A core/shell semiconductor nanoparticle structure comprises a core comprising a halide perovskite semiconductor and a shell comprising a semiconductor material that is not a halide perovskite (and that is substantially free of halide perovskites). The halide perovskite semiconductor core may be of the form AMX3, wherein: A is an organic ammonium such as CH3NH3+, (C8H17)2(CH3NH3)+, PhC2H4NH3+, C6H11CH2NH3+ or 1-adamantyl methyl ammonium, an amidinium such as CH(NH2)2+, or an alkali metal cation such as Li+, Na+, K+, Rb+ or Cs+; M is a divalent metal cation such as Mg2+, Mn2+, Ni2+, Co2+, Pb2+, Sn2+, Zn2+, Ge2+, Eu2+, Cu2+ or Cd2+; and X is a halide anion (F−, Cl−, Br−, I−) or a combination of halide anions.