Fig. 1: Fabrication of a VG on PDMS strain sensor and its characterization.

a Schematic illustration for the fabrication of a VG on PDMS strain sensor. A pristine VG film is directly grown on a SiO₂/Si substrate by PECVD (step I). A BFG layer is formed between the VG and SiO₂ substrate during the initial VG growth. A dilute PDMS mixture (10:1) is drop cast onto the VG film, followed by spin-coating (step II). The PDMS film is degassed using a vacuum desiccator and cured for 2 h at 70 °C. After curing, PDMS/VG is mechanically peeled off the SiO₂ substrate (step III). When flipped over, the final product, BFG/VG on PDMS is obtained (step IV). Insets of step IV: contact angle optical image of a water droplet on VG (upper left) and photograph (lower right) of VG on PDMS after removal from the SiO₂ substrate. b SEM image of an as-grown VG films on a SiO₂ substrate. Scale bar, 3 μm. The top inset shows an HR-TEM image of a VG films, confirming the interplanar spacing of 0.342 Å, comparable to that of graphite. Scale bar, 2 nm. The bottom inset shows a cross-sectional view of a VG films on SiO₂, revealing a tufted 3D network structure. Scale bar, 100 nm. c Raman spectra of VG on SiO₂/Si and PDMS. The clear peaks at around 1350 cm⁻¹ (D peak), 1585 cm⁻¹ (G peak), and 2690 cm⁻¹ (2D peak) are observed for both samples.