Fast One‐Step Fabrication of Highly Regular Microscrolls with Controllable Surface Morphology

Abstract Although rolling origami technology has provided convenient access to three‐dimensional (3D) microstructure systems, the high yield and scalable construction of complex rolling structures with well‐defined geometry without impeding functionality has remained challenging. The straightforward, one‐step fabrication that uses external mechanical stress to scroll micrometer thick, flexible planar films with centimeter lateral dimensions into tubular or spiral geometry within a few seconds is demonstrated. The method allows controlling the scrolls’ diameter, number of windings and nanostructured surface morphology, and is applicable to a wide range of functional materials. The obtained 3D structures are highly promising for various applications including sensors, actuators, microrobotics, as well as energy storage and electronic devices.


V2O5 nanofiber synthesis
A V2O5 nanofiber dispersion was prepared from ammonium meta-vanadate (1 g, Fluka) and acidic ion-exchange resin (10 g, Dowex 50WX8, Alfa Aesar) in deionized water. To achieve concentrations of 3.5 mg/mL and 7.6 mg/mL, the volume of used water was 200 mL and 100 mL respectively. Tis mixture was heated to 80 °C for 10 min, cooled to room temperature and then stored for 28 days under ambient conditions, and finally the dispersion was decanted from the ion exchanger. Thus obtained dispersion contains V2O5 nanofibers with a length of up to five μm. [1,2] V2O5 thin film fabrication Details of the fabrication steps have been previously reported. [1] Briefly, the V2O5 nanofiber dispersion was poured on Si(100) p-type wafer (Wacker, Sitronic) inside a glass beaker.
Evaporation of the water at ambient conditions induced self-assembly of the nanofibers into a micrometer thick film on the substrate. By using different amounts of V2O5 fiber solutions, it is possible to control the thickness of the resulting film in steps of approximately 0.2 μm. After complete evaporation of the water, the V2O5 thin film was used for scrolling. To obtain V2O5 film thicknesses of 500 nm, the dispersion with a concentration of 3.5 mg/mL was diluted with water in a ratio of 1:3. For 5 µm thick V2O5 films, the as-prepared dispersion with a concentration of 7.6 mg/mL was directly used.

Graphene oxide (GO) thin film fabrication
The fabrication is detailed in our previous publication. [3] The GO dispersion was purchased from Nanografi (Ankara, Turkey) with a GO concentration of 8 mg/mL and a GO sheet size of 1 to 5 µm an a thickness of 0.4 to 1.1nm. To obtain GO thin films with a thickness of 2.5 µm, the GO dispersion was diluted to 1.61 mg/mL and poured into a polyethylene beaker (whose size leads to 0.46 mL/cm²) equipped with a cleaned Si(100) p-type wafer (Wacker, Sitronic).
After complete drying, the GO thin film was used for scrolling.

Nanocellulose (NC) thin film synthesis
The CNFs gel was diluted to 1 mg/mL with deionized water and poured onto a cleaned Si (100) p-type wafer (Wacker, Sitronic) inside a polyethylene dish (55 mm diameter). The amount of the mixture was 0.38 ml/cm², yielding 1.5 µm thick CNF thin films. [5] Hybrid film synthesis A 4 µm thick heterotrilayer consisting of GO, NC and V2O5 films was fabricated by subsequent deposition of the films, starting with GO after complete drying, using the procedures described above for each layer.

Structure characterization
The micro/nanostructure of the films/scrolls was investigated by light microscopy (Keyence Digital Microscope VHX-2000) and scanning electron microscopy (Ultra 55 FEM Microscope Zeiss). Scroll formation was recorded with a Keyence Digital Microscope VHX-2000.
Electrostatic charging used for opening the V2O5 scrolls was accomplished by rubbing a cotton cloth against the glass rod.   As the razor blade is manually lowered onto the film's surface under a certain angle (θ), and fixed with a weight (up to 222g), this inevitably causes some lateral slide of the blade, accompanied by the spontaneous formation of a half-scroll with an irregular shape, due to the uncontrolled speed of this initial stage movement (movie S2). The geometry of this half-scroll governs the subsequent scrolling, i.e., whether more tube-or spiral-like scrolling occurs ( Figure   S5).      Movie S1. Scroll formation (real time) from a 2 µm thick V2O5 film under scrolling angle of 45°, over a length of 3 mm; speed 1 mm/s. real time Movie S2. Spontaneous formation (real time) of a half-scroll with an irregular shape, which is due to the uncontrolled speed of the initial stage movement.

Supplementary Text and Figures
Movie S3. Scrolling of a 2 µm thick V2O5 film (real time) under scrolling angle of 25°, over a length of 3 mm; speed 1 mm/s. Shown is the delamination of the film resulting in one winding with a large diameter of 1 µm.
Movie S4. Scrolling of a 2 µm thick V2Oy film (real time) using a slower speed of 0.1 mm/s, leading to full delamination of the film.
Movie S5. Electrostatic charging-induced opening and closing of a V2O5 scroll (real time, view 1 and 2).