An Ultra‐Reliable and Highly Sensitive Flexible Piezoelectric Pressure Sensor Based on Vertically Aligned Gallium Nitride Microrod Arrays on Graphene

ABSTRACT We report a robust and flexible piezoelectric pressure sensor based on gallium nitride (GaN) p–n homojunction microrod arrays on graphene. The microrods were synthesized via a two‐step metal‐organic chemical vapor deposition (MOCVD) process. First, catalyst‐free MOCVD was used to grow position‐controlled zinc oxide (ZnO) nanotubes on graphene. Subsequently, n ‐ and p ‐type GaN layers were conformally coated on ZnO nanotubes to obtain coaxial GaN p‐n homojunction microrods. FESEM confirmed position‐controlled microrod arrays, while X‐ray Diffraction (XRD) revealed their preferential c ‐axis orientation. To fabricate the device, the gaps between the microrods were filled with a polyimide layer, followed by the mechanical release of the microrods from the SiO ₂ /Si substrate, enabled by the weak van der Waals interface between the graphene and the substrate. The vertical device structure was completed by depositing an ohmic metal contact at the microrods tip, yielding a flexible piezoelectric pressure sensor. The pressure response of the sensor was evaluated by applying pressure using argon gas, demonstrating stable, rapid, and highly sensitive pressure responses. Long‐term sensor performance was quantitatively assessed, yielding consistent pressure responses after repeated thermal cycling and immersion in phosphate‐buffered saline, demonstrating the device's potential for operation in harsh environments, including in vivo biomedical applications.