Article pubs.acs.org/JACS
Atomic Layer Engineering of High‑κ Ferroelectricity in 2D Perovskites Bao-Wen Li, Minoru Osada,* Yoon-Hyun Kim, Yasuo Ebina, Kosho Akatsuka, and Takayoshi Sasaki World Premier International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan S Supporting Information *
ABSTRACT: Complex perovskite oxides offer tremendous potential for controlling their rich variety of electronic properties, including high-TC superconductivity, high-κ ferroelectricity, and quantum magnetism. Atomic-scale control of these intriguing properties in ultrathin perovskites is an important challenge for exploring new physics and device functionality at atomic dimensions. Here, we demonstrate atomic-scale engineering of dielectric responses using two-dimensional (2D) homologous perovskite nanosheets (Ca2Nam−3NbmO3m+1; m = 3−6). In this homologous 2D material, the thickness of the perovskite layers can be incrementally controlled by changing m, and such atomic layer engineering enhances the high-κ dielectric response and local ferroelectric instability. The end member (m = 6) attains a high dielectric constant of ∼470, which is the highest among all known dielectrics in the ultrathin region ( 300). We also find that higher m members exhibit local ferroelectric instability.
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RESULTS AND DISCUSSION Unit-Cell-Level Assembly of Perovskite Nanosheets. A homologous series of Ca2Nam−3NbmO3m+1 nanosheets (m = 3− 6) was prepared by delaminating layered perovskites (KCa2Nam−3NbmO3m+1) with an aqueous solution of tetrabuthylammonium hydroxide (TBAOH).16 The chemical compositions and atomic arrangements of the host layers are preserved in the exfoliated nanosheets. Characterization by atomic force Received: June 6, 2017 Published: July 12, 2017 10868
DOI: 10.1021/jacs.7b05665 J. Am. Chem. Soc. 2017, 139, 10868−10874
Article
Journal of the American Chemical Society
Figure 1. 2D high-κ dielectric perovskites. (a) Structures of Ca2Nam−3NbmO3m+1 nanosheets. (b−e) AFM images of individual nanosheets: (b) Ca2Nb3O10, (c) Ca2NaNb4O13, (d) Ca2Na2Nb5O16, (e) Ca2Na3Nb6O19. A tapping-mode atomic force microscope operating in a vacuum was used to evaluate the morphology of the nanosheets.
Figure 2. Unit-cell-level assembly of Ca2Nam−3NbmO3m+1 nanosheets. HRTEM images of monolayer (upper panels) and five-layer (lower panels) films of Ca2Nam−3NbmO3m+1 nanosheets on atomically flat SrRuO3 substrates: (a, e) Ca2Nb3O10, (b, f) Ca2NaNb4O13, (c, g) Ca2Na2Nb5O16, (d, h) Ca2Na3Nb6O19.
nm corresponds to the thickness of one NbO6 octahedron, which is consistent with the homologous structural aspect of layered perovskites. These near-ideal characteristics persisted in thicker films (Figure 2e−h), indicating the successful assembly of multilayer films in a unit-cell-upon-unit-cell manner. We performed the interface characterization using electron energyloss spectroscopy (EELS) showing an example of a (Ca2Na3Nb6O19)n/SrRuO3 film (n = 5) (Figure S2, Supporting Information). The EELS analysis also showed an atomically shaped and clean interface without interdiffusion. Notably, a compositional abruptness at the Ca2Na3Nb6O19/SrRuO3 interface was detected, suggesting the production of dead-layer-free nanofilms directly assembled on the SrRuO3 substrate. Such a superior interface property is not specific to Ca2Na3Nb6O19; a similar interface quality was achieved in other multilayer films (Ca2Nam−3NbmO3m+1)n with different layers (n) on other substrates such as Pt and Si. Atomic Layer Engineering of High-κ Perovskite Nanosheets. These highly ordered films allowed us to
microscopy (AFM) revealed a 2D morphology with a unique thickness for each sample (Figure 1b−e). The average thicknesses were 1.85, 2.30, 2.74, and 3.20 nm for m = 3, 4, 5, and 6, respectively, and the standard deviation was 0.05−0.07 nm. These thickness values are in good agreement with the crystallographic thicknesses of Ca2Nam−3NbmO3m+1 nanosheets. We used the Langmuir−Blodgett (LB) technique to perform layer-by-layer engineering of perovskite nanosheets.17,18 The LB approach with the use of an atomically flat SrRuO3 substrate is effective for room-temperature fabrication of high-quality monolayer films with a highly dense characteristic (Figure S1, Supporting Information). By repeated LB deposition of the monolayer, we fabricated the multilayer films (Ca2Nam−3NbmO3m+1)n with various thicknesses (n). High-resolution transmission electron microscopy (HRTEM) (Figure 2) resolved equidistant dark fringes, which corresponded to the elementary units of NbO6 octahedral layers. In the monolayer case (Figure 2a−d), the thicknesses were approximately 1.5, 1.9, 2.3, and 2.7 nm for m = 3, 4, 5, and 6, respectively. The increment of ∼0.4 10869
DOI: 10.1021/jacs.7b05665 J. Am. Chem. Soc. 2017, 139, 10868−10874
Article
Journal of the American Chemical Society
to that of bulk KCa2Nam−3NbmO3m+1 ceramics, where the εr values at high frequency are depleted due to charge separation.19 The εr values at high frequency (>100 kHz) were larger than the bulk values of KCa2Nam−3NbmO3m+1. Figure 4d shows the thickness dependence of εr in the multilayer films of Ca2Nam−3NbmO3m+1 nanosheets. The data for an m = 2 nanosheet (LaNb2O7) are also included. LaNb2O7 with m = 2 exhibited a rather low εr value (∼50), possibly due to the low m number and slight La defects. In contrast, in Ca2Nam−3NbmO3m+1 nanosheets, increasing the number of the octahedral units (m) resulted in higher εr values, and the end member (m = 6) attained the highest εr value of ∼470. In these films, the εr values remained at a constant level, irrespective of the film thickness. We also note that the thickness dependences of the reciprocal capacitance 1/C for perovskite nanosheets show a linear relationship across the zero, indicating a clean high-κ system without interfacial low-κ dead layers (Figure S4, Supporting Information). To further assess the utility of Ca2Nam−3NbmO3m+1 nanosheets, we considered the performance of Ca2Nam−3NbmO3m+1 nanosheets (Figure 4e) in comparison to typical high-κ perovskite thin films.20−27 Importantly, the high εr values of perovskite nanosheets persist even in the
