Patterning Oxide Nanopillars at the Atomic Scale by Phase

Letter pubs.acs.org/NanoLett

Patterning Oxide Nanopillars at the Atomic Scale by Phase Transformation Chunlin Chen,† Zhongchang Wang,*,† Frank Lichtenberg,‡ Yuichi Ikuhara,*,†,§,∥ and Johannes Georg Bednorz⊥ †

Advanced Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan Department of Materials, ETH Zürich, Zürich CH-8093, Switzerland § Institute of Engineering Innovation, The University of Tokyo, 2-11-16 Yayoi, Bunkyo-ku, Tokyo 113-8656, Japan ∥ Nanostructures Research Laboratory, Japan Fine Ceramics Center, 2-4-1 Mutsuno, Atsuta, Nagoya 456-8587, Japan ⊥ IBM Research Division, Zürich Research Laboratory, Rüschlikon CH-8803, Switzerland

Nano Lett. Downloaded from pubs.acs.org by GEORGETOWN UNIV on 09/01/15. For personal use only.

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S Supporting Information *

ABSTRACT: Phase transformations in crystalline materials are common in nature and often modify dramatically properties of materials. The ability to precisely control them with a high spatial precision represents a significant step forward in realizing new functionalities in confined dimensions. However, such control is extremely challenging particularly at the atomic scale due to the intricacies in governing thermodynamic conditions with a high spatial accuracy. Here, we apply focused electron beam of a scanning transmission electron microscope to irradiate SrNbO3.4 crystals and demonstrate a precise control of a phase transformation from layered SrNbO3.4 to perovskite SrNbO3 at the atomic scale. By purposely squeezing O atoms out of the vertex-sharing NbO6 octahedral slabs, their neighboring slabs zip together, resulting in a patterning of SrNbO3 nanopillars in SrNbO3.4 matrix. Such phase transformations can be spatially manipulated with an atomic precision, opening up a novel avenue for materials design and processing and also for advanced nanodevice fabrication. KEYWORDS: SrNbOx, nanopillars, phase transformation, transmission electron microscopy

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octahedral slabs for SrnNbnO3n+2: one is four (n = 4) and the other is five (n = 5) NbO6 octahedra thick along the c axis. The fully oxidized n = 4 compound SrNbO3.5 (Sr4Nb4O14) has a valency of Nb5+ and takes on ferroelectric insulating nature with a very high ferroelectric transition temperature (Tc = 1615 K).11,12 However, those oxides with a lower oxygen content, i.e., SrNbO3.5‑y (y > 0), show a mixed valency of Nb5+/Nb4+ and are electrically conducting.8,13−20 One representative example is the n = 5 compound SrNbO3.4 (Sr5Nb5O17), which is a diamagnetic quasi-1D metal.8,13−20 In light of these distinctions, we modify precisely the content and distribution of O in SrNbO3.4 by a focused electron beam in a STEM and demonstrate that phase transformations can be spatially controlled with atomic precision, giving rise to patterning of SrNbO3 nanopillars in SrNbO3.4 at the atomic scale. SrNbO3 shows paramagnetic properties at room temperature.21 Experimental and Image Simulation Details. Sample Preparation and Microscopic Observation. SrNbO3.4 was grown using the floating zone melting method under Ar atmosphere. The oxygen content of the single crystals was determined by the thermogravimetric analysis.8,14 Thin-foil specimens for TEM and STEM imaging were prepared by

hase transformations in crystalline materials are of primary fundamental interest and practical significance for a wide range of fields, including materials science,1−3 information storage,4,5 and geological science.6 To date, it remains highly desirable to precisely tailor the phase transformations in a material due to their potential impacts on macroscopic properties and thus many advanced applications. Despite decades of efforts, precisely controlling phase transformations at the atomic scale still poses a significant challenge owing to the intricacies in governing thermodynamic conditions with atomic precision. Recent technical advances in aberrationcorrected scanning transmission electron microscope (STEM) offer a fertile new ground for probing samples by a focused subAngström electron beam, opening an avenue for precisely triggering phase transformations. Strontium niobates SrNbOx represent such a species of materials: a slight modification of their oxygen content may induce a phase transformation, resulting in a remarkable shift in functionalities. Structurally, they belong to a homologous series AnBnO3n+2 (ABOx), which can be derived by intercalating additional oxygen atoms in between the NbO6 octahedra in the three-dimensional (3D) network of perovskite SrNbO3 along {110} planes.7−10 The main building blocks of orthorhombic SrnNbnO3n+2 (SrNbOx) are NbO6 octahedral slabs stacked along c axis, and their thickness can be tuned by the oxygen content x. There are so far two different species of NbO6 © XXXX American Chemical Society

