Fig. 1

a Simplified 3D schematic of the kernel structure of the e-KEH; b View of the full device from section SS’ in a; c The key design parameter of the hierarchical comb structure: side angle θ of the teeth. d Microscopic photographs of the KEHs

a Simplified 3D schematic of the kernel structure of the e-KEH; b View of the full device from section SS’ in a; c The key design parameter of the hierarchical comb structure: side angle θ of the teeth. d Microscopic photographs of the KEHs