Self-assembly is the spontaneous gathering and regular arrangement of atoms, molecules and particles and is observed in various natural environments. There are many reports describing basic and applied research into artificial self-assembly, in particular "self-assembled monolayers (SAMs)" formed by the aggregation of organic molecules on a solid surface. A family of organic silicon compounds, silane coupling agents, can form SAMs on glass, alumina and other inorganic oxide surfaces. Thiols (R-SH) are used to forms SAMs on the surface of metal nanoparticles and metal substrates because of their high affinity for gold, silver and other transition metals. Phosphonic acids (R-PO3H2) combine with the surfaces of many inorganic oxides to form stable SAMs.