NOVELTY - Preparing a semiconducting core-shell nanoparticles comprising elements of the groups IB, IIB, IIIA, IVA, VA or VIA, comprises: (a) dissolving core reaction mixture comprising at least one cation metal precursor and an anion precursor in a ligand and solvent mixture; (b) heating core reaction mixture; (c) maintaining heated reaction mixture for core growth; (d) adding shell reaction mixture; (e) heating the reaction mixture; (f) optionally separately preparing zinc sulfide shell preparation shell reaction mixture; (g) heating reaction mixture of step (f); and (h) cooling reaction mixture.    USE - The method is useful for preparing a semiconducting core-shell nanoparticles, which is used in: a formulation; a device, preferably an electronic device (all claimed); and in bio-labeling, light-emitting diode, thin-film solar cell and polymer solar cell.    ADVANTAGE - The method provides semiconducting core-shell nanoparticles which has an emission maximum at wavelengths of 580-800 nm and a quantum yield of greater than or equal to 60% (claimed).    DETAILED DESCRIPTION - Preparing a semiconducting core-shell nanoparticles comprising elements of the groups IB, IIB, IIIA, IVA, VA or VIA, comprises: (a) dissolving a core reaction mixture comprising at least one cation metal precursor and an anion precursor in a ligand and solvent mixture at a temperature that is still beneath the threshold at which nucleation takes place; (b) heating core reaction mixture above nucleation threshold temperature and then maintaining at the reaction temperature for core nucleation; (c) maintaining the heated reaction mixture at a temperature well suited for core growth but lower than the nucleation temperature; (d) adding shell reaction mixture prepared separately comprising at least one cation metal precursor or an anion precursor and heated at a temperature that is still beneath the threshold at which nucleation takes place to the reaction mixture of step (b) or (c) for shell coating of the core nanoparticles; (e) heating the reaction mixture of step (d) and holding to a shell growth temperature; (f) optionally separately preparing zinc sulfide shell preparation shell reaction mixture comprising zinc sulfide shell material and heating at a temperature that is still beneath the threshold at which nucleation takes place and adding to the reaction mixture of step (d) or (e) for shell coating of the nanoparticles; (g) heating reaction mixture of step (f) and holding to a shell growth temperature; and (h) cooling reaction mixture to prevent further particle growth, where the composition of the core nanoparticle comprises IB-VIA, IIB-VIA, IIIA-VIA, IB-IIIA-VIA or IB-IIB-IVA-VIA, IIB-IIIA-VIA and/or IIB-IVA-VA, and at least one shell comprises elements IIB or VIA, where in case the core composition is IIB-VIA the shell comprises at least a further element comprising IB, IIIA, IVA or VA. An INDEPENDENT CLAIM is also included for the semiconducting core-shell nanoparticle obtained by the method and having the formulae (-(AB1C12-AxByZn2-x-yC12), (-AxB1yZn2-x-yC12-ZnS), (-AB1C12-AxB1yZn2-x-yC12-ZnS), (-ZnS-AB1C12-AxB1yZn2-x-yC12-ZnS), (-ZnS-AxB1yZn2-x-yC12-ZnS) or (-ZnS-AxB1yZn2-x-yC12-AB1C12-AxB1yZn2-x-yC12-ZnS).    x = greater than 0;    y = less than 1 with no preference on the value for modulus of x-y with a gradual increase of x and y from the particle surface to the remaining core;    A = at least one element of the group IB, preferably Cu or Ag;    B1 = at least one element of the group IIIA, preferably Al, Ga or In; and    C1 = at least one element of the group VIA, preferably O, S, Se or Te.