View source for Physics:Two-dimensional materials

{{short description|Crystalline materials consisting of a single layer of atoms}}
'''Two-dimensional''' ('''2D''') '''materials''', sometimes referred to as '''single-layer materials''', are [[Chemistry:Crystalline|crystalline]] materials consisting of a single layer of atoms. These materials have found use in applications such as [[Chemistry:Photovoltaics|photovoltaics]],<ref>{{cite journal|last1=Ozdemir|first1=Burak|last2=Barone|first2=Veronica|date=2020|title=Thickness dependence of solar cell efficiency in transition metal dichalcogenides MX2 (M: Mo, W; X: S, Se, Te)|journal=Solar Energy Materials and Solar Cells|volume=212|pages=110557|doi=10.1016/j.solmat.2020.110557}}</ref> [[Physics:Semiconductor|semiconductor]]s, electrodes and [[Earth:Water purification|water purification]].

2D materials can generally be categorised as either 2D allotropes of various elements or compounds (consisting of two or more covalently bonding elements).<ref>{{cite journal|last1=Garcia|first1=J. C.|last2=de Lima|first2=D. B.|last3=Assali|first3=L. V. C.|last4=Justo|first4=J. F.|year=2011|title=Group IV graphene- and graphane-like nanosheets|journal=J. Phys. Chem. C|volume=115|issue=27|pages=13242–13246|arxiv=1204.2875|doi=10.1021/jp203657w}}</ref> 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.

The efficient integration of 2D functional layers with three-dimensional (3D) systems remains a significant challenge, limiting device performance and circuit design.<ref>{{cite journal|last1=Xu|first1=Yang|last2=Cheng|first2=Cheng|last3=Du|first3=Sichao|last4=Yang|first4=Jianyi|last5=Yu|first5=Bin|last6=Luo|first6=Jack|last7=Yin|first7=Wenyan|last8=Li|first8=Erping|last9=Dong|first9=Shurong|date=2016|title=Contacts between Two- and Three-Dimensional Materials: Ohmic, Schottky, and p–n Heterojunctions|journal=ACS Nano|volume=10|issue=5|pages=4895–4919|doi=10.1021/acsnano.6b01842|pmid=27132492|last10=Ye|first10=Peide|last11=Duan|first11=Xiangfeng}}</ref>

Some 700 2D materials have been predicted to be stable, although many remain to be synthesized.<ref>{{cite journal|last1=Ashton|first1=M.|last2=Paul|first2=J.|last3=Sinnott|first3=S. B.|last4=Hennig|first4=R. G.|year=2017|title=Topology-Scaling Identification of Layered Solids and Stable Exfoliated 2D Materials|journal=Phys. Rev. Lett.|volume=118|issue=10|pages=106101|arxiv=1610.07673|bibcode=2017PhRvL.118j6101A|doi=10.1103/PhysRevLett.118.106101|pmid=28339265}}</ref><ref>{{cite web|url=https://2dmaterialsweb.org|title=MaterialsWeb.org - Databases of Structural, Electronic, and Thermodynamic data for 2D and 3D Materials}}</ref> The global market for 2D materials is expected to reach US$390 million by 2025, mostly for graphene in the semiconductor, electronics, battery energy and composite materials markets.<ref>{{cite news|url=http://www.graphene-info.com/tags/market-reports|title=Graphene-Info Market Report|date=June 2015|accessdate=16 June 2015|publisher=Graphene-info}}</ref><ref>{{cite news|url=http://www.azonano.com/news.aspx?newsID=29510|title=Global Demand for Graphene after Commercial Production to be Enormous|date=28 February 2014|accessdate=24 July 2014|publisher=AZONANO.com}}</ref>
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== History ==
The first 2D material, [[Physics:Graphene|graphene]], a single layer of [[Chemistry:Graphite|graphite]], was isolated in 2004. Thereafter many other 2D materials were identified.

The first MXene was discovered in 2011 at Drexel University.<ref>{{cite journal|title= Two‐dimensional nanocrystals produced by exfoliation of Ti<sub>3</sub>AlC<sub>2</sub>|journal=Advanced Materials (Deerfield Beach, Fla.)|volume=23|issue=37|pages=4248–53|pmid=21861270|year=2011|last1=Naguib|first1=M.|last2=Kurtoglu|first2=M.|last3=Presser|first3=V.|last4=Lu|first4=J.|last5=Niu|first5=J.|last6=Heon|first6=M.|last7=Hultman|first7=L.|last8=Gogotsi|first8=Y.|last9=Barsoum|first9=M. W.|doi=10.1002/adma.201102306}}</ref>

The first post-graphene materials, i.e. [[Chemistry:Silicene|silicene]], was discovered in 2012. Thereafter, [[Chemistry:Germanene|germanene]], [[Physics:Stanene|stanene]], and [[Chemistry:Plumbene|plumbene]] were discovered in 2014, 2015, and 2018, respectively.

== 2D allotropes ==

=== Graphene ===
{{Main|Physics:Graphene}}
{{details|topic=[[Physics:Graphene|Graphene]]|Potential applications of graphene }}
[[File:Graphen.jpg|thumb|upright=1.25|Graphene is an [[Chemistry:Chicken wire|atomic-scale honeycomb lattice]] of carbon atoms.]]

[[Physics:Graphene|Graphene]] is a [[Earth:Crystal|crystal]]line allotrope of [[Chemistry:Carbon|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.<ref name="yahoo.155243022">{{cite news |url=https://news.yahoo.com/5-ways-graphene-change-gadgets-155243022.html |title=5 Ways Graphene Will Change Gadgets Forever |work=Laptop |date=14 April 2014 |first=Michael |last=Andronico }}</ref> It has the highest known thermal and electrical conductivity, displaying current densities 1,000,000 times that of [[Chemistry:Copper|copper]].<ref>{{cite web|url=http://www.graphene-battery.net/graphene-properties.htm |title=Graphene properties |publisher=www.graphene-battery.net |date=2014-05-29 |accessdate=2014-05-29}}</ref> It was first produced in 2004.<ref name="APS News">{{cite journal |year=2009 |url=http://www.aps.org/publications/apsnews/200910/loader.cfm?csModule=security/getfile&pageid=187967 |title=This Month in Physics History: October 22, 2004: Discovery of Graphene |page=2 |series=Series II |volume=18 |issue=9 |journal=APS News }}</ref>

