Formation model of the ultra-sharp nanotip array. (a) Photoresist coated and patterned on the silicon substrate (b) after 3-min etching, (c) after 4-min etching, and (d) after 6-min etching. (e) After 10-min etching, a pyramid-like tip was formed. (f) A 50-nm-thick silicon oxide was thermally grown. (g) After silicon oxide removal, an ultra-sharp nanotip is formed. (h) A 3-D schematic representation of the ultra-sharp silicon nanotip array.