“Self‐Peel‐Off” Transfer Produces Ultrathin Polyvinylidene‐Fluoride‐Based Flexible Nanodevices

Here, a new strategy, self‐peel‐off transfer, for the preparation of ultrathin flexible nanodevices made from polyvinylidene‐fluoride (PVDF) is reported. In this process, a functional pattern of nanoparticles is transferred via peeling from a temporary substrate to the final PVDF film. This peeling process takes advantage of the differences in the work of adhesion between the various layers (the PVDF layer, the nanoparticle‐pattern layer and the substrate layer) and of the high stresses generated by the differential thermal expansion of the layers. The work of adhesion is mainly guided by the basic physical/chemical properties of these layers and is highly sensitive to variations in temperature and moisture in the environment. The peeling technique is tested on a variety of PVDF‐based functional films using gold/palladium nanoparticles, carbon nanotubes, graphene oxide, and lithium iron phosphate. Several PVDF‐based flexible nanodevices are prepared, including a single‐sided wireless flexible humidity sensor in which PVDF is used as the substrate and a double‐sided flexible capacitor in which PVDF is used as the ferroelectric layer and the carrier layer. Results show that the nanodevices perform with high repeatability and stability. Self‐peel‐off transfer is a viable preparation strategy for the design and fabrication of flexible, ultrathin, and light‐weight nanodevices.


Videos of experimental work
Video-S1: The preparation of a typical flexible Au-Pd/PVDF antenna via SPOT.
Video-S2: The preparation of a typical flexible SWCNT/PVDF film via SPOT (with/without blowing by mouth).
2. Analysis of the mechanism of SPOT in the Supplementary Text, Scheme S1, Figure S1, and Table S1. S2 to S9. 4. References.

Scheme S1. Fabrication of PVDF-based bilayer films using SPOT. a) Explanation of the stress response of PVDF-based films; b) The self-peel-off behavior of PVDF-based films
attached on a PET substrate via van der Waals forces. (R T : the radius of curvature at temperature T; A: the deflection of PVDF based films; △F: the internal stress).

Supplementary Text: Mechanism of action
Here, we analyze the mechanism of SPOT more deeply, aiming to maximize the process's beneficial features. Specifically, SPOT results from the differential expansion between the different layers, which generates interlaminar stresses at the interface between the substrate and the nanomaterials/PVDF film.
Due to the high CTE value of PVDF (α = 127 × 10 -6 K -1 ), its layers experience extensive shrinkage during cooling. If the PVDF layer is attached to a substrate with a lower CTE, then we have a bimaterial membrane that deforms out of plane (bending) during the cooling phase.
The out-of-plane motion is responsible for the generation of the out-of-plane stresses at the interface between the two materials, which in turn induces delamination and subsequent self-3 peel-off behavior. The curvature of the bimaterial membrane (and as a consequence, the intensity of the residual stresses) depends on both the material's parameters and geometrical parameters, such as the thickness ratio between the different layers and the contrast in the elastic modulus between the layers. Timoshenko proposed an analytical solution based on thin structures bimaterial (Timoshenko, 1953) [S1]: where R T is the radius of curvature at temperature T and R T0 is the radius of curvature at the other temperature, T 0 , material 1 has the lower CTE (α 1 ) and material 2 has the higher CTE (α 2 ). is the thickness ratio between the two layers and is the the ratio of the elastic moduli of the materials. s is the total thickness of the film (t 1 + t 2 ). It is clear from equation S1 that the change in radius of curvature is directly proportional to the change in temperature and the difference in CTE values. Moreover, a narrower thickness also increases the bending of the film. In other words, to maximize bending in a given temperature change, the bimaterial film must in theory be as thin as possible.
X-ray Diffraction (XRD) was also used to further verify the above prediction, as shown below. It can be found that, compared with the XRD pattern of pure PVDF, the PVDF film deposited on graphene oxide (GO) has a weaker alpha peak (α) and a stronger beta peak, indicating the relative reaction between the chemical groups on GO and the fluorine group in the PVDF chain, which creates a positive affect on the polarization of PVDF during its drying process. Meanwhile, the behavior can be further confirmed through the shift of a typical GO peak from 12.6 deg to 11.3 deg. GO; c) LiFePO 4 . Note that before the functional ink is dropped on the square glass slide (2 cm × 2 cm), the slide is cleaned with ethanol and treated with plasma. Figure S8. a) A typical large-area PVDF rGO film prepared via SPOT with a total thickness of 3 ± 0.5 µm; b) a demonstration of a small piece of a flexible rGO/PVDF film attached to a water bubble, indicating its ultra-light performance.
13 Figure S9. A schematic of a typical printed antenna with a total length of 2.5 cm and total width of 1.25 cm (the width of the conductive line = 750 µm and interval = 750 µm). [S8]