[[Biography:Andre Geim|Andre Geim]] and [[Biography:Konstantin Novoselov|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 [[Engineering:Adhesive tape|adhesive tape]] and then transferring them onto a silicon wafer.<ref>{{cite web |title=The Nobel Prize in Physics 2010 |url=http://nobelprize.org/nobel_prizes/physics/laureates/2010/ |publisher=The Nobel Foundation |accessdate=2013-12-03 }}</ref>

=== Graphyne ===

[[Chemistry:Graphyne|Graphyne]] is another 2-dimensional carbon allotrope whose structure is similar to graphene's. It can be seen as a lattice of [[Chemistry:Benzene|benzene]] rings connected by [[Chemistry:Acetylene|acetylene]] bonds. Depending on the content of the acetylene groups, graphyne can be considered a mixed hybridization, sp<sup>n</sup>, where 1 < n < 2,<ref>{{cite journal | first1 = R.B. | last1 = Heimann | first2 = S.E. | last2 = Evsvukov | first3 = Y. | last3 = Koga | year = 1997 | title = Carbon allotropes: a suggested classification scheme based on valence orbital hybridization | journal = Carbon | volume = 35 | issue = 10–11 | pages = 1654–1658 | doi = 10.1016/S0008-6223(97)82794-7 }}</ref><ref>{{cite journal | first1 = Andrey N. | last1 = Enyashin | first2 = Alexander L. | last2 = Ivanovskii | year = 2011 | title = Graphene Allotropes | journal = Physica Status Solidi B | volume = 248 | issue = 8 | pages = 1879–1883 | doi = 10.1002/pssb.201046583 | bibcode = 2011PSSBR.248.1879E }}</ref> and versus graphene's (pure sp<sup>2</sup>) and [[Chemistry:Diamond|diamond]] (pure sp<sup>3</sup>).

First-principle calculations using [[Physics:Phonon scattering|phonon dispersion curves]] and ab-initio finite temperature, quantum mechanical molecular dynamics simulations showed graphyne and its [[Chemistry:Boron nitride|boron nitride]] analogues to be stable.<ref>{{cite journal|last=Özçelik|first=V. Ongun|author2=Ciraci, S. |title=Size Dependence in the Stabilities and Electronic Properties of α-Graphyne and Its Boron Nitride Analogue|journal=The Journal of Physical Chemistry C|date=January 10, 2013|doi=10.1021/jp3111869|volume=117|issue=5|pages=2175–2182|hdl=11693/11999|arxiv=1301.2593}}</ref>

The existence of graphyne was conjectured before 1960.<ref>{{cite journal|vauthors= Balaban AT, Rentia CC, Ciupitu E|title=Chemical graphs. 6. Estimation of relative stability of several planar and tridimensional lattices for elementary carbon|journal= Revue Roumaine de Chimie|year=1968|volume=13|issue=2|pages=231–}}</ref> It has not yet been synthesized. However, graphdiyne (graphyne with [[Chemistry:Diacetylene|diacetylene]] groups) was synthesized on copper substrates.<ref>{{cite journal | title = Architecture of graphdiyne nanoscale films | journal = Chemical Communications | volume = 46 | issue = 19 | pages = 3256–3258 | doi = 10.1039/B922733D | pmid = 20442882 | year = 2010 | last1 = Li | first1 = Guoxing | last2 = Li | first2 = Yuliang | last3 = Liu | first3 = Huibiao | last4 = Guo | first4 = Yanbing | last5 = Li | first5 = Yongjun | last6 = Zhu | first6 = Daoben | s2cid = 43416849 }}</ref> Recently it has been claimed to be a competitor for graphene, due to the potential of direction-dependent Dirac cones.<ref name="Gopalakrishnan 43–50">{{Cite journal|last1=Gopalakrishnan|first1=K.|last2=Moses|first2=Kota|last3=Govindaraj|first3=A.|last4=Rao|first4=C. N. R.|date=2013-12-01|title=Supercapacitors based on nitrogen-doped reduced graphene oxide and borocarbonitrides|journal=Solid State Communications|series=Special Issue: Graphene V: Recent Advances in Studies of Graphene and Graphene analogues|volume=175–176|pages=43–50|doi=10.1016/j.ssc.2013.02.005|bibcode=2013SSCom.175...43G}}</ref><ref>{{cite journal | journal =Physics | volume = 5 | issue = 24 | pages = 24 | title = Focus: Graphyne May Be Better than Graphene | first = Michael | last = Schirber | doi = 10.1103/Physics.5.24 |date=24 February 2012 |bibcode = 2012PhyOJ...5...24S }}</ref>

=== Borophene ===

[[File:Borophene.png|thumb|A {{chem|B|36}} cluster might be seen as smallest borophene; front and side view]]

[[Chemistry:Borophene|Borophene]] is a [[Earth:Crystal|crystal]]line atomic monolayer of [[Chemistry:Boron|boron]] and also known as ''boron sheet''. 
First predicted by theory in the mid-1990s in a freestanding state,<ref name="Boustani1997">{{cite journal |last1=Boustani |first1=Ihsan |title=New quasi-planar surfaces of bare boron |journal=Surface Science |date=January 1997 |volume=370 |issue=2–3 |pages=355–363 |doi=10.1016/S0039-6028(96)00969-7|bibcode=1997SurSc.370..355B }}</ref> and then demonstrated as distinct monoatomic layers on substrates by Zhang et al.,<ref>{{cite journal |last1=Zhang |first1=Z. |last2=Yang |first2=Y. |last3=Gao |first3=G. |last4=Yakobson |first4=B.I. |title=Two-Dimensional Boron Monolayers Mediated by Metal Substrates |journal=Angewandte Chemie International Edition |date=2 September 2015 |volume=54 |issue=44 |pages=13022–13026 |doi=10.1002/anie.201505425|pmid=26331848 }}</ref> 
different borophene structures were experimentally confirmed in 2015.<ref name="Mannix2015">
{{cite journal|last1=Mannix|first1=A. J.|last2=Zhou|first2=X.-F.|last3=Kiraly|first3=B.|last4=Wood|first4=J. D.|last5=Alducin|first5=D.|last6=Myers|first6=B. D.|last7=Liu|first7=X.|last8=Fisher|first8=B. L.|last9=Santiago|first9=U.|last10=Guest|first10=J. R.|last11=Yacaman|first11=M. J.|last12=Ponce|first12=A.|last13=Oganov|first13=A. R.|last14=Hersam|first14=M. C.|last15=Guisinger|first15=N. P.|display-authors=10|title=Synthesis of borophenes: Anisotropic, two-dimensional boron polymorphs|journal=Science|date=17 December 2015|volume=350|issue=6267|pages=1513–1516|doi=10.1126/science.aad1080|bibcode = 2015Sci...350.1513M|pmid=26680195|pmc=4922135}}
</ref><ref name="Feng2016">{{cite journal |last1=Feng |first1=Baojie |last2=Zhang |first2=Jin |last3=Zhong |first3=Qing |last4=Li |first4=Wenbin |last5=Li |first5=Shuai |last6=Li |first6=Hui |last7=Cheng |first7=Peng |last8=Meng |first8=Sheng |last9=Chen |first9=Lan |last10=Wu |first10=Kehui |title=Experimental realization of two-dimensional boron sheets |journal=Nature Chemistry |date=28 March 2016 |volume=8 |issue=6 |pages=563–568 |doi=10.1038/nchem.2491|pmid=27219700 |arxiv=1512.05029 |bibcode=2016NatCh...8..563F }}</ref>

