There¡¯s little that can surprise us anymore about the remarkable capabilities of carbon, in the atom-thin sheet of graphene ¨C but some simple inorganic compounds have structures with similar potential. High amongst them is the molybdenum-sulfur compound molybdenum disulfide.
As the name suggests, this is MoS2, occurring naturally as the ore molybdenite, from which molybdenum itself is derived. Not only does molybdenum disulfide have some similar tricks to graphene up its sleeve, it has a physical resemblance to graphite, as a crystalline black solid with highlights of shiny grey. Although the chemical nature of the compound was only discovered in the late eighteenth century with the isolation of molybdenum, the mineral has been used since ancient times, when it was often confused with graphite and sometimes with lead sulfide or galena (the word molybdenum itself comes from molybdaena which was a lead ore).
Molybdenum disulfide¡¯s thinnest structure is only three atoms deep, with a central layer of molybdenum atoms linked to layers of sulfur on either side. This form was only discovered in 2011 and was originally produced, like graphene, by using adhesive tape to peel off a single layer ¨C though more recently by growing sheets on a silicon wafer. These thin, transparent sheets have limited interaction, sliding over each other easily, making a dusting of small particles of molybdenum disulfide an excellent dry lubricant, capable of withstanding high temperatures ¨C up to around 400 ¡ãC. Its melting point is even higher at an impressive 1,185 ¡ãC, but above 400 degrees it tends to oxidize unless it¡¯s in an oxygen-free atmosphere. Molybdenum disulfide is also mixed into more conventional grease lubricants (often given the pet name ¡®Moly¡¯, spelled m-o-l-y) to give added anti-wear action.