A process for preparing an aqueous, alcoholic or colloidal hydro-alcoholic solution of amorphous nanoparticles of metal chalcogenide(s) of formula M-C wherein: - M has one or more identical or different first metals selected from Cu, Zn, Sn, Ge, Sb and Bi, preferably Cu, Zn, Sn and Sb, and - C represents one or more identical or different chalcogenide elements selected from S, Se and Te, characterized in that the following successive steps are carried out at a temperature of 0°C to 50°C, preferably of 20°C to 40°C, wherein: a) a first solution of at least one precursor of said first metal M, other than a chalcogenide C salt, is prepared in solution in a solvent consisting of pure acetonitrile or in a mixture with water and/or an alcohol other than methanol, preferably ethanol, and b) a second aqueous, alcoholic or hydro-alcoholic solution of at least one chalcogenide C precursor consisting of at least one chacogenide salt of a second metal, other than a first metal M, is prepared, the alcohol of said second solution being other than methanol, preferably ethanol, and c) both of said first and second precursor solutions are mixed at atmospheric pressure and room temperature until a crude colloidal solution comprising primary amorphous nanoparticles of sizes of less than 30 nm, preferably 3 to 20 nm, is obtained within a period of less than one minute, and d) the solid portion is separated from said colloidal solution of step c), preferably by centrifugation, to obtain a solid residue after removal of the liquid supernatant, and e) the solid residue obtained in step d) is rinsed by pouring an aqueous, alcoholic or hydro-alcoholic solution into it to form a colloidal solution, the alcohol of said aqueous, alcoholic or hydro-alcoholic colloidal solution being other than methanol, preferably ethanol, and f) the solid portion is separated again from said colloidal solution of step e), preferably by centrifugation, to obtain, after removal of the liquid supernatant, a rinsed solid residue in the form of a wet paste, and g) said wet paste of step f) is redispersed in a dispersion solvent comprising, preferably consisting of an aqueous, alcoholic or hydro-alcoholic solution, the alcohol of said alcoholic or hydro-alcoholic solution being, if need be, an alcohol other than methanol. - M has one or more identical or different first metals selected from Cu, Zn, Sn, Ge, Sb and Bi, preferably Cu, Zn, Sn and Sb, and - C represents one or more identical or different chalcogenide elements selected from S, Se and Te, a) a first solution of at least one precursor of said first metal M, other than a chalcogenide C salt, is prepared in solution in a solvent consisting of pure acetonitrile or in a mixture with water and/or an alcohol other than methanol, preferably ethanol, and b) a second aqueous, alcoholic or hydro-alcoholic solution of at least one chalcogenide C precursor consisting of at least one chacogenide salt of a second metal, other than a first metal M, is prepared, the alcohol of said second solution being other than methanol, preferably ethanol, and c) both of said first and second precursor solutions are mixed at atmospheric pressure and room temperature until a crude colloidal solution comprising primary amorphous nanoparticles of sizes of less than 30 nm, preferably 3 to 20 nm, is obtained within a period of less than one minute, and d) the solid portion is separated from said colloidal solution of step c), preferably by centrifugation, to obtain a solid residue after removal of the liquid supernatant, and e) the solid residue obtained in step d) is rinsed by pouring an aqueous, alcoholic or hydro-alcoholic solution into it to form a colloidal solution, the alcohol of said aqueous, alcoholic or hydro-alcoholic colloidal solution being other than methanol, preferably ethanol, and f) the solid portion is separated again from said colloidal solution of step e), preferably by centrifugation, to obtain, after removal of the liquid supernatant, a rinsed solid residue in the form of a wet paste, and g) said wet paste of step f) is redispersed in a dispersion solvent comprising, preferably consisting of an aqueous, alcoholic or hydro-alcoholic solution, the alcohol of said alcoholic or hydro-alcoholic solution being, if need be, an alcohol other than methanol.