=== Germanene ===

[[Chemistry:Germanene|Germanene]] is a two-dimensional allotrope of [[Chemistry:Germanium|germanium]], with a buckled honeycomb structure.<ref name=Bampoulis2014>{{Cite journal
| doi = 10.1088/0953-8984/26/44/442001
| volume = 26| issue = 44| pages = 442001 
| last = Bampoulis| first = P. | author2 = Zhang, L.| author3 =Safaei, A.| author4 =van Gastel, R.| author5 =Poelsema, B.| author6 = Zandvliet, H. J. W.
| title = Germanene termination of Ge<sub>2</sub>Pt crystals on Ge(110)
| journal = Journal of Physics: Condensed Matter
| date = 2014
|bibcode = 2014JPCM...26R2001B| pmid=25210978| arxiv = 1706.00697}}</ref> 
Experimentally synthesized [[Chemistry:Germanene|germanene]] exhibits a honeycomb structure.<ref name=Yuhara2018>{{Cite journal
| doi = 10.1021/acsnano.8b07006| pmid = 30371060| last = Yuhara| first = J. | author2 = Shimazu, H.| author3 =Ito, K.| author4 =Ohta, A.| author5 =Kurosawa, M.|author6=Nakatake, M.|author7 =Le Lay, Guy |title = Germanene Epitaxial Growth by Segregation through Ag(111) Thin Films on Ge(111)| journal = ACS Nano| volume = 12| issue = 11| pages = 11632–11637| date = 2018}}</ref> 
This honeycomb structure consists of two hexagonal sub-lattices that are vertically displaced by 0.2 A from each other.<ref name="Lee 4178–4182">{{Cite journal|last1=Lee|first1=Kangho|last2=Kim|first2=Hye-Young|last3=Lotya|first3=Mustafa|last4=Coleman|first4=Jonathan N.|last5=Kim|first5=Gyu-Tae|last6=Duesberg|first6=Georg S.|date=2011-09-22|title=Electrical Characteristics of Molybdenum Disulfide Flakes Produced by Liquid Exfoliation|journal=Advanced Materials|volume=23|issue=36|pages=4178–4182|doi=10.1002/adma.201101013|pmid=21823176}}</ref>

=== Silicene ===

[[File:Silicene-Ag STM5 crop.jpg|thumb|[[Engineering:Scanning tunneling microscope|STM]] image of the first (4×4) and second layers ({{radic|3}}×{{radic|3}}-''β'') of silicene grown on a thin silver film. Image size 16×16 nm.]]

[[Chemistry:Silicene|Silicene]] is a two-dimensional allotrope of [[Chemistry:Silicon|silicon]], with a [[Hexagon|hexagon]]al honeycomb structure similar to that of graphene.

=== Si<sub>2</sub>BN ===
In 2016 researchers predicted a 2d hexagonal, metallic allotrope of Si<sub>2</sub>BN with only sp<sup>2</sup> bonds.<ref>{{Cite journal|last=Andriotis|first=Antonis N.|date=2016-01-01|title=Prediction of a new graphenelike|journal=Physical Review B|volume=93|issue=8|pages=081413|doi=10.1103/PhysRevB.93.081413|bibcode=2016PhRvB..93h1413A|url=https://uknowledge.uky.edu/ccs_facpub/3}}</ref>

=== Stanene ===
{{Main|Physics:Stanene}}
[[File:Stanene TEM.jpg|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.]]
[[Physics:Stanene|Stanene]] is a predicted [[Physics:Topological insulator|topological insulator]] that may display dissipationless currents at its edges near [[Physics:Room-temperature superconductor|room temperature]]. It is composed of [[Chemistry:Tin|tin]] atoms arranged in a single layer, in a manner similar to graphene.<ref>{{cite journal | first1 = J. | last1 = Yuhara | first2 = Y. | last2 = Fujii | first3 = N. | last3 = Isobe | first4 = M. | last4 = Nakatake | first5 = X. | last5 = Lede | first6 = A. | last6 = Rubio | first7 = G. | last7 = Le Lay | year = 2018 | title = Large Area Planar Stanene Epitaxially Grown on Ag(111) | journal = 2D Materials | volume = 5 | issue = 2 | pages = 025002 | doi = 10.1088/2053-1583/aa9ea0
 | bibcode = 2018TDM.....5b5002Y | doi-access = free }}</ref> 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.<ref>{{cite journal|last1=Takahashi|first1=L.|last2=Takahashi|first2=K. |title=Low temperature pollutant trapping and dissociation over two-dimensional tin|journal=Physical Chemistry Chemical Physics|date=2015|volume=17|issue=33|pages=21394–21396|doi=10.1039/C5CP03382A|pmid=26226204|bibcode = 2015PCCP...1721394T}} [https://pdfs.semanticscholar.org/b744/7c16e5581d8473a0da0b6de9d5da5db15915.pdf?_ga=2.126974926.233582681.1588546944-821835483.1588546944 Supporting Information]</ref>
Recently structure determination of stanene is done using low energy electron diffraction and it shows very interesting result of ultra-flat stanene on Cu(111) surface.<ref>{{cite journal |last1=Ahmed |first1=Rezwan |last2=Nakagawa |first2=Takeshi |last3=Mizuno |first3=Seigi |title=Structure determination of ultra-flat stanene on Cu(111) using low energy electron diffraction. |journal=Surface Science |volume=691 |pages=121498 |doi=10.1016/j.susc.2019.121498 |year=2020 }}</ref>

