Bismuth telluride
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Bismuth telluride (Bi2Te3) is a gray powder that is a compound of
bismuth Bismuth is a chemical element with the Symbol (chemistry), symbol Bi and atomic number 83. It is a post-transition metal and one of the pnictogens, with chemical properties resembling its lighter group 15 siblings arsenic and antimony. Elemental ...
and
tellurium Tellurium is a chemical element with the symbol Te and atomic number 52. It is a brittle, mildly toxic, rare, silver-white metalloid. Tellurium is chemically related to selenium and sulfur, all three of which are chalcogens. It is occasionally fou ...
also known as bismuth(III) telluride. It is a
semiconductor A semiconductor is a material which has an electrical resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...
, which, when alloyed with
antimony Antimony is a chemical element with the symbol Sb (from la, stibium) and atomic number 51. A lustrous gray metalloid, it is found in nature mainly as the sulfide mineral stibnite (Sb2S3). Antimony compounds have been known since ancient time ...
or
selenium Selenium is a chemical element with the symbol Se and atomic number 34. It is a nonmetal (more rarely considered a metalloid) with properties that are intermediate between the elements above and below in the periodic table, sulfur and tellurium, ...
, is an efficient
thermoelectric The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice versa via a thermocouple. A thermoelectric device creates a voltage when there is a different temperature on each side. Conversely, when ...
material for refrigeration or portable power generation. Bi2Te3 is a
topological insulator A topological insulator is a material whose interior behaves as an electrical insulator while its surface behaves as an electrical conductor, meaning that electrons can only move along the surface of the material. A topological insulator is an ...
, and thus exhibits thickness-dependent physical properties.


Properties as a thermoelectric material

Bismuth telluride is a narrow-gap layered semiconductor with a trigonal unit cell. The valence and conduction band structure can be described as a many-ellipsoidal model with 6 constant-energy ellipsoids that are centered on the reflection planes. Bi2Te3 cleaves easily along the trigonal axis due to
Van der Waals bonding In molecular physics, the van der Waals force is a distance-dependent interaction between atoms or molecules. Unlike ionic or covalent bonds, these attractions do not result from a chemical electronic bond; they are comparatively weak and th ...
between neighboring tellurium atoms. Due to this, bismuth-telluride-based materials used for power generation or cooling applications must be polycrystalline. Furthermore, the
Seebeck coefficient The Seebeck coefficient (also known as thermopower, thermoelectric power, and thermoelectric sensitivity) of a material is a measure of the magnitude of an induced thermoelectric voltage in response to a temperature difference across that material ...
of bulk Bi2Te3 becomes compensated around room temperature, forcing the materials used in power-generation devices to be an alloy of bismuth, antimony, tellurium, and selenium. Recently, researchers have attempted to improve the efficiency of Bi2Te3-based materials by creating structures where one or more dimensions are reduced, such as nanowires or thin films. In one such instance n-type bismuth telluride was shown to have an improved
Seebeck coefficient The Seebeck coefficient (also known as thermopower, thermoelectric power, and thermoelectric sensitivity) of a material is a measure of the magnitude of an induced thermoelectric voltage in response to a temperature difference across that material ...
(voltage per unit temperature difference) of −287 μV/K at 54 °C, However, one must realize that Seebeck coefficient and electrical conductivity have a tradeoff: a higher Seebeck coefficient results in decreased carrier concentration and decreased electrical conductivity. In another case, researchers report that bismuth telluride has high
electrical conductivity Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allow ...
of 1.1×105 S·m/m2 with its very low lattice
thermal conductivity The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa. Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...
of 1.20 W/(m·K), similar to ordinary
glass Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenching) of ...
.


