This disclosure provides systems, methods, and apparatus related to heterojunctions in halide perovskite nanostructures. In one aspect, a nanostructure comprises a first region and a second region. The first region comprises ABX3, with A being selected from a group consisting of Cs and Rb, with B being selected from a group consisting of Sn and Pb, and with X being selected from a group consisting of Br, Cl, and Br and Cl. The second region comprises ABY3, with Y being selected from a group consisting of Br, I, and Br and I. Compositions of the first region of the nanostructure and the second region of the nanostructure are different.