2D Materials

2D Materials, sometimes referred to as Single layer materials are materials made consisting of a single layer of atoms or molecules. Since the isolation of graphene (a single-layer of graphite) in 2004, a large amount of research has been directed at isolating other 2D materials due to their unusual characteristics and for use in applications such as photovoltaics, semiconductors, water purification and many others.

2D materials can generally be catigorised as either elemental (2D allotropes of various elements) or compounds (usually consisting of two covalently bonding elements). The elemental 2D materials generally have –ene as the suffix to their names while the compounds have –ane or -ide as their suffixes.

The global market for such materials (mostly graphene) is reported to have reached $9 million by 2014, mostly in the semiconductor, electronics, battery energy and composites markets.[1]

2D Allotropes

Graphene

For more details on this topic, see Potential applications of graphene.
Graphene is an atomic-scale honeycomb lattice of carbon atoms.

Graphene is a crystalline allotrope of carbon in the form of a nearly transparent (to visible light) one atom thick sheet. It is hundreds of times stronger than most steels by weight.[2] It has the highest thermal and electrical conductivity known to man, displaying current densities 1,000,000 times that of copper.[3] It was first produced in 2004.[4]

Andre Geim and Konstantin Novoselov won the 2010 Nobel Prize in Physics "for groundbreaking experiments regarding the two-dimensional material graphene". They first produced it by lifting graphene flakes from bulk graphite with adhesive tape and then transferred them onto a silicon wafer.[5]

Graphyne

Graphyne is another 2-dimensional carbon allotrope whose structure is similar to graphene's. It can be seen as a lattice of benzene rings connected by acetylene bonds. Depending on the content of the acetylene groups, graphyne can be considered a mixed hybridization, spn, where 1 < n < 2,[6][7] and versus graphene's (pure sp2) and diamond (pure sp3).

First-principle calculations using phonon dispersion curves and ab-initio finite temperature, quantum mechanical molecular dynamics simulations showed graphyne and its boron nitride analogues to be stable.[8]

The existence of graphyne was conjectured before 1960.[9] It has not yet been synthesized. However, graphdiyne (graphyne with diacetylene groups) was synthesized on copper substrates.[10] Recently it has been claimed to be a concurrent for graphene, due to the potential of direction-dependent Dirac cones.[11][12]

Borophene

B
36° borophene, front and side view

Borophene is a proposed crystalline allotrope of boron. One unit consists of 36 atoms arranged in an 2-dimensional sheet with a hexagonal hole in the middle.[13][14]

Silicene

Structure of a typical silicene cluster showing ripples across the surface.

Silicene is a two-dimensional allotrope of silicon, with a hexagonal honeycomb structure similar to that of graphene.

Stanene

Stanene[15][16] is a predicted topological insulator that may display dissipationless currents at its edges near room temperature. It is composed of tin atoms arranged in a single layer, in a manner similar to graphene.[17] Stanene’s name is a portmanteau of stannum (the Latin name for tin) with '-'ene' used by graphene.[18][19]

The addition of fluorine atoms to the tin lattice could extend the critical temperature up to 100 °C.[20]

Phosphorene

Phosphorene is a 2-dimensional, crystalline allotrope of phosphorus. Its mono-atomic hexagonal structure makes it conceptually similar to graphene. However, phosphorene has substantially different electronic properties; in particular it possesses a nonzero band gap. This property potentially makes it a better semiconductor than graphene.[21]

Metals

Single atom layers of palladium,[22] and rhodium[23] have also been synthesized.

2D supracrystals

The supracrystals are defined as the supra atomic periodic structure where atoms in the nodes of a structure are represent by their symmetric complexes[24] [25]

Compounds

Graphane

Graphane

Graphane is a polymer of carbon and hydrogen with the formula unit (CH)
n where n is large. Graphane is a form of fully hydrogenated (on both sides) graphene.[26] Partial hydrogenation is then hydrogenated graphene.[27]

Graphane's carbon bonds are in sp3 configuration, as opposed to graphene's sp2 bond configuration. Thus graphane is a two-dimensional analog of cubic diamond.

The first theoretical description of graphane was reported in 2003[28] and its preparation was reported in 2009.

