In materials science, the term single-layer materials or 2D materials refers to crystalline solids consisting of a single layer of atoms.
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.
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).
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.
The existence of graphyne was conjectured before 1960.
It has not yet been synthesized.
However, graphdiyne (graphyne with diacetylene groups) was synthesized on copper substrates.
Recently, it has been claimed to be a competitor for graphene due to the potential of direction-dependent Dirac cones.
B: borophene
thumb|A  cluster might be seen as smallest borophene; front and side view
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.
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.
Si: silicene
Silicene is a two-dimensional allotrope of silicon, with a hexagonal honeycomb structure similar to that of graphene.
Sn: stanene
thumb|Lattice image of stanene flake, with the middle inset showing a large-area electron micrograph of the sample.
The right inset is an electron diffraction pattern confirming the hexagonal structure.
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..
P: phosphorene
thumb|Phosphorene structure: (a) tilted view, (b) side view, (c) top view.
Red (blue) balls represent phosphorus atoms in the lower (upper) layer.
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.
In a study made in 2018, antimonene modified screen-printed electrodes (SPE’s) were subjected to a galvanostatic charge/discharge test using a two-electrode approach to characterize their supercapacitive properties.
The best configuration observed, which contained 36 nanograms of antimonene in the SPE, showed a specific capacitance of 1578 F g−1 at a current of 14 A g−1.
Over 10,000 of these galvanostatic cycles, the capacitance retention values drop to 65% initially after the first 800 cycles, but then remain between 65% and 63% for the remaining 9,200 cycles.
The 36 ng antimonene/SPE system also showed an energy density of 20 mW h kg−1 and a power density of 4.8 kW kg−1.
These supercapacitive properties indicate that antimonene is a promising electrode material for supercapacitor systems.
Bi: bismuthene
Bismuthene, the two-dimensional allotrope of bismuth, was predicted to be a topological insulator.
It was predicted that bismuthene retains its topological phase when grown on silicon carbide in 2015.
The prediction was successfully realized and synthesized in 2016.
At first glance the system is similar to graphene, as the Bi atoms arrange in a honeycomb lattice.
However the bandgap is as large as 800mV due to the large spin-orbit-coupling of the Bi atoms and their interaction with the substrate.
Thus, room-temperature applications of the quantum spin Hall effect come into reach.
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.
Metals
right|thumb|3D AFM topography image of multilayered palladium nanosheet.
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 also been synthesized and characterized with atomic force microscopy and transmission electron microscopy.
2D alloys
Two-dimensional alloys is single atomic layer of alloy that is incommensurate with underlying substrate.
One example is the 2D ordered alloys of Pb with Sn and with Bi.
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.
Compounds
thumb|Graphane
Graphane,
Boron nitride nanosheet
thumb|Two alternately stacked layers of hexagonal boron nitride
Borocarbonitrides
MXenes
Transition metal dichalcogenide monolayers
2D silica
;Germanane  Germanane is a single-layer crystal composed of germanium with one hydrogen bonded in the z-direction for each atom.
Germanane’s structure is similar to graphane, 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.
The Ca sites in Zintl-phase CaGe2 interchange with the hydrogen atoms in the HCl solution, producing GeH and CaCl2.
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−1, 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 and 500 K, suggesting an unusual charge transport mechanism that has not been previously observed in organic semiconductors.
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 with conductivity values of 2 and 40 S cm−1, respectively.
Combinations
Single layers of 2D materials can be combined into layered assemblies.
For example, bilayer graphene is a material consisting of two layers of graphene.
One of the first reports of bilayer graphene was in the seminal 2004 Science paper by Geim and colleagues, in which they described devices "which contained just one, two, or three atomic layers".
Layered combinations of different 2D materials are generally called van der Waals heterostructures.
Twistronics is the study of how the angle (the twist) between layers of two-dimensional materials can change their electrical properties.
Characterization
Microscopy techniques such as transmission electron microscopy, 3D electron diffraction, scanning probe microscopy, scanning tunneling microscope, and atomic-force microscopy are used to characterize the thickness and size of the 2D materials.
Electrical properties and structural properties such as composition and defects are characterized by Raman spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy.
Applications
The major expectation held amongst researchers is that given their exceptional properties, 2D materials will replace conventional semiconductors to deliver a new generation of electronics.
Biological applications
Research on 2D nanomaterials is still in its infancy, with the majority of research focusing on elucidating the unique material characteristics and few reports focusing on biomedical applications of 2D nanomaterials.
Nevertheless, recent rapid advances in 2D nanomaterials have raised important yet exciting questions about their interactions with biological moieties.
2D nanoparticles such as carbon-based 2D materials, silicate clays, transition metal dichalcogenides (TMDs), and transition metal oxides (TMOs) provide enhanced physical, chemical, and biological functionality owing to their uniform shapes, high surface-to-volume ratios, and surface charge.
Two-dimensional (2D) nanomaterials are ultrathin nanomaterials with a high degree of anisotropy and chemical functionality.
2D nanomaterials are highly diverse in terms of their mechanical, chemical, and optical properties, as well as in size, shape, biocompatibility, and degradability.
These diverse properties make 2D nanomaterials suitable for a wide range of applications, including drug delivery, imaging, tissue engineering, and biosensors, among others.
However, their low-dimension nanostructure gives them some common characteristics.
For example, 2D nanomaterials are the thinnest materials known, which means that they also possess the highest specific surface areas of all known materials.
This characteristic makes these materials invaluable for applications requiring high levels of surface interactions on a small scale.
As a result, 2D nanomaterials are being explored for use in drug delivery systems, where they can adsorb large numbers of drug molecules and enable superior control over release kinetics.
Additionally, their exceptional surface area to volume ratios and typically high modulus values make them useful for improving the mechanical properties of biomedical nanocomposites and nanocomposite hydrogels, even at low concentrations.
Their extreme thinness has been instrumental for breakthroughs in biosensing and gene sequencing.
Moreover, the thinness of these molecules allows them to respond rapidly to external signals such as light, which has led to utility in optical therapies of all kinds, including imaging applications, photothermal therapy (PTT), and photodynamic therapy (PDT).
Despite the rapid pace of development in the field of 2D nanomaterials, these materials must be carefully evaluated for biocompatibility in order to be relevant for biomedical applications.
The newness of this class of materials means that even the relatively well-established 2D materials like graphene are poorly understood in terms of their physiological interactions with living tissues.
Additionally, the complexities of variable particle size and shape, impurities from manufacturing, and protein and immune interactions have resulted in a patchwork of knowledge on the biocompatibility of these materials.
Related articles
Two-dimensional semiconductor
Transition metal dichalcogenide monolayers
References
Additional reading
* Category:Condensed matter physics Category:Semiconductors Category:Monolayers
