Chemically-modified surfaces on unoxidized, bromine- or iodine-terminated carbon, silicon, and germanium substrates are disclosed. Visible light mediates the reaction of protected &ohgr;-modified, &agr;-unsaturated aminoalkenes (preferred) with bromine- or iodine-terminated carbon, silicon, or germanium surfaces. Removal of the protecting group yields an aminoalkane-modified silicon surface. These amino groups can be coupled to terminal-modified oligonucleotides using a bifunctional crosslinker, thereby permitting the preparation of modified surfaces and arrays. Methods for controlling the surface density of molecules attached to the substrate are also disclosed.