Properties as a topological insulator

Bismuth telluride is a well-studied topological insulator. Its physical properties have been shown to change at highly reduced thicknesses, when its conducting surface states are exposed and isolated. These thin samples are obtained through either
epitaxy Epitaxy refers to a type of crystal growth or material deposition in which new crystalline layers are formed with one or more well-defined orientations with respect to the crystalline seed layer. The deposited crystalline film is called an epit ...
or mechanical exfoliation. Epitaxial growth methods such as molecular beam epitaxy and metal organic chemical vapor deposition are common methods of obtaining thin Bi2Te3 samples. The stoichiometry of samples obtained through such techniques can vary greatly between experiments, so
Raman spectroscopy Raman spectroscopy () (named after Indian physicist C. V. Raman) is a spectroscopic technique typically used to determine vibrational modes of molecules, although rotational and other low-frequency modes of systems may also be observed. Raman sp ...
is often used to determine relative purity. However, thin Bi2Te3 samples are resistant to Raman spectroscopy due to their low melting point and poor heat dispersion. The crystalline structure of Bi2Te3 allows for mechanical exfoliation of thin samples by cleaving along the trigonal axis. This process is significantly lower in yield than epitaxial growth, but produces samples without defects or impurities. Similar to extracting
graphene Graphene () is an allotrope of carbon consisting of a single layer of atoms arranged in a hexagonal lattice nanostructure.
from bulk graphite samples, this is done by applying and removing adhesive tape from successively thinner samples. This procedure has been used to obtain Bi2Te3 flakes with a thickness of 1 nm. However, this process can leave significant amounts of adhesive residue on a standard Si/SiO2 substrate, which in turn obscure
atomic force microscopy Atomic force microscopy (AFM) or scanning force microscopy (SFM) is a very-high-resolution type of scanning probe microscopy (SPM), with demonstrated resolution on the order of fractions of a nanometer, more than 1000 times better than the op ...
measurements and inhibit the placement of contacts on the substrate for purposes of testing. Common cleaning techniques such as oxygen plasma, boiling
acetone Acetone (2-propanone or dimethyl ketone), is an organic compound with the formula . It is the simplest and smallest ketone (). It is a colorless, highly volatile and flammable liquid with a characteristic pungent odour. Acetone is miscib ...
and
isopropyl alcohol Isopropyl alcohol (IUPAC name propan-2-ol and also called isopropanol or 2-propanol) is a colorless, flammable organic compound with a pungent alcoholic odor. As an isopropyl group linked to a hydroxyl group (chemical formula ) it is the simple ...
are ineffective in removing residue.


Occurrence and preparation

The
mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. ( ...
form of Bi2Te3 is
tellurobismuthite Tellurobismuthite, or tellurbismuth, is a telluride mineral: bismuth telluride (bismuth, Bi2tellurium, Te3). It crystallizes in the trigonal system. There are natural cleavage planes in the (0001) direction as the crystal is effectively lamellar (l ...
which is moderately rare. There are many natural bismuth tellurides of different
stoichiometry Stoichiometry refers to the relationship between the quantities of reactants and products before, during, and following chemical reactions. Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equal ...
, as well as compounds of the Bi-Te-S-(Se) system, like Bi2Te2S (
tetradymite Tetradymite is a mineral consisting of bismuth, tellurium and sulfide, Bi2 Te2 S, also known as telluric bismuth. If sulfur is absent the mineral is tellurobismuthite and the formula is then Bi2Te3. Traces of selenium are usually present. Crysta ...
). Bismuth telluride may be prepared simply by sealing mixed powders of bismuth and tellurium metal in a quartz tube under vacuum (critical, as an unsealed or leaking sample may explode in a furnace) and heating it to 800 °C in a
muffle furnace A muffle furnace or muffle oven (sometimes retort furnace in historical usage) is a furnace in which the subject material is isolated from the fuel and all of the products of combustion, including gases and flying ash. After the development of ...
.


See also

*
Thermoelectric materials Thermoelectric materials show the thermoelectric effect in a strong or convenient form. The ''thermoelectric effect'' refers to phenomena by which either a temperature difference creates an electric potential or an electric current creates a t ...
*
Thermoelectric effect The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice versa via a thermocouple. A thermoelectric device creates a voltage when there is a different temperature on each side. Conversely, when ...
*
Topological insulator A topological insulator is a material whose interior behaves as an electrical insulator while its surface behaves as an electrical conductor, meaning that electrons can only move along the surface of the material. A topological insulator is an ...


References


External links

*
Bismuth Telluride and its Alloys (Chapter 4.5 of Martin Wagner dissertation)
{{Tellurides Bismuth compounds Tellurides Semiconductor materials Thermoelectricity