Table 2.

Nanostructure preparation techniques and their minimum feature size.

Fabrication method	Min. feature size	Advantages/limitations
Optical lithography [29,31,32,52]	~50 nm	Precise patterns can be created.
X-ray lithography [30,52]	~20 nm	Requires expensive optical lenses.
E-beam lithography [27,52–55]	5–10 nm	Controlled geometries can be achieved. Serial, time consuming process.
Colloidal lithography [52,56]	~300 nm	Random geometries.
Nano-embossing [34]	~100 nm	Controllable geometries and patterns.
Etching polymer [35]	>1 nm	Inexpensive method but produces random nanostructures.
Etching silicon wafer [36–38]	2–3 nm	No control on dimensions and pore sizes unless using aluminum template.
Reactive ion etching [39]	20–100 nm	High aspect ratio patterns.
Electrospinning [44]	40–2000 nm	Over 20 polymers can be used; limited to only fiber formation.
Chemical vapor deposition [45–48]	~2 nm	Require high vacuum furnaces, slow process especially in the case of epitaxy but layer thickness can be well controlled.
Vapor-phase coating [49]	100–500 nm	Easy fabrication method but the coatings are not very stable.
Gas foaming [50]	0.1–100 nm	Low-cost method but not widely applicable.
Phase separation [51]	>1 nm	Nanofeatures and pore sizes cannot be controlled precisely.