Received: May 11, 2015 Revised: August 15, 2015

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DOI: 10.1021/acs.nanolett.5b01847 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 1. Atomic and electronic structure of SrNbO3.4. (a) HAADF and (b) ABF STEM images taken along the [010] zone axis. A layered structure is revealed, which comprises alternately stacked chain-like (marked in blue) and zigzag-like (marked in red) slabs. All atomic columns can be identified clearly in the ABF STEM image, including O. An atomic model is superimposed on the ABF STEM image. (c) Simulated HAADF (left) and ABF (right) STEM images of SrNbO3.4 viewed along [010] zone axis. The simulated images are found to agree well with their experimental counterparts. (d) EELS spectra of the O−K edge. The upper spectrum (in blue) was taken from the chain-like slab and the lower one (in red) was taken from the zigzag-like slab.

cutting and grinding the crystal slices down to 20 μm. In the final Ar ion-beam thinning process, we applied an accelerating gun voltage of 1−4 kV and an incident beam angle of 4−5° to avoid radiation damage. The SAED pattern and TEM images were taken at 200 kV using the JEM-2010F (JEOL Co. Ltd.) microscope. HAADF and ABF images were obtained by using a 200 kV STEM (ARM200FC, JEOL) equipped with a probe corrector (CEOS Gmbh), which offers an unprecedented opportunity to probe structures with sub-Angström resolution. For the STEM imaging, a probe size of less than 1 Å and a probe convergence angle of ∼25 mrad were adopted. The high angle annular dark-field (HAADF) images were taken by a detector with a collection semiangle of 68−280 mrad. The annular bright-field (ABF) STEM images were observed by an annular-bright field detector with a collection semiangle of 12− 24 mrad. The probe current of the STEM electron beam during the irradiation was controlled by tuning probe size and condenser aperture. The in situ TEM experiments were conducted at 300 kV using the FEI TITAN80-300 electron microscope equipped with an image corrector. The applied spherical and over focus values were −50 μm and ∼20 nm, respectively. The dose rate of the electron beam was evaluated to be ∼1.3 × 1024 es−1 m−2. Image Simulations. HAADF and ABF STEM image simulations were conducted using the WinHREM package (HREM Research Inc.), which was based upon the multislice method.22 In the multislice method, samples were divided to a number of thin slices normal to incident electron beam, and the contribution to the cross section at every slice was calculated. A series of sample thicknesses were simulated by overlapping different number of subslices. For the HAADF image simulations, an acceleration voltage of 200 kV, a Cs of 0.02 mm, a defocus value of 20 Å, a probe convergence angle of 30

mrad, and a collection semiangle of 90−170 mrad were used. The collection angle for the ABF STEM image simulations spanned a range from 11 to 23 mrad. An orthorhombic SrNbO3.4 supercell with a dimension of 3.995 Å × 5.674 Å × 32.456 Å was adopted for the image simulations. The assumed slice and sample thicknesses were 1.4185 Å and 5.674 nm, respectively. For the image simulations of SrNbO3, an orthorhombic supercell with a dimension of 5.691 Å × 5.691 Å × 4.024 Å was used. The slice and sample thicknesses were set to be 1.423 Å and 5.691 nm, respectively. The pixel size for the image display was set to be 0.083 Å. Results and Discussion. Bright-field TEM images and diffraction patterns show that the as-prepared SrNbO3.4 crystals are of high crystallinity with no defects or secondary phases (Supplementary Figure 1). To reveal atomic-scale structure of SrNbO3.4 we first conduct a HAADF STEM imaging along [010] zone axis, as shown in Figure 1a. A layered structure is identified for SrNbO3.4, which comprises two distinct slabs alternately stacked along [001] direction: one has three atomic layers linked in a straight chain-like manner (marked in blue in Figure 1a) and the other is zigzag-like (marked in red in Figure 1a). Since the intensity of an atomic column in a HAADF image is directly proportional to Z1.7 (Z: atomic number),23 the contrast is brighter for heavier atoms, indicating that brighter spots in the image represent Nb atomic columns and that darker ones represent Sr atomic columns. The much lighter O atoms, however, are not scattered strongly enough to be visualized in the HAADF image, rendering it incomplete. To directly resolve all atomic columns, we also perform an ABF STEM imaging (Figure 1b), which allows a simultaneous imaging of light and heavy atoms with a good signal-to-noise ratio. 24 Apart from conveying the identical structural information as in the HAADF image, the ABF image, in B