=== Plumbene ===
[[Chemistry:Plumbene|Plumbene]] is a two-dimensional allotrope of [[Chemistry:Lead|lead]], with a [[Hexagon|hexagon]]al honeycomb structure similar to that of graphene.<ref>{{Cite journal |first1=J.|last1=Yuhara|first2=B.|last2=He|first3=G.|last3=Le Lay|title= Graphene's Latest Cousin: Plumbene Epitaxial Growth on a "Nano WaterCube"|journal= Advanced Materials|volume=31|issue=27|pages = 1901017|year=2019|doi=10.1002/adma.201901017|pmid = 31074927}}.</ref>

=== Phosphorene ===
[[File:Phosphorene structure.png|thumb|Phosphorene structure: (a) tilted view, (b) side view, (c) top view. Red (blue) balls represent phosphorus atoms in the lower (upper) layer.]]
[[Physics:Phosphorene|Phosphorene]] is a 2-dimensional, [[Earth:Crystal|crystal]]line allotrope of [[Chemistry:Phosphorus|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.<ref name=":0">{{Cite web|title = Beyond Graphene, a Zoo of New 2-D Materials|url = http://blogs.discovermagazine.com/crux/2015/07/17/beyond-graphene/#.Vane-PlVhBe |publisher=Discover Magazine |first=Andy |last=Berger |date= July 17, 2015 |accessdate = 2015-09-19}}</ref> This property potentially makes it a better semiconductor than graphene.<ref>{{Cite journal | doi = 10.1038/nnano.2014.35| title = Black phosphorus field-effect transistors| journal = Nature Nanotechnology| volume = 9| issue = 5| pages = 372–377| year = 2014| last1 = Li | first1 = L. | last2 = Yu | first2 = Y. | last3 = Ye | first3 = G. J. | last4 = Ge | first4 = Q. | last5 = Ou | first5 = X. | last6 = Wu | first6 = H. | last7 = Feng | first7 = D. | last8 = Chen | first8 = X. H. | last9 = Zhang | first9 = Y. 
|arxiv = 1401.4117 |bibcode = 2014NatNa...9..372L | pmid=24584274}}</ref> 
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<ref name="ReferenceA">{{Cite journal | doi = 10.1038/srep34095 | pmid = 27671093| pmc = 5037434| title = Large Area Fabrication of Semiconducting Phosphorene by Langmuir-Blodgett Assembly| journal = Sci. Rep.| volume = 6| pages = 34095| year = 2016| last1 = Ritu | first1 = Harneet | bibcode = 2016NatSR...634095K| arxiv = 1605.00875}}</ref>

=== Antimonene ===

Antimonene is a two-dimensional allotrope of [[Chemistry:Antimony|antimony]], with its atoms arranged in a buckled honeycomb lattice. Theoretical calculations<ref>{{Cite journal | first1 = S. | last1 = Zhang | first2 = Z. | last2 = Yan | first3 = Y. | last3 = Li | first4 = Z. | last4 = Chen | first5 = H. | last5 = Zeng | year = 2015 | title = Atomically Thin Arsenene and Antimonene: Semimetal-Semiconductor and Indirect-Direct Band-Gap Transitions | journal = Angew. Chem. Int. Ed. | volume = 54 |issue = 10| pages = 3112–3115 | doi = 10.1002/anie.201411246|pmid = 25564773}}</ref> 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<ref>{{Cite journal | first1 = P. | last1 = Ares | first2 = F. | last2 = Aguilar-Galindo | first3 = D. | last3 = Rodríguez-San-Miguel | first4 = D. A. | last4 = Aldave | first5 = S. | last5 = Díaz-Tendero | first6 = M. | last6 = Alcamí | first7 = F. | last7 = Martín | first8 = J. | last8 = Gómez-Herrero | first9 = F. | last9 = Zamora | year = 2016 | title = Mechanical Isolation of Highly Stable Antimonene under Ambient Conditions | journal = Adv. Mater. | volume = 28 |issue = 30| pages = 6332–6336 | doi = 10.1002/adma.201602128|pmid = 27272099|hdl = 10486/672484| arxiv = 1608.06859 | bibcode = 2016arXiv160806859A }}</ref> and it was found to be very stable under ambient conditions. Its properties make it also a good candidate for biomedical and energy applications.<ref>{{Cite journal | first1 = P. | last1 = Ares | first2 = J. J. | last2 = Palacios | first3 = G. | last3 = Abellán | first4 = J. | last4 = Gómez-Herrero | first5 = F. | last5 = Zamora | year = 2018 | title = Recent progress on antimonene: a new bidimensional material | journal = Adv. Mater. | volume = 30 |issue = 2| pages = 1703771 | doi = 10.1002/adma.201703771|pmid = 29076558|url = https://www.research.manchester.ac.uk/portal/en/publications/recent-progress-on-antimonene-a-new-bidimensional-material(e051769b-34be-41f7-a5de-ef6ebdc79756).html| hdl = 10486/688820 | hdl-access = free }}</ref>

In a study made in 2018,<ref>{{Cite journal|last1=Martínez‐Periñán|first1=Emiliano|last2=Down|first2=Michael P.|last3=Gibaja|first3=Carlos|last4=Lorenzo|first4=Encarnación|last5=Zamora|first5=Félix|last6=Banks|first6=Craig E.|date=2018|title=Antimonene: A Novel 2D Nanomaterial for Supercapacitor Applications|journal=Advanced Energy Materials|language=en|volume=8|issue=11|pages=1702606|doi=10.1002/aenm.201702606|issn=1614-6840|hdl=10486/688798|hdl-access=free}}</ref> 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<sup>−1</sup> at a current of 14 A g<sup>−1</sup>. 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&nbsp;ng antimonene/SPE system also showed an energy density of 20&nbsp;mW h kg<sup>−1</sup> and a power density of 4.8&nbsp;kW kg<sup>−1</sup>. These supercapacitive properties indicate that antimonene is a promising electrode material for supercapacitor systems.

