Superparamagnetic colloidal photonic structures

A method of forming chain-like colloidal assemblies, consisting of:

• mixing an iron salt precursor, a polar solvent, and a surfactant to form a first mixture;
• introducing a precipitation agent into the first mixture to initiate a hydrolysis reaction, wherein the precipitation agent is a base;
• controlling the hydrolysis reaction to occur at a temperature ranging from about 100° C. to about 320° C.;
• obtaining monodisperse superparamagnetic magnetite colloidal particles from the hydrolysis reaction, wherein each of the superparamagnetic magnetite colloidal particles is formed from a plurality of nanocrystals;
• controlling a size of the superparamagnetic magnetite colloidal particles from about 30 nanometers to about 300 nanometers based on a concentration of the base;
• rendering a surface of the superparamagnetic magnetite colloidal particles to be dispersible in nonaqueous solvents;
• dispersing the superparamagnetic magnetite colloidal particles in a nonaqueous polar solution;
• and
• applying an external magnetic field on the superparamagnetic magnetite colloidal particles so as to assemble the magnetite colloidal particles in chain-like structures that diffract light in the nonaqueous solution,
• wherein the colloidal particles within the chain-like structures are periodically arranged, with tunable periodicity and diffraction frequency under varying the external magnetic field.