A unique ID using optically visible carbon nanotubes with nanocrystal decoration is provided. In one aspect, a method for creating a unique ID includes: providing a substrate having an array of trenches; randomly placing carbon nanotubes throughout the array such that each trench either contains a carbon nanotube or does not, wherein the random placement of the carbon nanotubes throughout the array of trenches includes code information that forms the unique ID; and coating the carbon nanotubes with optically visible nanocrystals. A unique ID and authentication method using the unique ID are also provided.