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Figure 2. Structural transformation of SrNbO3.4 induced by electron irradiation. (a) HAADF STEM image taken before irradiation. The irradiation region is marked by a red open rectangle. (b) HAADF STEM image taken after irradiation for ∼180 s showing changes in the atomic structure in the irradiated region. The chain-like regions transform from three to four atomic layers. (c) Enlarged HAADF and (d) ABF STEM images showing the atomic structure of the newly formed phase under the irradiation. Note that the area of phase transformation can be controlled with atomic precision. A probe current of ∼400 pA was used.

while the two chain-like slabs at the border of the irradiated region transform to two atomic layers. As a result of the change in the chain-like slabs, the zigzag-like slabs in the irradiated area move outward by one atomic layer. It should be noted that those regions that are not irradiated remain intact. Due to the damage of intense electron beam irradiation, the sample thickness of the atomic layers labeled by “4” (irradiated) is thinner than that of the atomic layers labeled by “2” (without irradiation), which explains that the atomic layers labeled by “4” show a weaker intensity in the HAADF image. Upon a close inspection of the newly formed phase (Figure 2c,d), the layered SrNbO3.4 structure transforms from its original 3−3−3−3 (corresponding to SrnNbnO3n+2 with n = 5) to a 2−4−4−2 arrangement (corresponding to SrnNbnO3n+2 with n = 4 and 6). To the best of our knowledge, the n = 6 structure has never been reported in the SrnNbnO3n+2 family. However, we find interestingly that the n = 6 structure can be stable in a local area. Further comparison of the atomic structure before and after phase transformation reveals that the irradiation at a probe current of ∼400 pA triggers little change in oxygen concentration yet modifies oxygen distribution in SrNbO3.4. To squeeze O out of SrNbO3.4, we further increase the probe current of STEM electron beam to ∼700 pA and conduct a STEM imaging along the [010] zone axis. Figure 3a shows a HAADF STEM image before irradiation, in which the scanning area of electron beam is marked by a red line rectangle. Interestingly, a different phase transformation takes place in the scanning region after irradiation for ∼300 s (Figure 3b): the zigzag-like slab in between two chain-like slabs is now transformed to a chain-like slab, leading to the formation of eight consecutive straight atomic layers. Specifically, two neighboring chain-like slabs are zipped together to locally

which dark spots represent atomic columns, reveals all atomic columns including O, as verified by an atomic model of SrNbO3.4 (Figure 1b). As a final confirmation, we simulate both HAADF and ABF STEM images along [010] zone axis and compare them (Figure 1c) with their experimental counterparts (Figure 1a,b), finding good consistence between both images. To gain insights into valence states of Nb in SrNbO3.4, we conduct electron energy-loss spectroscopy (EELS) measurements of O−K edge (Figure 1d) from both the chain-like and the zigzag-like slabs (marked in blue and red in Figure 1a, respectively).25,26 The O−K edges are characterized mainly by two peaks with a different height, which are located just above the ionization edge (denoted “a” and “b” in Figure 1d). Interestingly, the “b” peak from the chain-like slab is much higher than the “a” peak, indicating that the Nb in the chain-like area has a valence state of +4. However, the two peaks from the zigzag-like slab show a similar height, implying that the Nb in the zigzag-like area has a valence state of +5. The SrNbO3.4 phase was stable during the STEM imaging, which was conducted with a probe current of ∼30 pA. However, once the probe current of the STEM electron beam was increased to ∼400 pA, a structural transformation occurs, which is induced by the electron irradiation. Figure 2a shows a typical HAADF STEM image viewed along [010] direction before electron irradiation, highlighting that each chain-like slab bears three atomic layers. Note that the free scanning mode of a STEM allows us to select a scanning area of electron beam using a rectangle box, where the size, shape, and position can be arbitrarily controlled. If the scanning area is fixed (red open box), one can find surprisingly that the irradiated region undergoes a structural transformation after electron irradiation for ∼180 s (Figure 2b): the two chain-like slabs at the center of irradiated region transform from three to four atomic layers, C


