Vertically Free-Standing Ordered Pb(Zr0.52Ti0.48)O3 Nanocup Arrays by Template-Assisted Ion Beam Etching

Zhang et al. Nanoscale Research Letters (2016) 11:225 DOI 10.1186/s11671-016-1369-x NANO EXPRESS Open Access Vertically Free-Standing Ordered Pb(Zr0.52Ti0.48)O3 Nanocup Arrays by Template-Assisted Ion Beam Etching Xiaoyan Zhang1, Dan Tang1, Kangrong Huang1, Die Hu1, Fengyuan Zhang1, Xingsen Gao1, Xubing Lu1, Guofu Zhou2,3, Zhang Zhang1* and Junming Liu1,4 Abstract In this report, vertically free-standing lead zirconate titanate Pb(Zr0.52Ti0.48)O3 (PZT) nanocup arrays with good ordering and high density (1.3 × 1010 cm−2) were demonstrated. By a template-assisted ion beam etching (IBE) strategy, the PZT formed in the pore-through anodic aluminum oxide (AAO) membrane on the Pt/Si substrate was with a cup-like nanostructure. The mean diameter and height of the PZT nanocups (NCs) was about 80 and 100 nm, respectively, and the wall thickness of NCs was about 20 nm with a hole depth of about 80 nm. Uppermost, the nanocup structure with low aspect ratio realized vertically free-standing arrays when losing the mechanical support from templates, avoiding the collapse or bundling when compared to the typical nanotube arrays. X-ray diffraction (XRD) and Raman spectrum revealed that the as-prepared PZT NCs were in a perovskite phase. By the vertical piezoresponse force microscopy (VPFM) measurements, the vertically free-standing ordered ferroelectric PZT NCs showed well-defined ring-like piezoresponse phase and hysteresis loops, which indicated that the high-density PZT nanocup arrays could have potential applications in ultra-high non-volatile ferroelectric memories (NV-FRAM) or other nanoelectronic devices. Keywords: Vertically free-standing, PZT nanocups, AAO, Ion beam etching Background In recent years, increasing efforts have been made to synthesize and understand ferroelectric nanostructures because of their peculiar physical properties, such as their finite size effects and unusual phase transitions [1, 2], offering a wide range of potential applications in nanoscale piezoelectric actuators [3], force and acceleration sensors [4, 5], ultrasonic transducers [6], and non-volatile ferroelectric random access memory (NV-FRAM) devices [7]. Due to the current trends of high integration and miniature in semiconductor industry, ferroelectric memories have been receiving more and more attention due to their unique advantages such as high density, low power consumption, and high read/write speed [8–10]. * Correspondence: 1 Institute for Advanced Materials and Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, China Full list of author information is available at the end of the article Lead zirconium titanate Pb(Zr0.52Ti0.48)O3 (PZT), a solid solution of the perovskites lead zirconate and lead titanate, is a prominent ferroelectric material that has stimulated tremendous fundamental and applied researches due to its high spontaneous polarization abilities, piezoelectric coefficient, dielectric permittivity, and pyroelectricity [11–13]. Applications of PZT nanostructures include tunable photonic crystals, ferroelectric random access memory (FRAMs), terahertz emission, fluidic delivery, and nanosensors [14–16]. In addition, the significant need of miniaturization of electronic devices leads to more extensive usage of PZT FRAMs based on the low-dimensional nanostructures [17]. Sol-gel process is one of the promising routes among many suitable methods for the preparation of nanostructured PZT materials, as it leads to products with high chemical homogeneity and purity at comparably low temperatures [18]. However, the synthesis speed of ferroelectric nanotubes (NTs) has been relatively slower than it has been for other materials, which © 2016 Zhang et al. Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Zhang et al. Nanoscale Research Letters (2016) 11:225 might due to the difficulties associated with the structural and stoichiometric complexity [19]. For large-scale memory device, high-density ordered ferroelectric NTs with a vertically aligned integration on the substrate is one essential. In recent years, high-density vertically aligned ordered ferroelectric NTs can be fabricated using template-assisted method [18]. In particular, anodic aluminum oxide (AAO) templates are with advantages of good ordering, large-area fabrication, paralleled pore arrangement, and tunable size [20]. Nevertheless, the clamping effect degrades the properties with the remaining template around the nanostructure [21, 22]. After getting rid of the template, however, the loss of the mechanical support from AAO and capillary force always resulted in the degradation of the ordered array alignment on the substrate [23, 24]. Obviously, the agglomeration of the nanostructures would greatly affect the device performance. And the agglomeration was mainly due to the high surface tension, which was closely related to the compressive stress of the nanostructures. In order to reduce surface tension, the highdensity nanostructures should be preferentially designed with low aspect ratio of height to diameter [25]. Up to now, there have been few reports addressing both the vertically free-standing ordered nanostructure arrays and the structure-property relations of the ferroelectric nanostructures. In this work, we have successfully developed highdensity and well-ordered vertically free-standing (VFS) PZT nanocup arrays on a conductive Pt/Si substrate, by a template-assisted ion beam etching (IBE) method. The low aspect ratio of height to diameter being critical to the vertically free-standing feature of PZT nanocups (NCs) could be well controlled by the ultrathin AAO templates and IBE process. X-ray diffraction (XRD) and Raman spectrum revealed that the as-prepared PZT NCs were in a perovskite phase. By the vertical piezoresponse force microscopy (VPFM) measurements, the well-ordered ferroelectric PZT nanocup arrays showed well-defined ring-like piezoresponse phase and hysteresis loops, which indicated that the nanostructure could have potential applications in ultra-high NV-FRAMs or other oxide nanoelectronic devices. Methods AAO Fabrication AAO templates are fabricated by a standard two-step anodization method. Firstly, high-purity aluminum foils (99.999 %, Good fellow Cambridge Limited) were degreased and then annealed at 450 °C for 3 h under argon atmosphere. Then, the Al foils are electrochemically polished in a solution of mixed ethanol and HClO4 (3:1 by volume) at 20 V for 5 min to form a mirror-like surface smoothness. The first (...truncated)