High Throughput Synthesis of Multifunctional Oxide Nanostructures

High Throughput Synthesis of Multifunctional Oxide Nanostructures within Nanoreactors Defined by Beam Pen Lithography Xing Liao,†,‡,⊥ Yi-kai Huang,†,⊥ Chad A. Mirkin,†,‡,§ and Vinayak P. Dravid*,†,‡ †

Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States International Institute for Nanotechnology, Evanston, Illinois 60208, United States § Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States ‡

S Supporting Information *

ABSTRACT: Reliably obtaining nanostructures of complex oxides over large area with nanoscale resolution and wellcontrolled shape, spacing, and pattern symmetry remains a major challenge. In this article, millions of nanowells have been routinely generated by beam pen lithography. Each attoliter volume nanowell functions as a “nanoreactor”, inside which oxide nanostructures are synthesized from their sol−gel precursors. Importantly, these nanometer scale entities are in single crystalline or textured forms and epitaxial to the underlying substrates, which promises functionalities including ferroelectricity, ferromagnetism, and multiferroicity. This method provides a general solution which allows one to rapidly screen structural parameters of oxide nanostructures comprising of three or more elements for prominent properties. KEYWORDS: beam pen lithography, sol−gel synthesis, oxide nanostructures, nanofabrication, multiferroics, ferroelectricity, ferrimagnetism

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EBL and DPN, combined with the control over microstructure afforded by the appropriate thermal conversion protocols, but all suffer from the inability to rapidly generate features over large areas and are not cost-effective. BPL, a recently introduced nanolithographic technique, overcomes the throughput barrier of scanning probe techniques by employing >104 apertures to simultaneously expose the underlying photosensitive substrates, therefore enabling one to rapidly generate millions of features within an hour, resulting in a throughput much higher than other direct-write tools like EBL and DPN. BPL also circumvents the diffraction limit of far-field optics by shining appropriate wavelength light (e.g., 405 UV employed here) through the sub-200 nm diameter apertures (100 nm in this case) to perform near-field lithography in a parallel fashion and produce features smaller than the wavelength of light. Furthermore, the heights of the nanopatterned features can be tuned by changing the thickness of the photoresist applied. In addition to the advantages in size and scalability, the registration, flexibility and reproducibility of

ultifunctional oxides continue to lead to important breakthroughs in diverse fields, spanning data storage, nonvolatile memory, and chemical sensing.1−4 However, emerging technological applications of multifunctional oxides often require complex nanopatterned architectures over large-areas, single crystal or textured forms consisting of millions of features, and controllable structural parameters with requisite consistency and reproducibility. Unfortunately, available patterning techniques based on either vapor-phase deposition5 or sol−gel synthesis6,7 fail to meet all of these required figures of merit. Herein, we report the use of beam-pen lithography (BPL),8,9 a cantilever-free, scanning probe lithographic technique,10 to generate large area periodic arrays of nanoscale wells that function as attoliter volume “nanoreactors” for sol−gel based syntheses of complex multicomponent oxides and their composite entities. Equally important, we show that these structures can be made in single crystal form with precise control over feature size, shape, and spacing, pattern symmetry, and epitaxy. Researchers have pioneered methods that combine top-down patterning techniques, such as e-beam lithography (EBL) and dip-pen nanolithography (DPN),11−15 with sol−gel chemistry in order to explore a universal way to synthesize complex oxide nanostructures.6,7,16−18 These methods enjoy the precision of © 2017 American Chemical Society

Received: January 6, 2017 Accepted: March 13, 2017 Published: March 13, 2017 4439

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Figure 1. Schematic of the synthesis of oxide nanostructures by beam pen lithography. (a) Nanoreactors generated by BPL in parallel with control over the size, spacing, and symmetry. (b−d) Sol−gel solution evolves within the nanoreactors with different surface contact angle. (e− g) Oxide nanostructures synthesized with different surface treatments corresponding to (b−d), respectively.