=== Bismuthene ===

Bismuthene, the two-dimensional allotrope of [[Chemistry:Bismuth|bismuth]], was predicted to be a [[Physics:Topological insulator|topological insulator]]. It was predicted that bismuthene retains its topological phase when grown on [[Chemistry:Silicon carbide|silicon carbide]] in 2015.<ref>{{Cite journal|last1=Hsu|first1=Chia-Hsiu|last2=Huang|first2=Zhi-Quan|last3=Chuang|first3=Feng-Chuan|last4=Kuo|first4=Chien-Cheng|last5=Liu|first5=Yu-Tzu|last6=Lin|first6=Hsin|last7=Bansil|first7=Arun|date=2015-02-10|title=The nontrivial electronic structure of Bi/Sb honeycombs on SiC(0001)|journal=New Journal of Physics|volume=17|issue=2|pages=025005|doi=10.1088/1367-2630/17/2/025005|bibcode=2015NJPh...17b5005H|doi-access=free}}</ref> The prediction was successfully realized and synthesized in 2016.<ref>{{Cite journal|title = Bismuthene on a SiC substrate: A candidate for a high-temperature quantum spin Hall material|date = July 21, 2017|journal = Science|doi = 10.1126/science.aai8142 |pmid =28663438|volume=357|issue = 6348|pages=287–290|bibcode =2017Sci...357..287R |pmc=| last1 = Reis | first1 = Felix | last2 = Li| first2 = Gang | last3 = Dudy | first3 = Lenart | last4 = Bauernfiend | first4 = Maximilian | last5 = Glass | first5 = Stefan | last6 = Hanke | first6 = Werner | last7 = Thomale | first7 = Ronny | last8 = Schaefer | first8 = Joerg | last9 = Claessen | first9 = Ralph|arxiv = 1608.00812}}</ref> 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 [[Physics:Quantum spin Hall effect|quantum spin Hall effect]] come into reach. Top-down exfoliation of bismuthene has been reported in various instances <ref>{{cite journal |last1=Qi-Qi |first1=Yang |title=2D bismuthene fabricated via acid-intercalated exfoliation showing strong nonlinear near-infrared responses for mode-locking lasers |journal=Nanoscale |date=2/10/2018 |volume=10 |page=21106|doi=10.1039/c8nr06797}}</ref><ref>{{cite journal |last1=Gusmao |first1=Rui |last2=Sofer |first2=Zdenek |last3=Bousa |first3=Daniel |last4=Pumera |first4=Martin |title=Pnictogens (As, Sb, Bi) Nanosheets by Shear Exfoliation Using Kitchen Blenders for Electrochemical Applications |journal=Angewandte Chemie International Edition |date=29/07/2017 |volume=56 |issue=46 |doi=10.1002/anie.201706389}}</ref> with recent works promoting the implementation of bismuthene in the field of electrochemical sensing.<ref>{{cite journal |last1=Martinez |first1=Carmen C. |last2=Gusmao |first2=Rui |last3=Sofer |first3=Zdenek |last4=Pumera |first4=Martin |title=Pnictogen-Based Enzymatic Phenol Biosensors: Phosphorene, Arsenene, Antimonene, and Bismuthene |journal=Angewandte Chemie International Edition |volume=58 |issue=1 |doi=10.1002/anie.201808846}}</ref><ref>{{cite journal |last1=Lazanas |first1=Alexandros Ch. |last2=Tsirka |first2=Kyriaki |last3=Paipetis |first3=Alkiviadis S. |last4=Prodromidis |first4=Mamas I. |title=2D bismuthene/graphene modified electrodes for the ultra-sensitive stripping voltammetric determination of lead and cadmium |journal=Electrochimica Acta |date=2020 |volume=336 |doi=10.1016/j.electacta.2020.135726}}</ref>

=== Metals ===
[[File:Palladium nanosheet on silicon wafer.jpg|right|thumb|3D AFM topography image of multilayered palladium nanosheet.<ref name="Pd" />]]
Single and double atom layers of [[Chemistry:Platinum|platinum]] in a two-dimensional film geometry has been demonstrated.<ref name=":5">{{Cite journal|last1=Abdelhafiz|first1=Ali|last2=Vitale|first2=Adam|last3=Buntin|first3=Parker|last4=deGlee|first4=Ben|last5=Joiner|first5=Corey|last6=Robertson|first6=Alex|last7=Vogel|first7=Eric M.|last8=Warner|first8=Jamie|last9=Alamgir|first9=Faisal M.|date=2018|title=Epitaxial and atomically thin graphene–metal hybrid catalyst films: the dual role of graphene as the support and the chemically-transparent protective cap|journal=Energy & Environmental Science|volume=11|issue=6|pages=1610–1616|doi=10.1039/c8ee00539g}}</ref><ref name=":7">{{Cite journal|last1=Abdelhafiz|first1=Ali|last2=Vitale|first2=Adam|last3=Joiner|first3=Corey|last4=Vogel|first4=Eric|last5=Alamgir|first5=Faisal M.|date=2015-03-16|title=Layer-by-Layer Evolution of Structure, Strain, and Activity for the Oxygen Evolution Reaction in Graphene-Templated Pt Monolayers|journal=ACS Applied Materials & Interfaces|volume=7|issue=11|pages=6180–6188|doi=10.1021/acsami.5b00182|pmid=25730297}}</ref> These atomically thin platinum films are epitaxially grown on [[Physics:Graphene|graphene]]<ref name=":5" /> which imposes a compressive strain that modifies the surface chemistry of the platinum, while also allowing charge transfer through the [[Physics:Graphene|graphene]].<ref name=":7" /> Single atom layer of [[Chemistry:Palladium|palladium]] with the thickness down to 2.6 Å,<ref name="Pd">{{cite journal|last1=Yin|first1=Xi|last2=Liu|first2=Xinhong|last3=Pan|first3=Yung-Tin|last4=Walsh|first4=Kathleen A.|last5=Yang|first5=Hong|title=Hanoi Tower-like Multilayered Ultrathin Palladium Nanosheets|journal=Nano Letters|date=November 4, 2014|doi=10.1021/nl503879a|pmid=25369350|bibcode = 2014NanoL..14.7188Y|volume=14|issue=12|pages=7188–7194}}</ref> and [[Chemistry:Rhodium|rhodium]] with the thickness of less than 4 Å <ref>{{cite journal|last1=Duan|first1=Haohong|last2=Yan|first2=Ning|last3=Yu|first3=Rong|last4=Chang|first4=Chun-Ran|last5=Zhou|first5=Gang|last6=Hu|first6=Han-Shi|last7=Rong|first7=Hongpan|last8=Niu|first8=Zhiqiang|last9=Mao|first9=Junjie|last10=Asakura|first10=Hiroyuki|last11=Tanaka|first11=Tsunehiro|last12=Dyson|first12=Paul Joseph|last13=Li|first13=Jun|last14=Li|first14=Yadong|title=Ultrathin rhodium nanosheets|journal=Nature Communications|date=17 January 2014|volume=5|pages=3093|doi=10.1038/ncomms4093|pmid=24435210|bibcode=2014NatCo...5.3093D|doi-access=free}}</ref> have also been synthesized and characterized with atomic force microscopy and transmission electron microscopy.

