Bendable Polycrystalline and Magnetic CoFe₂O₄ Membranes by Chemical Methods

Abstract

The preparation and manipulation of crystalline yet bendable functional complex oxide membranes has been a long-standing issue for a myriad of applications, in particular, for flexible electronics. Here, we investigate the viability to prepare magnetic and crystalline CoFe 2 O 4 (CFO) membranes by means of the Sr 3 Al 2 O 6 (SAO) sacrificial layer approach using chemical deposition techniques. Meticulous chemical and structural study of the SAO surface and SAO/CFO interface properties have allowed us to identify the formation of an amorphous SAO capping layer and carbonates upon air exposure, which dictate the crystalline quality of the subsequent CFO film growth. Vacuum annealing at 800 °C of SAO films promotes the elimination of the surface carbonates and the reconstruction of the SAO surface crystallinity. Ex-situ atomic layer deposition of CFO films at 250 °C on air-exposed SAO offers the opportunity to avoid hightemperature growth while achieving polycrystalline CFO films that can be successfully transferred to a polymer support preserving the magnetic properties under bending. Float on and transfer provides an alternative route to prepare freestanding and wrinkle-free CFO membrane films. The advances and challenges presented in this work are expected to help increase the capabilities to grow different oxide compositions and heterostructures of freestanding films and their range of functional properties.

On the Role of the Sr3−xCaxAl2O6 Sacrificial Layer Composition in Epitaxial La0.7Sr0.3MnO3 Membranes

“…The film after vacuum thermal annealing at 825°C results in a spotty pattern revealing that the surface recrystallizes, as previously reported. [ 31 ] Then, the LSMO film is deposited by PLD on the reconstructed SAO obtaining a new spotty pattern that discloses the formation of an oriented LSMO film, Figure 1g. The same process has been followed to study the behavior of the SC x AO series with x ⩽ 3 and the subsequent growth of the LSMO.…”

“…It can be identified areas with dark contrast, which corresponds to amorphous SAO and areas with outcrops (white arrows). Regarding the existence of amorphous SAO regions, it reveals that the post‐annealing in vacuum performed on SAO films [ 31 ] prior to LSMO deposition does not completely reconstruct the surface crystallinity although the spotty RHEED pattern shown in Figure 1f. A closer look at one of these outcrops, Figure 4b, shows the atomic ordering of SAO and LSMO, confirming that it corresponds to epitaxial (001) SAO and it acts as a nucleation site for epitaxial (001) LSMO.…”

“…[26] Ex Situ Synthesis of Epitaxial LSMO by PLD: Following the growth of SC x AO, upon sample air-exposure, the film was introduced in the PLD chamber to perform an in-vacuum annealing up to 825 °C for 30 min at an oxygen partial pressure PO 2 of 0.1 mbar to restore the SC x AO surface crystallinity. [31] Then, 60 nm thick LSMO films were epitaxially grown in situ by PLD at 725 °C and a PO 2 of 0.1 mbar using a KrF excimer laser. The same PLD growing conditions were used to prepare LSMO directly on a (001) STO substrate.…”

“…First, while its high reactivity with water makes it almost the perfect candidate for selective etching against many perovskite oxides, [ 23 ] it also helps the fast development of an amorphous capping layer when exposed to air. [ 31 ] This reactivity hinders the subsequent epitaxial growth of the perovskite oxide. The use of a protective SrTiO 3 layer [ 32 ] can partially help fixing the sacrificial instability issue, although it brings some limitations to prepare stacked membrane heterostructures with pristine interfaces.…”

Research progress of applications of freestanding single crystal oxide thin film

“…需求也在急速增长 [1][2][3][4][5] 。目前常见的柔性电子器件主要是由柔性有机支撑体和功 能体构成， 而传统硅基电子器件的核心功能材料往往是以刚性的功能氧化物或者 半导体为主 [6] 超弹的机械特性等特点，成为近年来柔性电子材料中的研究热点。目前已报道了 诸多自支撑氧化物薄膜钙钛矿结构 BaTiO 3 (BTO) [7] 、PbZr x Ti (1-x) O 3 (PZT) [8] 、 BiFeO 3 (BFO) [9] 和尖晶石结构 Fe 3 O 4 [10] 、CoFe 2 O 4 (CFO) [11] 。这些自支撑功能氧化 物薄膜在表现出其独特功能性的同时，还表现出柔性、轻质和稳定性好等优点。 随着对薄膜剥离技术的研究， 将功能性氧化物薄膜与衬底分离的方法也得到 了发展，目前主要有湿法刻蚀技术和机械剥离技术。湿法刻蚀技术是在目标薄膜 与衬底之间添加牺牲层，通过湿法刻蚀工艺分离薄膜与衬底，最后得到自支撑氧 化物薄膜，这是使用较多且效果较好的方法之一，其中常见的牺牲层有： Sr 3 Al 2 O 6 (SAO) [12] 、La 0.7 Sr 0.3 MnO 3 (LSMO) [13] 、Ba 3 Al 2 O 6 [14] 等。机械剥离技术主要 以激光剥离为主，激光剥离是将高能激光聚焦到衬底和外延层的界面处，通过激 光逐点扫描瞬间融化外延缓冲层，从而使衬底和外延层剥离，其常见的衬底有： 蓝宝石 [15] 、云母 [16] 、石墨烯 [17] 等。 自支撑功能氧化物薄膜已经被证实，当它们变成独立状态时，氧化物薄膜仍 能保持良好的热、力、声、电、磁等 [18][19][20][21][22][23] 性质，且由于薄膜晶格处于自由状态， 在外应力场作用时具有显著的调控效果。 因此自支撑氧化物薄膜在柔性电子器件 应用中具有独特优势。 本文总结概述了近年来自支撑氧化物薄膜在晶体管存储器、 铁电隧道结、纳米发电机、应变传感器、压电谐振器、储能器件、超导、二维电 子气、金属-绝缘体过渡、自支撑磁性氧化物、柔性三维屈曲结构、转角贴合等(图…”