2D materials

2D Topological Materials, sometimes referred to as single layer materials, are crystalline materials consisting of a single layer of atoms. 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, electrodes and water purification.

2D materials can generally be categorised as either 2D allotropes of various elements or compounds (usually consisting of two covalently bonding elements).^[1] The elemental 2D materials generally carry the -ene suffix in their names while the compounds have -ane or -ide suffixes. Layered combinations of different 2D materials are generally called van der Waals heterostructures. However, the efficient integration of 2D functional layers with three-dimensional (3D) systems remains a significant challenge, limiting device performance and circuit design.^[2]

While the first 2D material to be discovered was graphene, in 2004, some 500 2D materials may remain to be found.^[3] Research on these other materials has grown more rapidly than that on graphene since 2010.^[4]

The global market for 2D materials is expected to reach US$390 million within a decade, mostly for graphene in the semiconductor, electronics, battery energy and composites markets.^[5][6]

2D Allotropes

Graphene

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.^[7] It has the highest known thermal and electrical conductivity, displaying current densities 1,000,000 times that of copper.^[8] It was first produced in 2004.^[9]

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.^[10]

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,^[11][12] 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.^[13]

The existence of graphyne was conjectured before 1960.^[14] It has not yet been synthesized. However, graphdiyne (graphyne with diacetylene groups) was synthesized on copper substrates.^[15] Recently it has been claimed to be a competitor for graphene, due to the potential of direction-dependent Dirac cones.^[16][17]

Borophene

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.^[18][19] Borophene has also been synthesized, but with a different crystal structure.^[20]

Germanene

Germanene is a two-dimensional allotrope of germanium, with a buckled honeycomb structure.^[21] Experimentally synthesized germanene exhibits a honeycomb structure. This honeycomb structure consists of two hexagonal sub-lattices that are vertically displaced by 0.2 A from each other.^[22]

Silicene

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

Stanene

Stanene 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. Its buckled structure leads to high reactivity against common air pollutions such as NOx and COx and is able to trap and dissociate them at low temperature.^[23]

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 while displaying high electron mobility.^[4] This property potentially makes it a better semiconductor than graphene.^[24] The synthesis of phosphorene mainly consists of micromechanical cleavage or liquid phase exfoliation methods. The former has a low yield while the latter produce free standing nanosheets in solvent and not on the solid support. The bottom up approaches like chemical vapor deposition are still blank because of its high reactivity. Therefore, in the current scenario, the most effective method for large area fabrication of thin films of phosphorene consists of wet assembly techniques like Langmuir-Blodgett involving the assembly followed by deposition of nanosheets on solid supports.^[25]

Molybdenite

Metals

Single atom layers of palladium,^[26] and rhodium^[27] have also been synthesized.

2D supracrystals

The supracrystals of 2D materials have been proposed and theoretically simulated.^[28][29] These monolayer crystals are built of supra atomic periodic structures where atoms in the nodes of the lattice are replaced by symmetric complexes. For example, in the hexagonal structure of graphene patterns of 4 or 6 carbon atoms would be arranged hexagonally instead of single atoms, as the repeating node in the unit cell.

3D-topological structures (foams, aerogel substances and materials)

Aerographite

Aerogel

Nanogel

Carbon nanofoam

Compounds

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.^[30] Partial hydrogenation is then hydrogenated graphene.^[31]

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^[32] 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.^[33]

p-doped graphane is postulated to be a high-temperature BCS theory superconductor with a T_c above 90 K.^[34]

Hexagonal boron nitride

Boron nitride forms crystals of alternating atoms of boron and nitrogen, with a lattice spacing similar to that of carbon. It therefore also form similar allotropes like carbon, including a graphite-like hexagonal layered structure and graphene-like hexagonal monolayers. Unlike graphene, hexagonal boron nitride is electrically insulating, and can be combined with graphene and other 2D materials to make van der Waals heterostructure devices.

Germanane

Germanane is a single-layer crystal composed of germanium with one hydrogen bonded in the z-direction for each atom.^[35] Germanane’s structure is similar to graphane,^[36] 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.^[37] The Ca sites in Zintyl-phase CaGe
2 interchange with the hydrogen atoms in the HCl solution, producing GeH and CaCl2.

Transition metal Di-chalcogenides (TMDCs)

Molybdenum disulfide

Molybdenum disulfide is the inorganic compound with the formula MoS
2. In its multilayer form it is a silvery black solid that occurs as the mineral molybdenite, the principal ore for molybdenum.[1] MoS
2 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^[38] and Transistors.^[39]

Tungsten diselenide

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.^[40] Layers stack together via van der Waals interactions. WSe
2 is a stable semiconductor in the group-VI transition metal dichalcogenides.

Hafnium Disulphide

Hafnium Disulphide is a group IVB TMD with formula HfS
2. Like other TMDs, it possess a layered structure with strong covalent bonding between the Hf and S atoms in a layer and weak Van der Wall forces between layers. The compound has CdI
2 type structure and is an indirect band gap semiconducting material. The interlayer spacing between the layers is 0.56 nm, which is small compared to group VIB TMDs like MoS
2, making it difficult to cleave its atomic layers. However, recently its crystals with large interlayer spacing has grown using a chemical vapor transport route.^[41] These crystals exfoliate in solvents like N-Cyclohexyl-2-pyrrolidone (CHP) in a time of just some minutes resulting in a high-yield production of its few-layers resulting in increase of its indirect bandgap from 0.9 eV to 1.3 eV. As an application in electronics, its field-effect transistors has been realised using its few layers as a conducting channel material offering a high current modulation ratio larger than 10000 at room temperature. Therefore, group IVB TMDs also holds potential applications in the field of opto-electronics.

MXenes

MXenes are layered transition metal carbides and carbonitrides with general formula of M_n+1X_nT_x, where M stands for early transition metal, X stands for carbon and/or nitrogen and T_x 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.

Organic

Ni3(HITP)2 is an organic, crystalline, structurally tunable electrical conductor with a high surface area. HITP is an organic chemical (2,3,6,7,10,11-hexaaminotriphenylene). 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. Room temperature electrical conductivity is ~40 S/cm (Siemens per centimeter), comparable to that of bulk graphite and among the highest for any conducting Metal-organic frameworks (MOFs). The temperature dependence of its conductivity is linear at temperatures between 100 K (Kelvin) and 500 K, suggesting an unusual charge transport mechanism that has not been previously observed in organic semiconductors.^[42]

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.^[43]

Combinations

A 2015 study stacked two different TMD layers onto graphene. This composite displayed negative differential resistance – applying more voltage to the device reduced the current flowing through it.^[4]

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,^[44] 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.^[45]

References