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Figure 3. Atomic zip in SrNbO3.4. (a) HAADF STEM image taken before irradiation. The irradiation area is marked by a red open rectangle. (b) HAADF STEM image taken after the electron irradiation for ∼300 s showing changes in atomic structure in the irradiated region. The zigzag-like slab in the rectangle is transformed to a chain-like connected structure, resulting in atomic merging of the two neighboring chain-like slabs. The new phase has adopted the structure of SrNbO3. (c) Enlarged HAADF and (d) ABF STEM images showing the atomic structure of the exact position where the phase transformation initiates. (e) Simulated HAADF (left) and ABF (right) STEM images of a SrNbO3 phase viewed along the [110] zone axis. The simulated images are found to agree well with their experimental counterparts. (f) EELS spectra of the O−K edge obtained in three regions marked by A, B, and C, which are defined in panel c. The phase transformation can be well controlled with atomic precision. A probe current of ∼700 pA was used.

3c). In light of the relative height of the characteristic “a” and “b” peaks, the Nb cations in the transformed area have a valence state Nb4+, which is identical to that of the Nb cations in the chain-like slab, yet differs from that of the Nb cations in the zigzag-like slab whose valence state is Nb5+. These results verify that the new phase is SrNbO3, which is formed due to the release of O atoms in SrNbO3.4. To make a final confirmation, we conduct in situ high-resolution TEM (HRTEM) imaging in TEM mode and identify the irradiation-induced local transformation from SrNbO3.4 to SrNbO3 phase (Supplementary Figure 2). The ability to precisely manipulate phase transformations in a material with high spatial precision is not only of academic interest but also holds substantial promise for applications such as in nanodevices, provided that the patterning of the transformed phase in the matrix can be performed precisely (Figure 4a). The normal direction to the SrNbO3.4 thin film in Figure 4a is along the [010] zone axis, and the orientation of the SrNbO3 nanopillars is along the [110] zone axis. To test

form SrNbO3 phase by squeezing out O atoms in the original zigzag-like slab (Figure 3c,d). To further confirm the phase transformation, we simulate both HAADF and ABF STEM images of a SrNbO3 phase along [110] zone axis (Figure 3e) and compare them with their experimental counterparts (Figure 3c,d). We find a good agreement for the both images, verifying that the formed phase is SrNbO3. The crystallographic relationships between the two oxides are [110]SrNbO3// [010]SrNbO3.4 and [001]SrNbO3//[001]SrNbO3.4, consistent with our previous observation.27 From Figure 3b,c, one can see the obvious bending of the (001) crystal plane between these two oxides, which suggests that the SrNbO3 phase is locally under tensile strain and that the SrNbO3.4 phase is locally under compressive strain. Such strain tends to annihilate the SrNbO3 phase and retard its formation. To provide further evidence to the structural transformation, we perform EELS analyses (Figure 3f) of the O−K edge from the transformed region (marked by A) and nonirradiated chainlike (marked by B) and zigzag-like (marked by C) slabs (Figure D


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Figure 4. Controllable patterning of SrNbO3 nanopillars in a SrNbO3.4 matrix. (a) Sketch of the fabrication process of SrNbO3 nanopillars (in magenta), which are embedded in the SrNbO3.4 matrix (in green). (b−d) The HAADF STEM images show the fabrication of two, three, and four SrNbO3 nanopillars in the SrNbO3.4 matrix. The size and spacing of the SrNbO3 nanopillars are less than 15 nm.

