A method for forming a nanowhisker of, e.g., a III-V semiconductor material on a silicon substrate, comprises: preparing a surface of the silicon substrate with measures including passivating the substrate surface by HF etching, so that the substrate surface is essentially atomically flat. Catalytic particles on the substrate surface are deposited from an aerosol; the substrate is annealed; and gases for a MOVPE process are introduced into the atmosphere surrounding the substrate, so that nanowhiskers are grown by the VLS mechanism. In the grown nanowhisker, the crystal directions of the substrate are transferred to the epitaxial crystal planes at the base of the nanowhisker and adjacent the substrate surface. A segment of an optically active material may be formed within the nanowhisker and bounded by heterojunctions so as to create a quantum well wherein the height of the quantum well is much greater than the thermal energy at room temperature, whereby the luminescence properties of the segment remain constant without quenching from cryogenic temperatures up to room temperature.