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Electrospray Deposition of {200} Oriented RegularAssembly BaTiO3 Nanocrystal Films under an Electric Field Satoshi Suehiro, Teiichi Kimura, Makoto Tanaka, Seiji Takahashi, Kenichi Mimura, and Kazumi Kato Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03813 • Publication Date (Web): 27 Mar 2019 Downloaded from http://pubs.acs.org on March 31, 2019
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Electrospray Deposition of {200} Oriented Regular-Assembly BaTiO3 Nanocrystal Films under an Electric Field
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Satoshi Suehiroa*, Teiichi Kimuraa, Makoto Tanakaa, Seiji Takahashia, Ken-ichi Mimurab, and Kazumi Katoc.
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a
Materials Research and Development Laboratory, 2-4-1 Mutsuno, Atsuta-ku, Nagoya 456-8587, Japan Fine Ceramic Center (JFCC), Japan
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b
Inorganic Functional Material Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 2266-98 Anagahora, Shimoshidami, Moriyamaku, Nagoya, 4638560, Japan
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c
National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba 305-8560, Japan
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*Email:
[email protected] TEL: +81-52-871-3500
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ABSTRACT
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Highly oriented, regularly assembled nanocrystalline films have recently emerged as
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attractive new functional materials. In this study, we deposited a BaTiO3 (BT) nanocube dispersion
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on a Si substrate by electrospraying, resulting in a dense, regularly assembled BT nanocrystalline film.
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X-ray diffraction (XRD) analysis revealed that applying a voltage between the electrospray nozzle
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and the Si substrate during electrospraying caused the BT nanocubes to form a regular array in the
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200 plane aligned perpendicularly to the substrate. The volume fraction of BT nanocubes in the 200
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plane in the assembly was estimated by orientation distribution function (ODF) analysis to be about
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50%. The formation of this regularly assembled layer was determined to be linked to the interaction
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between the vaporized solvent and the substrate, enabled by the enhanced wettability under the electric
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field. Electrospray deposition has potential applications in the manufacture of nanocrystalline
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assembled films for nanofunctional devices.
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INTRODUCTION
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Various colloidal nanocrystals (NCs), such as metals, metal oxides, and chalcogenides, have
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been synthesized with tight control of particle size and shape through the employment of advanced
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synthesis technology [1-4]. Recently, the development of self-assembly processes for NCs has made
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them particularly attractive for use in electrical, dielectric, semiconductor, and magnetic devices [5-
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7]. Indeed, novel two- and three-dimensional regular self-assembled NC structures have the potential
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to be new functional materials. For example, the application of a regular self-assembled film of Co
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nanorods formed on a substrate yielded a high-density magnetic recording device with capacities above
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10 Tbits/in2. [8]. Employment of aligned CdSe/CdS nanorod assemblies in photo-electrical devices
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has also been suggested [9]. The dielectric constant (r > 3000) of a regular cube-like BaTiO3
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nanocrystal (BT nanocube) assembly is higher than that of a single BaTiO3 crystal [10]. Thus, the
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fabrication of self-assembled BT nanocrystal films and the mechanism responsible for the higher
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dielectric constant of BT nanocubes have been of interest [11, 12].
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The phenomenon of nanocrystal self-assembly is thought to be caused by interactions induced
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by van der Waals, Coulombic, and capillary forces, such as those present during the evaporation of
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solvents. Self-assembly of NCs has been observed during typical solution based processes, such as
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dip-coating [13], the Langmuir–Blodgett technique [14], and the spin-coating method [15]. On the
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other hand, formation of an oriented assembly layer of NCs can occur on a substrate with assistance
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from an external electric field [16]. For instance, CdS and CdSe nanorods form anisotropic assemblies 3 ACS Paragon Plus Environment
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owing to dipole moments exhibited along the electric fields of their crystals [17, 18]. In addition,
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ferroelectric nanoparticles such as BaTiO3 can be oriented in an organized structure by an external
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electric field [19]. However, fabrication of a film comprising an oriented BaTiO3 nanocrystal assembly
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has not been reported. In order to employ a semiconductor NC in a functional device, it is important
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to obtain a densely packed and highly oriented assembled nanocrystalline film.
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One of the techniques used to fabricate dense thin films using wet-chemical process is
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electrospray deposition [20-22]. Recently, the assembly of nanoparticles using electrospray deposition
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has been studied. For instance, Nie et al. reported a Langmuir–Blodgett-type assembly on water by
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electrospraying [23], and Ghafouri et al. stated that electrospraying may provide the capability for
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building monolayer assemblies [24]. In this study, we investigated a fabrication route for self-
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assembled BT nanocube films using electrospray deposition. By applying a voltage between the
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electrospray nozzle and a Si substrate, a packed and regularly assembled BT nanocube layer was
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formed on the substrate with a perpendicular orientation.
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.