=== 2D Sodium Chloride (NaCl) ===

NaCl is one of the simplest compounds and was thought to be well-understood, and yet, unexpected complexities related to it were uncovered at high pressure and in low-dimensional states. Here, exotic hexagonal NaCl thin films on the (110) diamond surface were crystallized in the experiment following a theoretical prediction based on ab initio evolutionary algorithm USPEX.<ref name="ACS">{{cite journal|doi=10.1021/acs.jpclett.0c00874|title= Exotic Two-Dimensional Structure: The First Case of Hexagonal NaCl.|journal=The Journal of Physical Chemistry Letters|pages=3821–3827|year=2020|last1= Tikhomirova|first1=K.|last2=Tantardini|first2=C.|last3=Sukhanova|first3=E.|last4=Popov|first4=Z.|last5=Evlashin|first5=S.|last6=Tarkhov|first6=M.|last7=Zhdanov|first7=V.|last8=Dudin|first8=A.|last9=Organov|first9=A.|last10=Kvashnin|first10=D.|last11=Kvashniv|first11=A.}}</ref>

=== 2D alloys ===

Two-dimensional alloys is single atomic layer of alloy that is incommensurate with underlying substrate. The 2D ordered alloy of Pb and Sn has been synthesized and characterized with scanning tunneling microscopy and low-energy electron diffraction in 2003.<ref>{{cite journal |first1=J. |last1= Yuhara |first2=M. |last2= Schmid |first3= P. |last3= Varga |year=2003 |title= A two-dimensional alloy of immiscible metals, The single and binary monolayer films of Pb and Sn on Rh(111) |journal= Phys. Rev. B |volume= 67 |issue= 19 |pages= 195407 |doi= 10.1103/PhysRevB.67.195407 |bibcode= 2003PhRvB..67s5407Y }}</ref> Moreover, the 2D all proportional solid solution alloy of Pb and Bi has been synthesized in 2011.<ref>{{cite journal |first1= J. |last1= Yuhara |first2= M. |last2= Yokoyama |first3= T. |last3= Matsui |year=2011 |title= Two-dimensional solid solution alloy of Bi-Pb binary films on Rh(111) |journal= J. Appl. Phys. |volume= 110 |issue= 7 |pages= 074314–074314–4 |doi= 10.1063/1.3650883 |bibcode= 2011JAP...110g4314Y }}</ref>

=== 2D supracrystals ===

The supracrystals of 2D materials have been proposed and theoretically simulated.<ref>{{Cite journal |doi= 10.1088/1742-6596/345/1/012007 |title= 2D supracrystals as a promising materials for planar nanoacoustoelectronics |journal= Journal of Physics: Conference Series |volume= 345 |issue= 1 |pages = 012007| year = 2012 |last1= Kochaev |first1= A. I. |last2= Karenin |first2= A.A. |last3= Meftakhutdinov |first3= R.M. |last4= Brazhe |first4= R.A. |bibcode = 2012JPhCS.345a2007K |doi-access= free }}</ref><ref>{{Cite journal | doi = 10.1134/S1063783412080069| title = Flexural waves in graphene and 2D supracrystals| journal = Physics of the Solid State| volume = 54| issue = 8| pages = 1612–1614| year = 2012| last1 = Brazhe | first1 = R. A.| last2 = Kochaev | first2 = A. I.|bibcode = 2012PhSS...54.1612B }}</ref> These monolayer crystals are built of supra atomic periodic structures where atoms in the nodes of the [[Lattice (group)|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 [[Chemistry:Crystal structure#Unit cell|unit cell]].

== Compounds ==

=== Graphane ===

[[File:graphane.png|thumb|Graphane]]

[[Chemistry:Graphane|Graphane]] is a [[Chemistry:Polymer|polymer]] of carbon and [[Chemistry:Hydrogen|hydrogen]] with the [[Chemistry:Formula unit|formula unit]] {{chem|(CH)|n}} where ''n'' is large. Graphane is a form of fully [[Chemistry:Hydrogenation|hydrogenated]] (on both sides) graphene.<ref>{{cite journal| author= Sofo, Jorge O.|title = Graphane: A two-dimensional hydrocarbon| journal = Physical Review B| year = 2007| volume = 75 | issue = 15 |pages = 153401–4 | doi = 10.1103/PhysRevB.75.153401|arxiv = cond-mat/0606704 |bibcode = 2007PhRvB..75o3401S |display-authors=0}}</ref> Partial hydrogenation is then hydrogenated graphene.<ref name="Graphane">{{cite journal|title =Control of Graphene's Properties by Reversible Hydrogenation: Evidence for Graphane| journal = Science|year =2009|volume = 323| doi = 10.1126/science.1167130| pmid= 19179524| issue= 5914| bibcode=2009Sci...323..610E| pages= 610–3|arxiv = 0810.4706 |display-authors=0| last1 = Elias| first1 = D. C.| last2 = Nair| first2 = R. R.| last3 = Mohiuddin| first3 = T. M. G.| last4 = Morozov| first4 = S. V.| last5 = Blake| first5 = P.| last6 = Halsall| first6 = M. P.| last7 = Ferrari| first7 = A. C.| last8 = Boukhvalov| first8 = D. W.| last9 = Katsnelson| first9 = M. I.| last10 = Geim| first10 = A. K.| last11 = Novoselov| first11 = K. S.}}</ref>

Graphane's carbon bonds are in ''sp''<sup>3</sup> configuration, as opposed to graphene's ''sp''<sup>2</sup> bond configuration. Thus graphane is a two-dimensional analog of cubic [[Chemistry:Diamond|diamond]].