barium titanate (BaTiO3BTO), cobalt ferrite (CoFe2O4 CFO), bismuth ferrite (BiFeO3BFO), and their composite analogs. The nanoscale reactors are used to nucleate and grow crystals within them such that the size and geometry of the crystals are defined by the dimensions of the wells (Figure 1a) and the appropriate epitaxial substrates, such as strontium titanate (SrTiO3STO), magnesium oxide (MgO), and Si/ SiOx. The nanoreactors are fabricated with 100 to 500 nm diameters and depths between 100 and 400 nm, which correspond to reactor volumes ranging from ∼0.8 to 80 attoliters. After the wells are fabricated, the substrate and well surfaces are rendered hydrophobic by thermally depositing octadecyltrichlorosilane (OTS) or poly(tetrafluoroethylene oxide-co-difluoromethylene oxide) α,ω-diisocyanate (PFPE, average Mn ∼ 3000) onto them. OTS deposition at 100 °C for 20 min is generally sufficient for most material systems (except for those with larger lattice mismatch). PFPE was employed just for those cases (CFO on STO, in this contribution), which require prolonged deposition time (>8 h) and higher temperature (140 °C). Indeed, the surface contact angle increases from ∼30° to >70−80° upon surface modification with the hydrophobic treatments (Figure S2). Following surface treatment, sol−gel solutions were spin-coated onto the substrates to fill-in the attoliter nanoreactors (Figure 1b−d) with the precursor formulations summarized in Table S1. Unlike the formation of “coffee stain” like features (Figure 1b and e), as seen in previous reports,22,23 the sol−gel droplets on our patterned substrates tend to form a hemispherical morphology that gradually shrinks inward and detaches from the photoresist walls during baking at 120 °C. This shrinkage behavior is a consequence of both the high contact angle and the Marangoni effect caused by an increase in temperature and the mixture of different solvents in solution (water and acetic acid).24,25 The shrinking droplets finally yield well-formed, dense dots with dimensions approximately 1/27 to 1/125 (1/33

BPL are ensured by the piezo-actuated stages in X, Y, and Z directions with nanometer scale precision.

RESULTS AND DISCUSSIONS Synthesis of Oxide Nanostructures within Nanoreactors. When combined with sol−gel synthesis of oxide materials, BPL allows patterning of oxide nanostructures over cm2 areas, large enough for bulk characterization methods, such as X-ray diffraction (XRD) and superconducting quantum interference device (SQUID), to probe and quantitatively investigate the collective effects of nanopatterned architectures with adequate signal-to-noise ratio. Furthermore, compared to template-directed synthesis methods, BPL provides flexibility over the macro-, micro-, and nanometer length scales by stitching patterns generated by thousands of pens together to form an arbitrary pattern that covers centimeter scale and potentially even larger areas.9 These attributes also make BPL an ideal platform to investigate how structural parameters affect the phase conversion and subsequent properties/performance of oxide nanopatterns in a variety of technical areas. One of the key challenges in sol−gel and other precursorbased synthesis methods is to control the “internal microstructure” during the solid-state conversion of sol−gel patterns.11 During the conversion, the mass loss can be more than 60%, which often results in poor integrity, uncontrolled cracking or porosity of the resulting pattern rather than its development into a single crystal or textured form.19−21 We show that control over internal microstructure in BPL-derived nanopatterns can be achieved through a combination of surface treatment of nanoreactor wells, followed by appropriate thermal conversion protocols to convert the gelated structures to their single crystal and textured forms. Toward meeting these complex and diverse requirements, we define millions of nanowells by BPL with deliberate control over size and spacing of the wells in addition to the pattern symmetry. We demonstrate the synergy of BPL and the sol−gel approach with prototypical multifunctional complex oxides: 4440

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Figure 2. Oxide nanostructures with controlled structural parameters. (a, b) Photograph and SEM image showing large-area patterning (1 cm2). (c) XRD spectrum of the BFO line array. (d−g) Dot patterns with different sizes, symmetries, and separations, respectively.