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EXPERIMENTAL
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Assembly of BaTiO3 nanocrystal using electrospray
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BT NCs were synthesized using a typical hydrothermal method described elsewhere [25],
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then dispersed in methylene to obtain a dispersion of BT nanocubes at a concentration of about 0.3
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mg/mL. The NCs had a cubic-like structure with each side about 20 nm in length, as measured by
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transmission electron microscopy (TEM). Electrospray deposition was carried out using the process
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followed in a previous study [20]. A vertical cold-wall-type CVD chamber combined with an
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electrostatic atomizer was used for the deposition, as shown in Figure S1. The dispersion was fed into
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a stainless-steel nozzle via a syringe pump and atomized by applying a DC voltage from +4 kV to 12
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kV to attain electrostatic atomization. In this study, voltages of 0 kV,-2 kV, and -4 kV were applied to
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the sample stage on which the substrate was placed. The sample stage was heated to 60 °C, and then
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the BT nanocube dispersion was deposited onto the substrate by electrospraying for 60 s. The voltage
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was sustained until the solvent had evaporated completely. After the deposition, the film was heated
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under atmospheric pressure at 350 °C for 2 h to remove the organic component.
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Material characterization
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The morphologies of the films were examined using field emission scanning electron
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microscopy (FE-SEM, SU-8000, HITACHI, Tokyo, Japan). The crystalline phases and orientations of
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the deposited films were determined by X-ray diffraction (XRD) analysis using a diffractometer 5 ACS Paragon Plus Environment
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equipped with a Cu K radiation source (RINT2000, Rigaku, Tokyo, Japan). Orientation distribution
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function (ODF) analysis was performed with the arbitrarily defined cell (ADC) method [26, 27] in
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conjunction with a software package (LaboTex 3.0, LaboSoft s.c., Krakow, Poland). For ODF analysis,
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the tilt was varied from 15° to 90° in steps of 5°, while the angle of rotation was varied from 0° to 360°
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in steps of 5°. Three incomplete pole figures, namely {110}, (111), and {200}, were measured to avoid
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the diffraction peak of the Si substrate. The volume fractions (Vf) were calculated using the ODF
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within a 10° distance in Euler space from the ideal component.
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RESULTS AND DISCUSSION
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The SEM images in Figure 1 show the surface morphologies of the assembled BT nanocube
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films deposited by electrospraying under various applied voltages. When a voltage of (a) +4 kV or (b)
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+8 kV was applied to the nozzle, we observed locally self-assembled BT nanocube structures in the
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~100 nm range. However, when a higher voltage of around +12 kV was applied, no assembly layer
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was formed owing to the discharge of electricity. On the other hand, BT nanocubes formed densely
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packed regular assemblies in the few-micrometer range when a positive voltage was applied to the
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nozzle (+4 kV) and a negative voltage (–4 kV) to the substrate. The thickness of the film formed under
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this condition was estimated to be 500 nm, as shown in Figure S2 (SI).
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In order to evaluate the compactness of the assemblies, we used SEM binary images (Figure
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S3, SI) to estimate their apparent densities from gaps in the measurement areas. Table 1 lists 6 ACS Paragon Plus Environment
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representative apparent densities estimated from the binary images of films deposited under several
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applied voltage conditions. The apparent densities of the films deposited without voltage applied to
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the substrate (i.e., grounded) were relatively low, at about 80 %. On the other hand, the films deposited
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with voltage applied to the substrate had relatively high apparent densities of over 90 %.
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Table 1. Apparent densities estimated from the binary images of films deposited under several applied
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voltage conditions.
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Voltage (kV)
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8
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4 (−2)*
4 (−4)*
Density (%)
79
82
69
91
96
* voltage applied to the substrate
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In order to understand how the BT nanocube assemblies formed, we electrosprayed BT
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nanocube dispersions onto substrates for different durations. The SEM images in Figure 2 show how
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the surface morphology of the assembled nanocube layer progresses with deposition time. Soon after
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the deposition started at 10 s, the nanocubes aggregated and formed staircase-like structures on the
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edge of the substrate. The assembly gradually grew out from the edge of the substrate, until a regular
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assembly domain was formed with scaled regions of with a size of ~10 m. The cracks appear owing
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to drying shrinkage as the solvent would be unlikely to evaporate until reaching the substrate, owing
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to the relatively high boiling point of the dispersion used (~156 °C). On the other hand, when BT 7 ACS Paragon Plus Environment
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nanocubes were deposited using ethanol, the microstructure had an irregular granular structure (Figure
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S4). This is probably due to complete droplet evaporation prior to it reaching the substrate. The fact
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that the droplet is able to reach the substrate before evaporating is one of the key enablers for the
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formation of assembled films by electrospraying; a regular assembly was attained owing to the
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shrinkage behavior of the vaporized droplets on the substrate. In addition, as the dispersion is subjected
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to an electric field on the substrate, the attraction force between the nanoparticles is increased owing
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to the distribution of the charge elements, which initiates the formation of a dense assembly at the
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nanoscale [28].