The first theoretical description of graphane was reported in 2003<ref>{{cite journal | last1 = Sluiter | first1 = Marcel | last2 = Kawazoe | first2 = Yoshiyuki | title = Cluster expansion method for adsorption: Application to hydrogen chemisorption on graphene | journal = Physical Review B | volume = 68 | page = 085410 | year = 2003 | doi = 10.1103/PhysRevB.68.085410|bibcode = 2003PhRvB..68h5410S | issue = 8 }}</ref> 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.<ref>{{cite journal| author= Ilyin, A. M. |title =Computer simulation and experimental study of graphane-like structures formed by electrolytic hydrogenation| journal = Physica E|year =2011|volume =43 | doi = 10.1016/j.physe.2011.02.012|issue= 6|pages= 1262–65|bibcode = 2011PhyE...43.1262I |display-authors=0}}</ref>

p-doped graphane is postulated to be a [[Physics:High-temperature superconductor|high-temperature]] [[Physics:BCS theory|BCS theory]] [[Physics:Superconductor|superconductor]] with a T<sub>c</sub> above 90 [[Kelvin|K]].<ref>{{cite journal| author= Savini, G. |title =Doped graphane: a prototype high-Tc electron-phonon superconductor |journal=Phys Rev Lett|volume=105|issue =5 |pages =059902 |year=2010|id= | arxiv=1002.0653|display-authors=0|doi=10.1103/physrevlett.105.059902|bibcode=2010PhRvL.105e9902S}}</ref>

=== Hexagonal boron nitride ===
[[File:Boron-nitride-(hexagonal)-side-3D-balls.png|thumb|Two alternately stacked layers of hexagonal boron nitride]]

==== Properties ====

===== Structural =====
2D [[Chemistry:Boron nitride|boron nitride]] is an sp<sup>2</sup>-conjugated compound that forms a honeycomb structure of alternating [[Chemistry:Boron|boron]] and [[Chemistry:Nitrogen|nitrogen]] atoms with a lattice spacing of 1.45Å.<ref name=":2">{{Cite journal|last1=Li|first1=Lu Hua|last2=Chen|first2=Ying|title=Atomically Thin Boron Nitride: Unique Properties and Applications|journal=Advanced Functional Materials|volume=26|issue=16|pages=2594–2608|doi=10.1002/adfm.201504606|arxiv=1605.01136|year=2016|bibcode=2016arXiv160501136L}}</ref><ref name=":02">{{Cite book|title=Semiconductors and Semimetals|last1=Bhimanapati|first1=G. R.|last2=Glavin|first2=N. R.|last3=Robinson|first3=J. A.|date=2016-01-01|publisher=Elsevier|editor-last=Francesca Iacopi|editor-first=John J. Boeckl and Chennupati Jagadish|series=2D Materials|volume=95|pages=101–147|doi=10.1016/bs.semsem.2016.04.004|chapter=2D Boron Nitride|isbn=9780128042724}}</ref> It adopts the hexagonal (h-BN) allotrope of the three possible crystalline forms of boron nitride because it is the most ubiquitous and stable structure.<ref name=":02" /> Boron nitride nanosheets contain two different edges. In the armchair edge structure, the edge consists of either boron or nitrogen atoms.<ref name=":02" /> In the zig-zag edge structure, the edge consists of alternating boron and nitrogen atoms.<ref name=":02" /> These 2D structures can stack on top of each other and are held by [[Physics:Van der Waals force|Van der Waal forces]] to form what is called few-layer boron nitride nanosheets.<ref name=":02" /><ref name=":1">{{Cite journal|last1=Lin|first1=Yi|last2=Connell|first2=John W.|date=2012-10-29|title=Advances in 2D boron nitride nanostructures: nanosheets, nanoribbons, nanomeshes, and hybrids with graphene|journal=Nanoscale|volume=4|issue=22|pages=6908–39|doi=10.1039/c2nr32201c|pmid=23023445|bibcode=2012Nanos...4.6908L}}</ref> In these structures, the boron atoms of one sheet are positioned on top or below the nitrogen atoms due to electron deficient nature of boron and electron rich nature of nitrogen, respectively.<ref name=":02" /><ref name=":1" /> Due to several similar structural similarities with [[Physics:Graphene|graphene]], boron nitride nanosheets are considered graphene analogs, often called “white graphene".<ref name=":1" /><ref name=":3">{{Cite journal|last1=Pakdel|first1=Amir|last2=Zhi|first2=Chunyi|last3=Bando|first3=Yoshio|last4=Golberg|first4=Dmitri|date=2012-06-01|title=Low-dimensional boron nitride nanomaterials|journal=Materials Today|volume=15|issue=6|pages=256–265|doi=10.1016/S1369-7021(12)70116-5|doi-access=free}}</ref>

Boron nanosheets (BNNS) defined as single or few layers of boron nitride<ref name=":02" /><ref name=":3" /><ref name=":4">{{Cite journal|last1=Wang|first1=Xuebin|last2=Zhi|first2=Chunyi|last3=Weng|first3=Qunhong|last4=Bando|first4=Yoshio|last5=Golberg|first5=Dmitri|date=2013-01-01|title=Boron Nitride Nanosheets: novel Syntheses and Applications in polymeric Composites|journal=Journal of Physics: Conference Series|volume=471|issue=1|pages=012003|doi=10.1088/1742-6596/471/1/012003|bibcode=2013JPhCS.471a2003W|doi-access=free}}</ref> whose aspect ratio is small.<ref name=":02" /> There are a few variations of 2D boron nitride structure.<ref name=":02" /> Boron nitride nanoribbons (BNNR) are boron nitride nanosheets with significant edge effects<ref name=":1" /> and have widths that are smaller than 50 nanometers.<ref name=":02" /><ref name=":54">{{Cite journal|last1=Rao|first1=C. N. R.|last2=Ramakrishna Matte|first2=H. S. S.|last3=Maitra|first3=Urmimala|date=2013-12-09|title=Graphene Analogues of Inorganic Layered Materials|journal=Angewandte Chemie International Edition|volume=52|issue=50|pages=13162–13185|doi=10.1002/anie.201301548|pmid=24127325}}</ref> Boron nitride nanomeshes (BNNM) are boron nitride nanosheets that are placed upon specific metal substrates.<ref name=":1" />

