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
MoTe2 commonly exists in three crystalline forms with rather similar layered structures: hexagonal α (2H-MoTe2), monoclinic β (1T-MoTe2) and orthorhombic β' (1T'-MoTe2). At room temperature it crystallises in the hexagonal system similar to molybdenum disulfide. Crystals are platy or flat. MoTe2 has unit cell sizes of a=3.519 Å c=13.964 Å and a specific gravity of 7.78 g·cm−3. Each molybdenum atom is surrounded by six tellurium atoms in a trigonal prism with the separation of these Mo and Te atoms being 2.73 Å. This results in sublayers of molybdenum sandwiched between two sublayers of tellurium atoms, and then this three layer structure is stacked. Each layer is 6.97 Å thick. Within this layer two tellurium atoms in the same sublayer subtend an angle of 80.7°. The tellurium atoms on one sublayer are directly above those in the lower sublayer, and they subtend an angle of 83.1° at the molybdenum atom. The other Te-Mo-Te angle across sublayers is 136.0°. The distance between molybdenum atoms within a sublayer is 3.518 Å. This is the same as the distance between tellurium atoms in a sublayer. The distance between a tellurium atom in one sublayer and the atom in the other sublayer is 3.60 Å.
The layers are only bonded together with van der Waals force. The distance between tellurium atoms across the layers is 3.95 Å. The tellurium atom at the bottom of one layer is aligned with the centre of a triangle of tellurium atoms on the top of the layer below. The layers are thus in two different positions. The crystal is very easily cleaved on the plane between the three layer sheets. The sizes change with temperature, at 100 K a=3.492 Å and at 400 K is 3.53 Å. In the same range c changes from 13.67 Å to 14.32 Å due to thermal expansion. The hexagonal form is also called 2H-MoTe2, where "H" stands for hexagonal, and "2" means that the layers are in two different positions. Every second layer is positioned the same.
At temperatures above 900 °C MoTe2 crystallises in the monoclinic 1T form (β–MoTe2), with space group P21/m with unit cell sizes of a=6.33 Å b=3.469 Å and c=13.86 Å with the angle β=93°55′. The high-temperature form has rod shaped crystals. The measured density of this polymorph is 7.5 g·cm−3, but in theory it should be 7.67 g·cm−3. Tellurium atoms form a distorted octahedron around the molybdenum atoms. This high-temperature form, termed β–MoTe2 can be quenched to room temperature by rapid cooling. In this metastable state β-MoTe2 can survive below 500 °C. When metastable β–MoTe2 is cooled below −20 °C, its crystal form changes to orthorhombic. This is because the monoclinic angle c changes to 90°. This form is called β' or, misleadingly, Td.
The transition from α- to β-MoTe2 happens at 820 °C, but if Te is reduced by 5% the required transition temperature increases to 880 °C. K. Ueno and K. Fukushima claim that when the α form is heated in a low or high vacuum that it oxidises to form MoO2 and that reversible phase transitions do not take place.
In bulk, MoTe2 can be produced as a single crystal with difficulty, but can also be made as a powder, as a polycrystalline form, as a thin film, as a nanolayer consisting of a few TeMoTe sheets, a bilayer consisting of two sheets or as a monolayer with one sheet.
Thin nanolayer forms of α-MoTe2 have different symmetry depending on how many layers there are. With an odd number of layers the symmetry group is D13h without inversion, but for an even number of layers, the lattice is the same if inverted and the symmetry group is D33d. Nanotubes with a 20–60 nm diameter can be made by heat treating amorphous MoTe2.
Electrical N-type bulk α-MoTe2 has an electrical conductivity of 8.3 Ω−1cm−1 with 5×1017 mobile electrons per cubic centimeter. P-type bulk MoTe2 has an electrical conductivity of 0.2 Ω−1cm−1 and a hole concentration of 3.2×1016 cm−3. The peak electrical conductivity is around 235 K, dropping off slowly with decreasing temperatures, but also reducing to a minimum around 705 K. Above 705 K conductivity increases again with temperature. Powdered MoTe2 has a much higher resistance.
