This method relates to the preparation by ALD of a thin film of formula MYx, x being in the range from 1.5 to 3.1.According to this method, MYx is deposited by ALD on a substrate, from at least one precursor of metal M, and at least one precursor of element Y; M being tungsten and/or molybdenum; the degree of oxidation of metal M in the precursor of metal M being in the range from 3 to 6; the metal of the precursor of metal M only including simple or multiple bonds M-Z and/or M-M with Z=C, N, H, and any combination of these atoms; Y being sulfur and/or selenium; the substrate temperature being lower than or equal to 350° C.