA) (Figure S3a) indicates that the films are made of two pure components: Ni and BZY. E

DOI: 10.1021/acsami.6b05999 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. Anisotropy properties and universal method demonstration. (a) Out-of-plane and in-plane resistivity as a function of measured temperature. Pt is used as bottom electrode for vertical measurement. In the out-of-plane direction, the curve shows a metallic resistivity vs temperature characteristic. The resistivity along the in-plane direction is extremely high. The different resistivities along out-of-plane and in-plane directions are expected because the Ni pillars are not connected. (b) Plan-view TEM image for Ni-CeO2 film illustrating achievement of metal nanopillar in CeO2 matrix structure. It indicates that different metal and ceramic combinations can be obtained. (c) Cross-sectional TEM image for thick Ni-BZY film (>700 nm). (d) Top area of thick Ni-BZY film demonstrating a straight bottom to top pillar growth. There is no thickness limitation within the micrometer range.

pillars in the films is obvious with the overall resistivity ranging from 2.5 to 6 Ohm cm. In the in-plane direction, the resistivity for in-plane is extremely high (out of range of the instrument) due to the highly insulating BZY matrix dominating the overall transport properties in-plane. The electrical resistivity difference between in-plane and out-of-plane configurations is estimated to be more than 1000-fold. These unique anisotropic electrical transport properties could also enable novel thermoelectric materials by designing certain metal−ceramic nanocomposites. Our approach creates Ni-based metal−ceramic composite thin films with self-assembled perpendicular arrays of singlecrystal-like ferromagnetic Ni nanopillars embedded in the BZY matrix. This method enables a large selection of metal−ceramic composite films in nanometer scale, as long as a good phase compatibility can be found between the metallic and ceramic phases. Our preliminary results have shown that embedded Ni nanopillars can also be easily grown in other oxide matrix such as CeO2 (Figure 4b). In addition, the growth mechanism for metal−ceramic composite film is quite stable without any thickness limitation. It is clearly observed in Figure 4c,d that Ni nanopillars grow perfectly from bottom to top for the film exceeding 700 nm in thickness. Figure 4d shows the enlarged top area of the Ni-BZY thick film, which exhibits epitaxial growth of the two distinct phases without detectable

In addition, Curie temperature (Tc) of Ni-BZY film is ∼627 K (Figure S3b), which is the same as that of pure Ni, suggesting the formation of pure Ni nanopillars. The total energy from magnetostrictive, dipolar interaction and shape effect is consistent with the effective anisotropy measured from the experimental results. It demonstrates that the enhanced room-temperature ferromagnetic property of ultrathin Ni nanopillars results from effective strain coupling, large aspect ratio, and high crystallinity of the self-assembled Ni-BZY nanocomposite films. Because of the small diameter of Ni nanopillars measured from STEM images, a high pillar density of (10.8 ± 0.3) × 1012 in.−2 is obtained for the films deposited at 700 °C. Such a high density far exceeds the proposed storage density of 1 Tb in.−2 for high-density storage media.7,37 These results demonstrate the great potential of these high-quality and ordered metal−ceramic nanocomposites for high-density magnetic storage applications. Because of the unique architecture of metallic nanopillars embedded in the ceramic matrix, one would expect very different transport properties between out-of-plane and inplane directions. For example, Figure 4a shows significant difference of the electrical resistivity of the films both in-plane and out-of-plane measured from 25 K to room temperature. For the out-of-plane direction, the response of the metallic F

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drift detectors) operated at 200 kV was used for STEM imaging and EDS chemical mapping. EDS spectral image data was acquired in the BZY [001] zone axis with an electron probe of size 140 Gbit/in(2). IEEE Trans. Magn. 2001, 37, 1649−1651. (9) Melosh, N. A.; Boukai, A.; Diana, F.; Gerardot, B.; Badolato, A.; Petroff, P. M.; Heath, J. R. Ultrahigh-density Nanowire Lattices and Circuits. Science 2003, 300, 112−115. (10) Fert, A.; Piraux, L. Magnetic Nanowires. J. Magn. Magn. Mater. 1999, 200, 338−358. (11) Thurn-Albrecht, T.; Schotter, J.; Kastle, C. A.; Emley, N.; Shibauchi, T.; Krusin-Elbaum, L.; Guarini, K.; Black, C. T.; Tuominen, M. T.; Russell, T. P. Ultrahigh-density Nanowire Arrays Grown in Selfassembled Diblock Copolymer Templates. Science 2000, 290, 2126− 2129. (12) Chen, A. P.; Bi, Z. X.; Jia, Q. X.; MacManus-Driscoll, J. L.; Wang, H. Y. Microstructure, Vertical Strain Control and Tunable Functionalities in Self-assembled, Vertically Aligned Nanocomposite Thin Films. Acta Mater. 2013, 61, 2783−2792. (13) Zheng, H.; Wang, J.; Lofland, S. E.; Ma, Z.; Mohaddes-Ardabili, L.; Zhao, T.; Salamanca-Riba, L.; Shinde, S. R.; Ogale, S. B.; Bai, F.; Viehland, D.; Jia, Y.; Schlom, D. G.; Wuttig, M.; Roytburd, A.; Ramesh, R. Multiferroic BaTiO3-CoFe2O4 Nanostructures. Science 2004, 303, 661−663. (14) Su, Q.; Yoon, D.; Chen, A. P.; Khatkhatay, F.; Manthiram, A.; Wang, H. Y. Vertically Aligned Nanocomposite Electrolytes with Superior Out-of-plane Ionic Conductivity for Solid Oxide Fuel Cells. J. Power Sources 2013, 242, 455−463. (15) Dix, N.; Muralidharan, R.; Rebled, J. M.; Estrade, S.; Peiro, F.; Varela, M.; Fontcuberta, J.; Sanchez, F. Selectable Spontaneous Polarization Direction and Magnetic Anisotropy in BiFeO3-CoFe2O4 Epitaxial Nanostructures. ACS Nano 2010, 4, 4955−4961. (16) MaCmanus-Driscoll, J. L.; Zerrer, P.; Wang, H. Y.; Yang, H.; Yoon, J.; Fouchet, A.; Yu, R.; Blamire, M. G.; Jia, Q. X. Strain Control and Spontaneous Phase Ordering in Vertical Nanocomposite Heteroepitaxial Thin Films. Nat. Mater. 2008, 7, 314−320. (17) Harrington, S. A.; Zhai, J. Y.; Denev, S.; Gopalan, V.; Wang, H. Y.; Bi, Z. X.; Redfern, S. A. T.; Baek, S. H.; Bark, C. W.; Eom, C. B.; Jia, Q. X.; Vickers, M. E.; MacManus-Driscoll, J. L. Thick Lead-free Ferroelectric Films with High Curie Temperatures through Nanocomposite-induced Strain. Nat. Nanotechnol. 2011, 6, 491−495. (18) Su, Q.; Yoon, D.; Sisman, Z.; Khatkhatay, F.; Jia, Q. X.; Manthiram, A.; Wang, H. Y. Vertically Aligned Nanocomposite La0.8Sr0.2MnO3−δ/Zr0.92Y0.08O1.96 Thin Films as Electrode/Electrolyte Interfacial Layer for Solid Oxide Reversible Fuel Cells. Int. J. Hydrogen Energy 2013, 38, 16320−16327. H

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DOI: 10.1021/acsami.6b05999 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX