In materials science, the term single-layer materials or 2D materials refers to crystalline solids consisting of a single layer of atoms. More broadly, these materials also include structures in which individual monolayers are held together by interlayer van der Waals interactions. These materials are promising for some applications but remain the focus of research. Single-layer materials derived from single elements generally carry the -ene suffix in their names, e.g. graphene. Single-layer materials that are compounds of two or more elements have -ane or -ide suffixes. 2D materials can generally be categorized as either 2D allotropes of various elements or as compounds (consisting of two or more covalently bonding elements).
It is predicted that there are hundreds of stable single-layer materials. The atomic structure and calculated basic properties of these and many other potentially synthesisable single-layer materials can be found in computational databases. 2D materials can be produced using mainly two approaches: top-down exfoliation and bottom-up synthesis. Exfoliation refers to the reduction of interlayer van der Waals interactions in bulk layered materials, leading to monolayer detachment from the sample surface. The exfoliation methods include sonication, mechanical, hydrothermal, electrochemical, laser-assisted, and microwave-assisted exfoliation.
Single element materials
C: graphene and graphyne 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. It has the highest known thermal and electrical conductivity, displaying current densities 1,000,000 times that of copper. It was first produced in 2004.
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 transferring them onto a silicon wafer.
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, compared to graphene (pure sp2) and diamond (pure sp3).
The existence of graphyne was conjectured before 1960. In 2010, graphdiyne (graphyne with diacetylene groups) was synthesized on copper substrates. In 2022 a team claimed to have successfully used alkyne metathesis to synthesise graphyne though this claim is disputed. However, after an investigation the team's paper was retracted by the publication citing fabricated data. Later during 2022 synthesis of multi-layered γ‑graphyne was successfully performed through the polymerization of 1,3,5-tribromo-2,4,6-triethynylbenzene under Sonogashira coupling conditions. Recently, it has been claimed to be a competitor for graphene due to the potential of direction-dependent Dirac cones.
B: borophene
Borophene is a crystalline atomic monolayer of boron and is also known as boron sheet. First predicted by theory in the mid-1990s in a freestanding state, and then demonstrated as distinct monoatomic layers on substrates by Zhang et al., different borophene structures were experimentally confirmed in 2015. First-principle calculations predict that a bilayer Kagome-phase borophene is an anisotropic superconductor with strong electron-phonon coupling and a critical temperature on the order of 17-35K.
Ge: germanene Germanene is a two-dimensional allotrope of germanium with a buckled honeycomb structure. 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. Experiments have demonstrated that germanene's quantum spin Hall edge states persist at room temperature and can be switched off by electrical field, indicating a robust and highly tunable topological phase.
Si: silicene
Silicene is a two-dimensional allotrope of silicon, with a hexagonal honeycomb structure similar to that of graphene. Its growth is scaffolded by a pervasive Si/Ag(111) surface alloy beneath the two-dimensional layer. By fabricating silicene between a 2D tin buffer layer, an encapsulated silicene sheet with stability under air is achieved.
Sn: 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 pollutants such as NOx and COx and it is able to trap and dissociate them at low temperature. Supporting Information A structure determination of stanene using low energy electron diffraction has shown ultra-flat stanene on a Cu(111) surface.
Pb: plumbene Plumbene is a two-dimensional allotrope of lead, with a hexagonal honeycomb structure similar to that of graphene.. Because of its heavy atomic mass and strong spin-orbit coupling, plumbene is predicted to have a band gap ~0.2eV and to behave as a robust 2D topological insulator, potentially enabling the quantum spin Hall effect at room temperature.
P: 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. This property potentially makes it a better semiconductor than graphene. 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 (CVD) 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.
Sb: antimonene Antimonene is a two-dimensional allotrope of antimony, with its atoms arranged in a buckled honeycomb lattice. Theoretical calculations predicted that antimonene would be a stable semiconductor in ambient conditions with suitable performance for (opto)electronics. Antimonene was first isolated in 2016 by micromechanical exfoliation and it was found to be very stable under ambient conditions. Its properties make it also a good candidate for biomedical and energy applications.
Antimonene has shown great promise in both energy storage and electrochemical sensing applications. In supercapacitors, antimonene-based electrodes have achieved a high specific capacitance (~1578F g−1) along with an energy density of 20 Wh/kg and a power density of 4.8 kW/kg. Furthermore, antimonene has been integrated into electroanalytical platforms to enhance detection of analytes.
Bi: bismuthene Bismuthene is a two-dimensional topological insulator formed by a honeycomb lattice of bismuth atoms, first synthesized on silicon carbide in 2016. Its large bandgap (~800mV), driven by strong spin-orbit coupling, supports room-temperature quantum spin Hall behavior, making it one of the most robust natural-state 2D topological insulators. Top-down exfoliation of bismuthene has been reported in various instances with recent works promoting the implementation of bismuthene in the field of electrochemical sensing. Mechanical studies on bismuthene reveal this material combines high fracture strength, moderate stiffness and low thermal conductivity, making it a strong candidate for thermoelectric and nanoelectronic devices.
Au: goldene On 16 April 2024, scientists from Linköping University in Sweden reported that they had produced goldene, a single layer of gold atoms 100 nm wide. Lars Hultman, a materials scientist on the team behind the new research, is quoted as saying "we submit that goldene is the first free-standing 2D metal, to the best of our knowledge", meaning that it is not attached to any other material, unlike plumbene and stanene. Researchers from New York University Abu Dhabi (NYUAD) previously reported to have synthesised Goldene in 2022, however various other scientists have contended that the NYUAD team failed to prove they made a single-layer sheet of gold, as opposed to a multi-layer sheet. Goldene is expected to be used primarily for its optical properties, with applications such as sensing or as a catalyst.
Metals
Single and double atom layers of platinum in a two-dimensional film geometry has been demonstrated. These atomically thin platinum films are epitaxially grown on graphene, which imposes a compressive strain that modifies the surface chemistry of the platinum, while also allowing charge transfer through the graphene. Single atom layers of palladium with the thickness down to 2.6 Å, and rhodium with the thickness of less than 4 Å have been synthesized and characterized with atomic force microscopy and transmission electron microscopy.
A 2D titanium formed by additive manufacturing (laser powder bed fusion) achieved greater strength than any known material (50% greater than magnesium alloy WE54). The material was arranged in a tubular lattice with a thin band running inside, merging two complementary lattice structures. This reduced by half the stress at the weakest points in the structure.
2D supracrystals The supracrystals of 2D materials have been proposed and theoretically simulated. 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.
2D alloys Two-dimensional alloys (or surface alloys) are a single atomic layer of alloy that is incommensurate with the underlying substrate. One example is the 2D ordered alloys of Pb with Sn and with Bi. Surface alloys have been found to scaffold two-dimensional layers, as in the case of silicene.
Compounds
Boron nitride nanosheet Titanate nanosheet Borocarbonitrides MXenes 2D silica Niobium bromide and Niobium chloride ()
Transition metal dichalcogenide monolayers
Monolayer MoS2, a prototypical transition metal dichalcogenide(TMD), consists of a single layer of molybdenum atoms sit between two layers of sulfur atoms in a hexagonal lattice. It is 0.65nm thick, yet with remarkable electronic properties. Unlike bulk MoS2, which has an indirect band gap of 1.2eV, the monolayer is a direct band gap semiconductor with a gap roughly 1.8eV. The direct band gap leads to great enhancement on photoluminescence, which means it emits light more efficiently than the bulk material.
Monolayer MoS2 also lacks inversion symmetry, which couples the electron's spin with the distinct valley states. These unique structural and electronic features make it broadly useful for a wide range of applications.
MoS2 monolayer has been used to create field-effect transistors with high on/off current ratio. The direct band gap of the monolayer leads to an efficient photodetectors with high sensitivity in the visible light range. Furthermore, the atomic thickness and mechanically flexible make it a good material for flexible circuits or wearable sensors.