Method for producing thin film having high refractive index and high transparency, and thin film produced by the method

A method for producing a thin film without using any resin binder, comprising:

• a first step of forming, on a substrate, a coating using a sol comprising a metal oxide, wherein the metal of the metal oxide is at least one metal selected from the group consisting of titanium, zirconium, hafnium, aluminum, zinc, and tin, modified with a phosphorus compound of the following formula (1):
• wherein:
• R1 is a hydrogen atom or a linear or branched alkyl group having 1 to 5 carbon atoms;
• R2 is a divalent organic residue of the following formula (2):
• YOp (2)
• wherein Y is a linear or branched alkylene group having 1 to 10 carbon atoms;
• and p is an integer of 1 to 10;
• and
• n is 1 or 2,
• a second step of curing the coating formed on the substrate obtained in the first step by light irradiation to obtain a cured film, and
• a third step of further adding energy to the cured film obtained in the second step by heating and/or light irradiation.