Tuning Electronic Structure and Piezoresistivity of Graphene by Monovacancy Defect Concentration: A First-Principles Investigation.

Graphene, with its excellent mechanical and electrical properties, is an ideal candidate material for constructing high-performance piezoresistive sensors. However, lattice defects inevitably introduced during its preparation and transfer processes can significantly alter its electronic structure, thereby affecting the sensing performance of the devices. Based on first-principles calculations, this work systematically investigates the effects of monovacancy defect concentrations ranging from 2% to 8% on the geometric structure, electronic structure, and piezoresistive performance of graphene. The results show that monovacancy defects induce local lattice distortions and bond reconstructions, forming 5-9 non-hexagonal ring structures at defect concentrations of 4% and 8%. In terms of electronic structure, the defects break the lattice symmetry and open a band gap. High concentrations of defects lead to severe overlapping of electronic states, causing the band gap to first increase and then decrease with increasing defect concentration, reaching a maximum value of 0.697 eV at a concentration of 6%. Meanwhile, the defects introduce localized electronic states, enhance the electron localization effect, and render the system p-type doped. Regarding piezoresistive performance, monovacancy defects significantly improve the gauge factor of graphene. At a defect concentration of 6%, the gauge factor reaches 118.23, which is approximately 36 times that of pristine graphene. These findings reveal the microscopic mechanism of strain-dependent electronic modulation in defective graphene and provide theoretical support for defect engineering design in high-performance graphene-based piezoresistive sensors.