Fig. 2

Characterization of the FPG. a Simulation results of temperature distribution in PI under fluence of 8, 18, and 22 J cm⁻². b Surface temperature and depth of carbonization by COMSOL simulation for each lamp fluence. c Top-view SEM images of PI irradiated at a fluence of pristine, 8, and 22 J cm⁻². d XPS spectra with atomic percent (at%), e FTIR spectra obtained at a fluence of pristine (black line), 8 (green line), and 22 (blue line) J cm⁻². f Raman spectra of the FPG at a fluence of 8 (green line), 18 (orange line), and 22 (blue line) J cm⁻². Inset shows the I_D/I_G and I_2D/I_G ratio at each lamp fluence. g Top-view HRTEM images of FPG with domain spacing of 3.3 Å at a lamp fluence of 22 J cm⁻². h Cross-sectional SEM image reveals a ~ 62 μm thick hollow pillar FPG at a lamp fluence of 22 J cm⁻². i Magnified SEM image showing the porous hollow pillar morphology including ~ 5 µm pores in the FPG