An optoelectronic device comprising: a substrate ; a first layer or first portions covering the substrate , the first layer or the first portions having a thickness greater than or equal to 15 nm and comprising a first material having an extinction coefficient greater than or equal to 3 for any wavelength in the range from 380 nm to 650 nm; a second layer or second portions covering the first layer or the first portions and in contact with the first layer or with the first portions, the second layer or the second portions having a thickness smaller than or equal to 20 nm and comprising a second material having a refraction index in the range from 1 to 3 and an extinction coefficient smaller than or equal to 1.5 for any wavelength in the range from 380 nm to 650 nm; and assemblies having a structure of light-emitting diodes (LED) comprising wire, conical, or frustoconical semiconductor elements , the semiconductor elements being in contact with the second layer or with the second portions, the first layer or the first portions being interposed between the substrate and the semiconductor elements , wherein the second material is selected from the group comprising a nitride, a carbide, or a boride of a transition metal of column IV, V, or VI of the periodic table of elements or a combination of these compounds or is specifically selected from the group comprising aluminum nitride (AlN), boron (B), boron nitride (BN), titanium (Ti), titanium nitride (TiN), tantalum (Ta), tantalum nitride (TaN), hafnium (Hf), hafnium nitride (HfN), niobium (Nb), niobium nitride (NbN), zirconium (Zr), zirconium borate (ZrB 2 ), zirconium nitride (ZrN), silicon carbide (SiC), tantalum carbonitride (TaCN), magnesium nitride in Mg x N y form, where x is approximately equal to 3 and y is approximately equal to 2, for example, magnesium nitride in Mg 3 N 2 form or magnesium gallium nitride (MgGaN), tungsten (W), tungsten nitride (WN) or a combination of these compounds. a substrate ; a first layer or first portions covering the substrate , the first layer or the first portions having a thickness greater than or equal to 15 nm and comprising a first material having an extinction coefficient greater than or equal to 3 for any wavelength in the range from 380 nm to 650 nm; a second layer or second portions covering the first layer or the first portions and in contact with the first layer or with the first portions, the second layer or the second portions having a thickness smaller than or equal to 20 nm and comprising a second material having a refraction index in the range from 1 to 3 and an extinction coefficient smaller than or equal to 1.5 for any wavelength in the range from 380 nm to 650 nm; and assemblies having a structure of light-emitting diodes (LED) comprising wire, conical, or frustoconical semiconductor elements , the semiconductor elements being in contact with the second layer or with the second portions, the first layer or the first portions being interposed between the substrate and the semiconductor elements , wherein the second material is selected from the group comprising a nitride, a carbide, or a boride of a transition metal of column IV, V, or VI of the periodic table of elements or a combination of these compounds or is specifically selected from the group comprising aluminum nitride (AlN), boron (B), boron nitride (BN), titanium (Ti), titanium nitride (TiN), tantalum (Ta), tantalum nitride (TaN), hafnium (Hf), hafnium nitride (HfN), niobium (Nb), niobium nitride (NbN), zirconium (Zr), zirconium borate (ZrB 2 ), zirconium nitride (ZrN), silicon carbide (SiC), tantalum carbonitride (TaCN), magnesium nitride in Mg x N y form, where x is approximately equal to 3 and y is approximately equal to 2, for example, magnesium nitride in Mg 3 N 2 form or magnesium gallium nitride (MgGaN), tungsten (W), tungsten nitride (WN) or a combination of these compounds. <EOS>