Platinum diselenide
Platinum diselenide is a transition metal dichalcogenide (TMDC) consisting of the metal platinum and the non-metal selenium with the formula PtSe. Being a layered substance, PtSe can be split into thin layers down to three atoms thick called monolayers. PtSe is a semimetal or semiconductor depending on thickness. Formation. Minozzi was the first to report synthesising platinum diselenide from the elements in 1909. Platinum diselenide can be formed by heating thin foils of platinum in selenium vapour at 400 °C. A platinum 111 surface exposed to selenium vapour at 270 °C forms a monolayer of  PtSe. In addition to these selenization methods, PtSe can be made by precipitation in water solution of Pt(IV) treated with hydrogen selenide, or by heating platinum tetrachloride with elemental selenium. Natural occurrence. Platinum diselenide occurs naturally as the mineral Sudovikovite. It was named after Russian petrologist, N.G. Sudovikov who lived from 1903 to 1966. The mineral's hardness is 2 to 2. Sudovikovite was found in the Srednyaya Padma mine, Velikaya Guba uranium-vanadium deposit, Zaonezhie peninsula, Karelia Republic, Russia. Properties. Platinum diselenide forms crystals in the cadmium iodide structure. This means that the substance forms layers. Each of the  monolayers has a central bed of platinum atoms, with a sheet of selenium atoms above and below. This structure is also called "1T" and has an trigonal structure. The layers are only weakly bonded together, and it is possible to exfoliate layers to bilayers or monolayers. In bulk the material is semi-metallic, but when reduced to few layers it becomes a semiconductor. The conductivity of the bulk material is 620,000 S/m. The XPS spectrum shows a peak at 72.3 eV from Pt 4f core, and also has peaks from Pt 5p and Se 3d and 3d at  55.19and 54.39 eV. Phonon vibrations are designated by the infrared active A (Se vibrating out of plane opposite to Pt), E (in layer vibration, Se opposite to Pt), and Raman active A (Se top and bottom atoms moving out of plane in opposite directions 205 cm), and E (In plane, top and bottom Se atoms moving opposite 175 cm). In the Raman spectrum, the A is lessened when stimulated emissions polarised perpendicular to the incoming rays are measured. The E mode is red-shifted when more layers are stacked. (166 cm for bilayer and 155 cm for bulk material) The  A emission only has a slight change when thickness varies. The band gap is calculated as 1.2 eV for monolayers, and 0.21 eV for bilayers. For a trylayer or thicker the substance loses a bandgap and becomes semimetallic. PtSe can change its conductance in the presence of particular gases, such as nitrogen dioxide. Within a few seconds, NO absorbs on the surface of the PtSe material and lowers the resistance. When the gas is absent, high resistance returns again in about a minute. The Seebeck coefficient of PtSe is 40 μV/K. Although pristine platinum diselenide is nonmagnetic, the presence  of platinum vacancies and strain were predicted to induce magnetism. Later magneto-transport studies have indeed shown that defective PtSe exhibits magnetic properties. Due to RKKY interaction between magnetic Pt-vacancies, this results in layer-dependent ferromagnetic or anti-ferromagnetic behavior. Monolayers of platinum diselenide show helical spin texture, which is not expected for centrosymmetric materials such as this. This property could be due to a local dipole induced Rashba effect. It means that PtSe is a potential spintronics material. Reactions. Water can physisorb to the surface of platinum diselenide with an energy of −0.19 eV, and similarly for oxygen  with energy −0.13 eV. Water and oxygen do not react at toom temperature, because significant energy would be required to break apart the molecules. Comparison. Palladium diselenide has a different modified pyrite structure. Palladium ditelluride has a similar structure to platinum diselenide. Platinum disulfide is a semiconductor, and platinum ditelluride is metallic in nature. More complex substances with platinum and selenium also exist, including the quaternary chalcogenides RbPtUSe and CsPtUSe Jacutingaite is a ternary platinum selenide HgPtSe. Use. Platinum diselenide is proposed for use as a mid-infra red detector superior to black phosphorus in durability in air. Also it  can work as a catalyst, and can be built into field effect transistors. Combined with graphene it can be a photocatalyst, converting water and oxygen to reactive hydroxyl radical and superoxide. This reaction works when photons produce holes and electrons. The holes can neutralise hydroxide to make hydroxyl, and the electrons attach to oxygen to make superoxide. These reactive species can mineralise organic matter.