Freestanding oxide membranes: synthesis, tunable physical properties, and functional devices

Figure  1.  (a) Structural unit of mica. Surface structure of muscovite mica: (b) (001) projection and (c) (100) projection. Reproduced with permission from Ref. [34]. Copyright 2017, Elsevier. (d) Summary of vdW oxide heteroepitaxy. Reproduced with permission from Ref. [37]. Copyright 2017, Springer Nature.

Figure  2.  (a) Process schematic for oxide membrane release and transfer. Reproduced with permission from Ref. [71]. Copyright 2019, Springer Nature. (b) Cubic lattice structure of SAO and Al₆O₁₈¹⁸⁻ rings consisting of AlO₄ tetrahedra. Reproduced with permission from Ref. [22]. Copyright 2016, Springer Nature. (c) (Top) Top 1/4 of the SAO unit cell projected onto the (001) plane, dashed circles indicate vacancy sites, and 4×4 unit cells of the STO crystal structure projected onto the (001) plane. Reproduced with permission from Ref. [22]. Copyright 2016, Springer Nature. (Bottom) Schematic illustrating the pseudocubic unit cell of SAO. Reproduced with permission from Ref. [61]. Copyright 2017, American Institute of Physics.

Figure  3.  Schematics of four different wet-etching-based methods for fabricating oxide films. Reproduced with permission from Ref. [68]. Copyright 2021, Springer Nature.

Figure  4.  Phase diagram of biaxially strained and uniaxially strained La_0.7Ca_0.3MnO₃ membranes. (a) Electric potential mapping of the central van der Pauw (vdP) geometry in membranes. (b) Phase diagram of biaxially strained LCMO membranes. (c) Electric potential mapping of the central vdP geometry in membranes under uniaxial strain with 1:100 resistivity anisotropy. (d) Phase diagram of uniaxially strained LCMO membranes. Reproduced with permission from Ref. [23]. Copyright 2020, American Association for the Advancement of Science.

Figure  5.  (a) Schematic of strain gradient elasticity in freely suspended STO crystalline membrane drumheads. Reproduced with permission from Ref. [82]. Copyright 2021, American Chemical Society. (b) Illustration of the experimental configuration of the conductive AFM (c-AFM) test, which was used to examine the flexoelectricity of the transferred BTO_3-δ films. Reproduced with permission from Ref. [84]. Copyright 2022, Springer Nature. (c, d) The out-of-plane polarizations at the neutral layer (where the in-plane strain is near zero) P_Z-NL and their corresponding strain gradients $ {\epsilon }_{xx,z} $ from the bent BFO and STO membranes, respectively, where P_S is spontaneous polarization. The error bars represent the standard deviation of the measured unit cells. Reproduced with permission from Ref. [86]. Copyright 2022, Springer Nature. (e) Polarization-electric field (P-E) hysteresis loops under various tensile and compressive bending radii. Reproduced with permission from Ref. [18]. Copyright 2017, American Association for the Advancement of Science.

Figure  6.  (a) Series of SEM images of the bending process of a BTO nanobelt (20 μm × 4 μm × 120 nm). Reproduced with permission from Ref. [88]. Copyright 2019, American Association for the Advancement of Science. (b, c) PFM images and measurements of wrinkled BTO films. (b) IP-PFM images. (c) OOP-PFM images. (d) Line profiles of the corresponding height and amplitude data (average of 20 pixels) along the blue dotted lines in (c) and (d). Reproduced with permission from Ref. [24]. Copyright 2020, John Wiley and Sons. (e, f) Electromechanical cycles in BTO. (e) Schematic illustration of the membrane’s electromechanical structure. (f) Displacement of the membrane fold front versus dose (equivalent to the electric field) for cycling results in a reversible and reproducible electromechanical response. Reproduced with permission from Ref. [92]. Copyright 2020, American Chemical Society.

Figure  7.  (a) Cross-sectional HAADF images of a three-unit-cell BFO film before (top) and after (bottom) releasing the film, showing an R-like phase with polarization along the <111> direction and a T-like phase with polarization along the <001> direction, respectively. (b) Calculated giant polarization and lattice distortion in ultrathin freestanding BFO films. The off-center displacement (δ_cz) is defined as the distance along the out-of-plane direction between the centers of the neighboring Bi ions (dotted black line) and Fe ions (dotted blue line). Reproduced with permission from Ref. [71]. Copyright 2019, Springer Nature. (c) XRD θ–2θ scans for the strained and freestanding superlattice films. Schematics showing the relationship between the lattice parameters for the bulk materials (left), the favored polarization configuration for the strained heterostructure (middle inset), and the mismatch between the in-plane lattice parameters of the freestanding superlattice and the bulk lattice parameter of the SRO electrode, which causes the heterostructure to bend (right). Reproduced with permission from Ref. [96]. Copyright 2022, John Wiley and Sons.

Figure  8.  (a,b) MoS₂ transistors with transferred STO top-gate dielectrics. (a) Double-sweep I_DS–V_TG characteristics of the device with the back gate grounded. The gate sweeping directions are indicated by the arrows. The inset shows an optical image of the device. The boundaries of graphene and MoS₂ are outlined by dashed lines. Scale bar, 10 µm. (b) Output curves (I_DS–V_DS) with VTG varying from 0 to 1.0 V at steps of 0.1 V. Reproduced with permission from Ref. [100]. Copyright 2022, Springer Nature. (c) Freestanding oxide FTJ memories transferred onto SiO₂-coated Si substrates (left). Tunnel current measured at 0.2 V DC voltage after a gradually increasing and decreasing pulsed bias showing characteristic hysteresis loops for two different junctions (right). Reproduced with permission from Ref. [62]. Copyright 2019, American Chemical Society. (d, e) Flexible energy harvester based on the freestanding PZT thin film. (d) Bending and unbending motions of flexible energy harvester devices by a linear motor. During mechanical stimulation, the voltage and current produced from (e) the flexible PZT thin-film generator made from the MgO wafer. Insets: instantaneous output power levels of the PZT thin-film energy harvester as a function of the external load resistance. Reproduced with permission from Ref. [50]. Copyright 2017, Springer Nature.