Figure 3. Single crystalline oxide nanodots epitaxially synthesized on substrates. (a−e) CFO on MgO, CFO on STO, BFO on STO, BTO on STO, and BTO−CFO on STO, respectively. Arrows represent crystal directions of substrate.

formation of stable droplets on the flat surface; while on the photoresist wall, where the contact angle is a little smaller, droplets tend to lose solvent and shrink toward the wall upon baking, resulting in near-perfect rings. The as-synthesized oxide nanorings have an outer diameter that is close to the diameter of the BPL-defined nanowells, and the diameter of the rings can be deliberately controlled to as small as 30 nm (Figure 1g). Structural Characterization. Several oxide nanostructure systems were synthesized and patterned to examine the versatility and feasibility of this technique for other materials and geometries. A 1 × 1 cm2 BFO line array was synthesized and characterized by XRD (Figure 2a−c). The large area

to 1/53) the volume of the nanoreactors. This behavior is different from the conventional drying approaches that yield “coffee stain” patterns with features size comparable to the initial droplet size. Even larger shrinkage ratios can be achieved using solsolutions with lower concentrations of sol−gel precursors. Using this approach, the smallest size solid-state pattern obtained was ∼50 nm (Figure S4b). Surprisingly, perfect rings are obtained with no material left in the center (Figure 1d and g) by changing the water-to-acid ratio of the sol−gel solution to further increase the contact angle on SiO2 (>90°). The extremely high contact angle on SiO2 prevents the 4441

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Figure 4. Structural characterizations of synthesized oxide nanodots. (a, b) Low-mag cross-sectional TEM images of a BTO dot on STO and a CFO dot on STO, respectively. (c) HRTEM along interface between CFO and STO, inset shows the FFT images of the two materials. (d) Filtered HRTEM image shows the CFO(400) and STO(200) planes, misfit dislocations are marked along the interface. (e) Low-mag cross-sectional TEM of a BTO− CFO dot on STO. The Ti elemental mapping and Fe elemental mapping of the heterostructure are shown in f and g, respectively.

Property Characterization. Localized functional characteristics of individual oxide nanostructures were measured for the BTO, CFO, and BFO nanodots, and the consistency of ferroelectric and ferrimagnetic behaviors provide further evidence of the nature and high quality of the oxide nanopatterns (Figure 5). Single ferrimagnetic and ferroelectric domains were observed in the CFO/STO and BTO/STO system by magnetic force microscopy (MFM) and piezoelectric force microscopy (PFM), respectively (Figure 5a−d). Also, both ferrimagnetic and ferroelectric signals have been observed on the same BFO nanodot, which suggests the existence of multiferroic behavior expected for BFO (Figure 5e−g). In addition to the phase imaging, for BTO and BFO, d33 P-E measurements were conducted. The hysteresis loops are clearly seen with the coercive field of about 0.7 V for BTO and 2 V for BFO, which corresponds to 70 kV/cm and 500 kV/cm, respectively, which is in good agreement with previously reported values.16,28,29 These representative examples of individual measurements of nanopatterns provide evidence that despite large area coverage of the patterns, each one is functional in its localized form.

CONCLUSION In summary, a patterning approach, combining BPL and sol− gel synthesis, has been developed for the high throughput synthesis of oxide nanomaterials. We have demonstrated the 4442

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Figure 5. Functionality measurements. (a, b) Topology and MFM phase images of synthesized CFO nanodot. (c, d) Topology and PFM phase images of synthesized BTO nanodot on Nb:STO substrates. (e−g) Topology, MFM phase, and PFM phase images of the same BFO nanodot synthesized on Nb:STO substrate. (h) P-E loops of the BTO (black) and BFO (red) nanodots. The curves were smoothed for clarity. (Ba(CH3COO)2) and titanium isopropoxide (Ti((CH3)2CHO)4) was used. For CFO synthesis, a 0.3 M solution of cobalt nitrate and iron nitride was used. For BFO synthesis, a 0.3 M solution of bismuth nitrate and iron nitrate was prepared. For achieving features smaller than 100 nm (Figure S4b), a 0.1 M solution was also employed. The sol−gel solutions were spin-coated on the patterned substrate at 4000 rpm for 40 s. Then the substrates were baked at 120 °C for 1 h, followed by lift-off of the resist layers by immersing the substrates in Remover PG (MicroChem Corps., USA) at 80 °C overnight. They were then washed thoroughly with isopropanol and DI water before blowing dry with nitrogen. The substrates were placed in a tube furnace, and the temperature was increased to 200 °C and held for 1 h to remove any remaining solvent. Then, the materials left on the substrate were sintered at 1050 °C for ∼8 h to form the final crystals. Scanning Probe Characterization. The electrical and magnetic properties of the synthesized oxide nanostructures were examined by piezoresponse force microscopy (PFM) and magnetic force microscopy (MFM) modules equipped on a Dimension ICON AFM system (Veeco, USA). PFM measurements were performed with EFM probes with a resonant frequency of 75 kHz and a spring constant of 2.5 N/m (Nano World, USA) working in contact mode. MFM measurements were performed with MFMR probes with a resonance frequency of 75 kHz and a spring constant of 2.8 N/m (Nano World, USA), working in tapping mode.