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XRD patterns of the assembled BT nanocube films deposited by electrospraying are shown
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in Figure 3. The nanocubes were estimated to have a pseudocubic structure. When deposited without
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voltage applied to the substrate, a slight {200} orientation was indicated in the pattern of the resulting
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film, as shown in Figure 3(a). On the other hand, when voltage was applied to the substrate, orientation
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along the {200} planes was clearly indicated. This can be seen in Figure 3(b). The diffraction profiles
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of the {200} pole figures (Figure 4) show the pole densities in the center of each pole figure of the BT
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nanocube assembly on the Si {111} substrate. These results indicate that the assemblies are oriented
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in the {100} direction perpendicular to the substrate. The Vf values of the {200} films deposited
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without and with −4 kV voltage applied to the substrate were estimated to be 18 % and 50 %,
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respectively.
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Based on the SEM and XRD results, we analyzed the mechanism behind the formation of the
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BT nanocube regular assembly by electrospray deposition when applying a negative voltage to the
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substrate. The schematic illustration in Figure 5 depicts how the nanocube assembly may have formed
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during electrospraying. The spray of droplets causes the dispersion to be deposited incrementally onto
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the substrate surface, producing a thin layer of liquid. When the liquid surface is exposed to an electric
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field (i.e., when a negative voltage is applied to the substrate), the ferroelectric BT nanocubes assemble
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on the edge of substrate owing to Marangoni flow. The Marangoni effect has been noted previously
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during electrospray deposition under certain conditions [24]. We observed that wettability was
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improved between a BT dispersion in water and the Si substrate when a voltage was applied to the
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substrate (Figure S5). Therefore, self-assembly of BT nanocubes occurred from the edge of the
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substrate owing to the change in surface tension induced by Marangoni flow, by exposing the liquid
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phase to an electric field. On the other hand, it has been reported that BaTiO3 nanoparticles can be
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arranged to a structure by an external electric field of 1 kV due to the ferroelectric character [19]. The
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arrangement of nanoparticles was determined by the competition between the electric field and a
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favorable interface energy. Hence, it is important to evaluate the domain structure of ferroelectric
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BaTiO3 under an external electric field. In this paper, we described a nanoparticle array technology
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using the electrospray technique. We will report on the orientation of the nanocube and the domain
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structure of the sintered body, along with the evaluation results of its ferroelectricity, in the near future.
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CONCLUSION
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In this study, we deposited assembled nanocrystal films by electrospraying a BaTiO3
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nanocube dispersion. When a DC voltage was applied between the electrospray nozzle and the Si
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substrate, a dense film composed of regularly arranged BT nanocubes was deposited. The BT
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nanocrystal film had a strong {200} orientation, as indicated by the pole density and a high diffraction
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intensity of the {200} fraction. The volume fraction of {200} was estimated to be 50% by ODF
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analysis. The regularly assembled layer was determined to be formed by the interaction between the
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vaporized solvent and substrate under the electric field, induced by the wettability. It is concluded that
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electrospray deposition is likely to be an effective deposition process for future nanomaterial devices.
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ACKNOWLEDGMENTS
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This work was supported by Adaptable and Seamless Technology Transfer Program through Target-
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driven R&D (AS282I007e), Japan Science and Technology Agency.
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Figure 1. SEM images of the surface morphology of BaTiO3 nanocube assembly layers formed by electrospray deposition under various applied voltages: (a) +4 kV, (b) +8 kV, (c) +12 kV, (d) +4 kV (*−2 kV), and (e) +4 kV (*–4 kV) *voltage applied to the substrate
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Figure 2. SEM images of the surface morphology of BaTiO3 nanocube assembly layers under various deposition times: (a) 10 s, (b) 30 s, (c) and (d) 60 s
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Figure 3. XRD patterns of the BT nanocube assembly films deposited by electrospraying (a) without and (b) with voltage applied to the substrate.
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Figure 4. {200} pole figures showing the pole densities of BT nanocube assembly films deposited by electrospraying (a) without and (b) with voltage applied to the substrate.
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Figure 5. Schematic of the formation of regular assembly layers using electrospray deposition with voltage applied to substrate.
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Figure 1. SEM images of the surface morphology of BaTiO3 nanocube assembly layers formed by electrospray deposition under various applied voltages: (a) +4 kV, (b) +8 kV, (c) +12 kV, (d) +4 kV (*−2 kV), (e) +4 kV (*–4 kV) *voltage applied to the substrate 394x213mm (150 x 150 DPI)
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Figure 2. SEM images of the surface morphology of BaTiO3 nanocube assembly layers under various deposition times: (a) 10 s, (b) 30 s, (c) and (d) 60 s 346x262mm (150 x 150 DPI)
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Figure 3. XRD patterns of the BT nanocube assembly films deposited by electrospraying (a) without and (b) with voltage applied to the substrate. 358x302mm (150 x 150 DPI)
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Figure 3. {200} pole figures showing the pole densities of BT nanocube assembly films. (a) electrospray; (b) electrospray with a voltage applied to the substrate 410x222mm (150 x 150 DPI)
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Figure 5. Schematic of the formation of regular assembly layers using electrospray deposition with voltage applied to substrate. 410x339mm (150 x 150 DPI)
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