Molybdenum ditelluride

Molybdenum(IV) telluride, molybdenum ditelluride or just molybdenum telluride is an inorganic compound with formula MoTe2. It is a semiconductor, and can fluoresce. It is one of the transition metal dichalcogenides. As a semiconductor the band gap lies in the infrared region. It has potential use as a semiconductor in electronics or an infrared detector. MoTe2 is black. Although sometimes described as Mo4+, 2Te2-, it is not ionic but highly covalent.

Preparation MoTe2 can be prepared by heating the correct ratio of the elements together at 1100 °C in a vacuum. Another method is via vapour deposition, where molybdenum and tellurium are volatilised in bromine gas and then deposited. Using bromine results in forming an n-type semiconductor, whereas using tellurium only results in a p-type semiconductor. The amount of tellurium in molybdenum ditelluride can vary from 1.97 to 2. Excess tellurium deposited during this process can be dissolved off with sulfuric acid. It can crystallise in two dimensional sheets which can be thinned down to monolayers that are flexible and almost transparent. By annealing molybdenum film in a tellurium vapour at 850 to 870 K for several hours, a thin layer of MoTe2 is formed. An amorphous form can be produced by sonochemically reacting molybdenum hexacarbonyl with tellurium dissolved in decalin. Molybdenum ditelluride can be formed by electrodeposition from a solution of molybdic acid (H2MoO4) and tellurium dioxide (TeO2). The product can be electroplated on stainless steel or indium tin oxide. Tellurization of thin Mo film at 650 °C by chemical vapor deposition (CVD) leads to the hexagonal, semiconducting α-form (2H-MoTe2) while using MoO3 film produces the monoclinic, semimetallic β-form (1T'-MoTe2) at the same temperature of 650 °C.

Physical properties Very thin crystals of MoTe2 can be made using sticky tape. When they are thin around 500 nm thick red light can be transmitted. Even thinner layers can be orange or transparent. An absorption edge occurs in the spectrum with wavelengths longer than 6720 Å transmitted and shorter wavelengths heavily attenuated. At 77 K this edge changes to 6465 Å. This corresponds to deep red.

Infrared MoTe2 reflects about 43% in the infrared band but has a peak at 234.5 cm−1 and a minimum at 245.8 cm−1. As the temperature is lowered the absorption bands become narrower. At 77 K there are absorption peaks at 1.141, 1.230, 1.489, 1.758, 1.783, 2.049, 2.523, 2.578, and 2.805 eV. Exciton energy levels are at 1.10 eV, called A, and 1.48 eV, called B, with a difference of 0.38 eV.

Raman spectrum The Raman spectrum has four lines with wavenumbers of 25.4, 116.8, 171.4, and a double one at 232.4 and 234.5 cm−1. The peak at 234.5 cm−1 is due to E12g mode, especially in nanolayers, but the thicker forms and the bulk has the second peak at 232.4 cm−1 also perhaps due to the E21u phonon mode. The peak near 171.4 cm−1 comes from the A1g. 138 and 185 cm−1 peaks may be due to harmonics. B12g is assigned to a peak around 291 cm−1 in nanolayers with few layers. The E12g frequency increases as the number of layers decreases to 236.6 cm−1 for single layer. The A1g mode lowers its frequency as the number of layers decreases, becoming 172.4 cm−1 for the monolayer.

Crystal form

… excerpt ends here. Continue reading the full article.