focused sub-Angström electron beam to any selectable regions. As a result of the release of O atoms, the neighboring vertexsharing NbO6 octahedral slabs in SrNbO3.4 can be zipped together at the atomic scale, giving rise to patterning of SrNbO3 nanopillars in SrNbO3.4 matrix. Such concept of a precise control of phase transformations with an atomic spatial precision should be, in principle, applicable not only to SrNbO3.4/SrNbO3 but also to other materials, finding applications in material processing and nanodevice fabrication.

this scenario, we consider SrNbO3.4/SrNbO3 as a model system, which represents an example of the systems MOy/ MOy−z, where MOy is the matrix material and MOy−z is the phase that arises from MOy by phase transformation. If, for example, MOy−z is ferromagnetic and MOy is para- or diamagnetic, such a system can be adopted to create a magnetic device for information storage. We test the fabrication of such nanostructures for SrNbO3.4/SrNbO3. Interestingly, we prepare two, three, and four SrNbO3 nanopillars within the SrNbO3.4 matrix via a phase transformation from SrNbO3.4 to SrNbO3 (Figure 4b−d). The regularly arranged SrNbO3 nanopillars have a small size and interspacing of ∼5 nm (Figure 4c,d), and the size, configuration, and position of the SrNbO3 nanopillars can be controlled spatially with atomic precision. The formation of the SrNbO3 phase is throughout the film in that there appear no Morié fringes in the recorded images. Supposing that one can seek a system in which the matrix is para- or diamagnetic, while the transformed phase is ferromagnetic, such a nanodevice may find applications as media for perpendicular magnetic recording, which is currently widely utilized for information storage owing to its huge storage density.28,29 Since the state-of-the-art magnetic storage devices are composed of magnetic islands with a size and interspacing of larger than 20 nm,30 nanodevices with magnetic areas of a much smaller dimension may show a much higher storage density and thus satisfy the ever-growing needs for data storage by minimizing surface area of individual magnetic bits. Although phase transformation is not unusual and widely used for material processing because it can cause drastic changes in material properties, the ability to precisely tailor it in a material with atomic precision should represent a remarkable step forward in realizing new functionalities at confined dimension. We have demonstrated a successful control of a phase transformation from the layered SrNbO3.4 to the perovskite SrNbO3 with atomic precision by manipulating a

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b01847. Characterizations: diffraction patterns, TEM and HRTEM images; annular bright-field imaging (PDF) Chemical identification: electron energy-loss spectroscopy (ZIP)

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AUTHOR INFORMATION

Corresponding Authors

*Tel: +81-22-217-5931. Fax: +81-22-217-5930. E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by the JSPS Grant-in-Aid for Scientific Research on Innovative Areas “Nano Informatics” (grant no. 26106503) and A (15H02290), the “Nanotechnology Platform” (project no. 12024046) at the University of Tokyo from the Ministry of Education, Culture, Sports, E


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(26) Bach, D.; Schneider, R.; Gerthsen, D. Microsc. Microanal. 2009, 15, 524−538. (27) Chen, C.; Lv, S.; Wang, Z.; Akagi, K.; Lichtenberg, F.; Ikuhara, Y.; Bednorz, J. G. Appl. Phys. Lett. 2014, 105, 221602. (28) Whitney, T. M.; Jiang, J. S.; Searson, P. C.; Chien, C. L. Science 1993, 261, 1316−1319. (29) Krusin-Elbaum, L.; Shibauchi, T.; Argyle, B.; Gignac, L.; Weller, D. Nature 2001, 410, 444−446. (30) Stipe, B. C.; Strand, T. C.; et al. Nat. Photonics 2010, 4, 484− 488.

Science and Technology (MEXT) of Japan. C.C. acknowledges support from the Grant-in-Aid for Young Scientists (B) (grant no. 26820288). Z.W. thanks the financial supports from the Scientific Research (B) (grant no. 15H04114), the Challenging Exploratory Research (grant no. 15K14117), the NSFC (grant no. 11332013), the JSPS and CAS under Japan-China Scientific Cooperation Program, and the Shorai Foundation for Science and Technology. F.L. thanks Nicola Spaldin for her support.


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Patterning Oxide Nanopillars at the Atomic Scale by Phase

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