Graphane can be formed by electrolytic hydrogenation of graphene, few-layer graphene or high-oriented pyrolytic graphite. In the last case mechanical exfoliation of hydrogenated top layers can be used.[29]

p-doped graphane is postulated to be a high-temperature BCS theory superconductor with a Tc above 90 K.[30]

Germanane

Germanane is a single-layer crystal composed of germanium with one hydrogen bonded in the z-direction for each atom.[31] Germanane’s structure is similar to graphane,[32] Bulk germanium does not adopt this structure. Germanane is produced in a two-step route starting with calcium germanide. From this material, the calcium (Ca) is removed by de-intercalation with HCl to give a layered solid with the empirical formula GeH.[33] The Ca sites in Zintyl-phase CaGe
2 interchange with the hydrogen atoms in the HCl solution, producing GeH and CaCl2.

Nickel HITP

Ni
3(HITP)
2 is a two-dimensional metal organic framework, a combination of nickel and an organic compound called HITP (2,3,6,7,10,11-hexaaminotriphenylene). Its (two) constituents naturally self-assemble. It shares graphene’s hexagonal honeycomb structure. Multiple layers naturally form perfectly aligned stacks, with identical 2-nm openings at the centers of the hexagons. The material was claimed to be the first of a group formed by switching metals and/or organic compounds. The material can be isolated as a powder or a film. Conductivity values of 2 and 40 S·cm–1, respectively.[34]

Transition metal Di-chalcogenides (TMDCs)

Tungsten diselenide

WSe
2

Tungsten diselenide is an inorganic compound with the formula WSe
2. The compound adopts a hexagonal crystalline structure similar to molybdenum disulfide. Every tungsten atom is covalently bonded to six selenium ligands in a trigonal prismatic coordination sphere, while each selenium is bonded to three tungsten atoms in a pyramidal geometry. The tungsten – selenium bond has a bond distance of 2.526 Å and the distance between selenium atoms is 3.34 Å.[35] Layers stack together via van der Waals interactions. WSe
2 is a stable semiconductor in the group-VI transition metal dichalcogenides.

Molybdenum disulfide

Molybdenum disulfide is the inorganic compound with the formula MoS2. In its multilayer form it is a silvery black solid that occurs as the mineral molybdenite, the principal ore for molybdenum.[1] MoS2 is relatively unreactive. It is unaffected by dilute acids and oxygen. In appearance and feel, molybdenum disulfide is similar to graphite. It is widely used as a solid lubricant because of its low friction properties and robustness. As a transition metal di-chalcogenide, MoS
2 possesses some of graphene's desirable qualities (such as mechanical strength and electrical conductivity), and can emit light, opening possible applications such as photodetectors.[36]

MXenes

MXenes are layered transition metal carbides and carbonitrides with general formula of Mn+1XnTx, where M stands for early transition metal, X stands for carbon and/or nitrogen and Tx stands for surface terminations (mostly =O, -OH or -F). MXenes have high electric conductivity (1500 Scm−1) combined with hydrophilic surfaces. This materials show promise in energy storage applications and composites.

Applications

As of 2014, none of these materials has been used for large scale commercial applications (with the possible exception of graphene). Despite this, many are under close consideration for a number of industries, in areas including electronics and optoelectronics, sensors, biological engineering, filtration, lightweight/strong composite materials, photovoltaics, medicine, quantum dots, thermal management, ethanol distillation and energy storage,[37] and have enormous potential.

Graphene has been the most studied. In small quantities it is available as a powder and as a dispersion in a polymer matrix, or adhesive, elastomer, oil and aqueous and non-aqueous solutions. The dispersion is claimed to be suitable for advanced composites, paints and coatings, lubricants, oils and functional fluids, capacitors and batteries, thermal management applications, display materials and packaging, inks and 3D-printers’ materials, and barriers and films.[38]

References

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  2. ↑ Andronico, Michael (14 April 2014). "5 Ways Graphene Will Change Gadgets Forever". Laptop.
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  8. ↑ Özçelik, V. Ongun; S. Ciraci (January 10, 2013). "Size Dependence in the Stabilities and Electronic Properties of α-Graphyne and Its Boron Nitride Analogue". The Journal of Physical Chemistry C 117 (5): 2175. doi:10.1021/jp3111869.
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