β–MoTe2 has a much lower resistivity than α–MoTe2 by more than a thousand times with values around 0.002 Ω·cm. It is much more metallic in nature. In the β form the molybdenum atoms are closer together so that the conduction band overlaps. At room temperature resistivity is 0.000328 Ω·cm.
Orthorhombic MoTe2 has a resistance about 10% lower than the β form, and the resistance shows hysteresis of several degrees across the transition point around 250 K. The resistance drops roughly linearly with decreasing temperature. At 180 K resistivity is 2.52×10−4 Ω·cm, and at 120 mK the material becomes a superconductor. Since orthorhombic MoTe2 breaks spatial inversion symmetry, it exhibits ferroelectricity which can be coupled to its innate superconductivity. This coupling was leveraged to create a superconducting switch with MoTe2.
At low electric current levels the voltage is proportional to the current in the α form. With high electric currents MoTe2 shows negative resistance, where as the current increases the voltage across the material decreases. This means there is a maximum voltage that can be applied. In the negative resistance region the current must be limited, otherwise thermal runaway will destroy the item made from the material.
The Hall constant at room temperature is around 120 cm3/Coulomb for stochiometric α-MoTe2. But as Te is depleted the constant drops to close to 0 for compositions in the range MoTe1.94 to MoTe1.95.
The Seebeck coefficient is about 450 μV/K at room temperature for pure MoTe2, but this drops to 0 for MoTe1.95. The Seebeck coefficient increases as temperature drops.
Band gap In the bulk α form of MoTe2 the material is a semiconductor with a room temperature indirect band gap of 0.88 eV and a direct band gap of 1.02 eV. If instead of bulk forms, nanolayers are measured, the indirect band gap increases as the number of layers is reduced. α-MoTe2 changes from an indirect to a direct band gap material in very thin slices. It is a direct bandgap material when it is one or two layers (monolayer or bilayer).
The band gap is reduced for tellurium-deficient MoTe2 from 0.97 to 0.5. The work function is 4.1 eV.
Magnetism α–MoTe2 is diamagnetic whereas β–MoTe2 is paramagnetic.
X-ray X-ray photoelectron spectroscopy on clean MoTe2 crystal surfaces show peaks at 231 and 227.8 eV due to molybdenum 3d3/2 and 3d5/2; with 582.9 and 572.5 due to tellurium 3d3/2 and 3d5/2 electrons.
The X-ray K absorption edge occurs at 618.41±0.04 X units compared to molybdenum metal at 618.46 xu.
Microscopy Atomic force microscopy (AFM) of the van der Waals surface of α-MoTe2 shows alternating rows of smooth balls, which are the tellurium atoms. AFM images are often done on a silica (SiO2) surface on silicon. A monolayer of α-MoTe2 has its surface 0.9 nm above the silica, and each extra layer of α-MoTe2 adds 0.7 nm.
Scanning tunneling microscopy (STM) of α-MoTe2 reveals a hexagonal grid like chicken wire, where the molybdenum atoms are contributing to the current. Higher bias voltages are required to get an image, either over 0.5 V or below −0.3 V.
β-MoTe2 surfaces examined with scanning tunneling microscopy can show either a pattern of tellurium atoms or a pattern of molybdenum atoms on different parts. When the scanning tip is further from the surface only tellurium atoms are visible. This is explained by the dz2 orbitals from molybdenum penetrating up through the surface layer of tellurium. The molybdenum can supply a much bigger current than tellurium. But at greater distance only the p orbital from tellurium can be detected. Lower voltages than used for α form still produce atomic images.
Friction force microscopy (FFM) has been used to get a slip-stick image at a resolution below that of the unit cell.
Thermal