ability to make nanopatterns of complex oxides with different morphologies, sizes, symmetries, and spacing. The patterned coverage is large enough to be analyzed by bulk techniques, such as XRD with adequate signal-to-noise ratio. Furthermore, by applying surface treatment, we are able to not only counteract “coffee stain” behavior, but also shrink the nanopatterns to be smaller than dimensions originally defined by patterning technique. The BPL approach, combined with sol−gel chemistry, provides a generalizable, massively parallel platform for generating nanostructures of multifunctional oxides, which may further promote fundamental research in the field and prospects for technological realization. Indeed, it underscores how these cantilever-free scanning probe techniques can be used to rapidly generate libraries of nanostructures30 with a diverse set of desirable properties.

METHODS BPL Patterning. The BPL patterning was performed on an actuated BPL patterning apparatus customized on an AFM platform (XE-100 ParkAFM, Korea) with piezo-actuated head carrying a 1 × 1 cm2 pen array. A BPL pen array with a ∼ 200 nm aperture size was fabricated following a previously published protocol.9 AZ1505 (MicroChemicals GmbH, Germany) and LOR 5A resist (MicroChem Inc., USA) are spin-coated on substrates with 4000 rpm and baked at 115 and 180 °C for 1 and 5 min, respectively. Samples were then mounted in the actuated BPL apparatus and leveled following an optical leveling procedure.8,9 Patterns were then exposed in a frameby-frame manner with an exposure time that was deliberately varied from 2 to 10 s to generate nanoreactors with diameters from 200 nm to 1 μm, respectively and subsequently developed in MF24A (MicroChem Inc., USA). Final patterns were characterized using optical microscopy (Axiovert, Zeiss, USA), and scanning electron microscopy (S-4800-II and Su-8030, Hitachi, Japan). Surface Treatment and Sol−gel Synthesis. Octadecyltrichlorosilane (OTS) and poly(tetrafluoroethylene oxide-co-difluoromethylene oxide) α,ω-diisocyanate (Mn ∼ 3000) (Sigma-Aldrich, USA) were thermally evaporated and deposited on the patterned area in a sealed glass Petri dish to increase the surface contact angle. The deposition temperatures are 90 and 140 °C, respectively. Oxygen plasma was employed when small contact angle was required. Sol−gel solutions were made by first dissolving the starting precursors in either 1:2 or 2:1 v/v mixture of acetic acid and DI water with thorough stir for the synthesis of nanorings and nanodots, respectively. Then the bottle was sealed and heated for 2 h at 60 °C. For BTO nanostructures a 0.3 M solution of barium acetate

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b00124. Detailed mechanism of the nanocrystal formation and more experimental methods; discussions on the uniformity of the patterns; table of the composition of precursors; contact angel measurements; more SEM images of the synthesized nanostructures; and PFM and MFM images of large nanodots that have multiple grains (PDF) Movie showing how CFO droplets evaporate on untreated silicon substrates (AVI) Movie showing how CFO droplets evaporate on OTStreated silicon substrates (AVI)

AUTHOR INFORMATION Corresponding Author

*[email protected]. 4443

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Xing Liao: 0000-0002-0405-1564 Chad A. Mirkin: 0000-0002-6634-7627 Vinayak P. Dravid: 0000-0002-6007-3063 Author Contributions ⊥

X.L. and Y.H. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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