Table 3.

The currently reported SERS substrates with different fabrication processes.

Strategy	Method	Substrates	Structure	Fabrication Time	Refs.
Bottom-up	chemical vapor deposition	Si nanowire	Si nanowire around 2–3 μm long, with an average diameter at the bottom of about 40–70 nm	-	[120,143]
		Fractal structured TiO2 network	TiO2 nano fractal structured	Form TiO2 for 20 min at 65 °C, and calcination process at 650 °C for 1 h in an air atmosphere.	[129]
	chemical bath deposition	MoO3	MoO3 sea urchin-like microstructures	The reaction mixture was heated to 90 °C for 3 h.	[144]
	3D printing	photosensitive resin transfer to PDMS	Micro needle: the height, bottom diameter, and top diameter of the needle are about 880, 470, and 25 μm, respectively	-	[124]
			3D arrays of pores of 2 μm in diameter and 1.4, 1.8 and 2.2 μm in depth	-	[145]
	Self-assembly particles	PS beads	1.5 μm-diameter monodisperse PS spheres	24 h for PS beads Self-assembly	[123]
	Chemical synthesis	CuO nanorods	CuO nanorods structure has a smooth surface with a diameter ranging from 50 to 100 nm, and the length of the rods is estimated to be 5 μm.	heated at 500 °C for 120 min	[126]
		AAO nanopores	46–72 nm nanopores in 4 cm2	3 min anodization and 10 min etching	[130]
			nanopores with 100 nm depth and a 80 nm diameter.	7 h anodization and total 5 h etching	[135]
Top-down	Chemical etching	Si micropyramid	8.4 µm Si micropyramid in 4 inches wafer	85 °C for 90 min etching in KOH	[119]
	Chemical etching	Porous Au	the width of the Au ligaments is in the range of 40 nm	copper etching of the stacked multilayers at room temperature for 300 min to form the porous Au	[149]
	Metal assisted chemical etching	Si nanowires	440–480 nm Si nanowires	Etching in the solution containing AgNO3,hydrofluoric acid (HF), and DI water for 6 min.	[121]
	SF6/O2 gas reactive ion etching	Si nanopillar	Si nanopillar with height of 639–2217 nm, apex thickness of 112–144 nm.	SF6/O2 gas reactive ion etching for 18–30 min.	[150]
	Laser processing	ZnO	Nanostrip, nanopillar array, nanogrooves, nanocavities.	-	[125]