===== Electrical =====
Boron nitride nanosheets have a wide [[Physics:Band gap|bandgap]] that ranges from 5 to 6 eV<ref name=":02" /><ref name=":1" /><ref name=":3" /> and can be changed by the presence of [[Chemistry:Stone–Wales defect|Stone-Wales defects]] within the structure,<ref name=":1" /> by doping<ref name=":1" /> or functionalization,<ref name=":1" /> or by changing the number of layers.<ref name=":2" /><ref name=":1" /> Due to this large bandgap and tunability as well as its surface flatness,<ref name=":2" /> boron nitride nanosheets are considered to be an excellent electric insulators and are often used as dielectrics in electrical devices.<ref name=":1" /><ref name=":3" /><ref name=":4" /><ref name=":54"/>

===== Thermal =====
2D boron nitride structures are excellent thermal conductors,<ref name=":02" /><ref name=":1" /><ref name=":3" /> with a [[Physics:Thermal conductivity|thermal conductivity]] range of 100–270 W/mK.<ref name=":2" /><ref name=":02" /> It has been suggested that single layer boron nitride nanosheets have a greater thermal conductivity<ref name=":2" /><ref name=":1" /> than other forms of boron nitride nanosheets due to decreased [[Physics:Phonon scattering|phonon scattering]]<ref name=":1" /> from subsequent layers.

The thermal stability of boron nitride nanosheets is very high due to the high thermal stability properties of hexagonal boron nitride.<ref name=":2" /><ref name=":02" /><ref name=":3" /><ref name=":6">{{Cite journal|last1=Li|first1=Lu Hua|last2=Cervenka|first2=Jiri|last3=Watanabe|first3=Kenji|last4=Taniguchi|first4=Takashi|last5=Chen|first5=Ying|date=2014-02-25|title=Strong Oxidation Resistance of Atomically Thin Boron Nitride Nanosheets|journal=ACS Nano|volume=8|issue=2|pages=1457–1462|doi=10.1021/nn500059s|pmid=24400990|bibcode=2014arXiv1403.1002L|arxiv=1403.1002}}</ref> As single layer and few-layer boron nitride nanosheets begin to oxidize and lose their electrical properties at 800&nbsp;°C, they are excelle

==== Synthesis ====

=====Chemical vapor deposition' =====
{{Main|Chemistry:Chemical vapor deposition}}

Chemical vapor deposition (CVD) is a popular synthesis method to produce boron nitride because it is a highly controllable process that produces high quality and defect free monolayer and few-layer boron nitride nanosheets.<ref name=":1" /><ref name=":3" /><ref name=":4" /><ref name=":54"/><ref name=":8">{{Cite journal|last1=Wang|first1=Zifeng|last2=Tang|first2=Zijie|last3=Xue|first3=Qi|last4=Huang|first4=Yan|last5=Huang|first5=Yang|last6=Zhu|first6=Minshen|last7=Pei|first7=Zengxia|last8=Li|first8=Hongfei|last9=Jiang|first9=Hongbo|date=2016-06-01|title=Fabrication of Boron Nitride Nanosheets by Exfoliation|journal=The Chemical Record|volume=16|issue=3|pages=1204–1215|doi=10.1002/tcr.201500302|pmid=27062213}}</ref> In the majority of CVD methods, boron and nitride precursors react with a metal substrate at high temperature.<ref name=":1" /><ref name=":3" /> This allows for nanosheets of a large area as the layers grow uniformly on the substrate.<ref name=":1" /><ref name=":3" /><ref name=":6" /> There is a wide range of boron and nitride precursors such as [[Chemistry:Borazine|borazine]] and selection of these precursors depend on factors such as toxicity,<ref name=":1" /> stability,<ref name=":02" /><ref name=":1" /> reactivity,<ref name=":1" /> and the nature of the CVD method.<ref name=":02" /><ref name=":1" /><ref name=":3" /><ref name=":54"/> However, despite the high quality of the nanosheets synthesized by CVD, it is not a good method for the large scale production of boron nitride nanosheets for applications.<ref name=":8"/>

===== Mechanical cleavage =====
While there are several mechanical cleaving methods to produce boron nitride nanosheets, they employ the same principle: using shear forces to break the Van der Waals interactions between the layers of boron nitride.<ref name=":02" /> The advantage of mechanical cleavage is that the nanosheets isolated from these techniques have few defects and retain the lateral size of the original substrate.<ref name=":02" /><ref name=":1" />

Inspired by its use in the isolation of graphene, micromechanical cleavage, also known as the Scotch-tape method, has been used to consistently isolate few-layer and monolayer boron nitride nanosheets by subsequent exfoliation of the beginning material with adhesive tape.<ref name=":02" /><ref name=":1" /><ref name=":4" /><ref name=":8"/> However, the disadvantage of this technique is that it is not scalable for large scale production.<ref name=":02" /><ref name=":1" />

Ball milling is another technique used to mechanically exfoliate boron nitride sheets from the parent substrate.<ref name=":2" /><ref name=":02" /><ref name=":1" /><ref name=":3" /><ref name=":4" /><ref name=":54"/><ref>{{Cite journal|last1=Li|first1=Lu Hua|last2=Chen|first2=Ying|last3=Behan|first3=Gavin|last4=Zhang|first4=Hongzhou|last5=Petravic|first5=Mladen|last6=Glushenkov|first6=Alexey M.|s2cid=41206042|date=2011-08-03|title=Large-scale mechanical peeling of boron nitride nanosheets by low-energy ball milling|journal=Journal of Materials Chemistry|volume=21|issue=32|doi=10.1039/c1jm11192b|page=11862}}</ref><ref name=":8"/> In this process, shear forces are applied on the face of bulk boron nitride by rolling balls, which break the Van der Waal interactions between each layer.<ref name=":02" /><ref name=":1" /><ref name=":54"/><ref name=":8" /> While the ball milling technique may allow for large quantities of boron nitride nanosheets, it does not allow for control the size or the number of layers of the resulting nanosheets.<ref name=":02" /><ref name=":1" /> Furthermore, these nanosheets have more defects due to the aggressive nature of this technique.<ref name=":2" /><ref name=":8" /> However, improvements such as the addition of a milling agent such as benzyl benzoate<ref name=":02" /><ref name=":8" /> or the use of smaller balls<ref name=":02" /> has allowed for a greater yield of higher quality nanosheets.<ref name=":02" /><ref name=":8" />

Boron nitride nanosheets have also been isolated by using a vortex fluidic device, which uses centripetal force to shear off layers of boron nitride.<ref name=":8" />

===== Unzipping of boron nitride nanotubes =====
Boron nitride nanosheets may also be synthesized by the unzipping of boron nitride nanotubes (BNNT).<ref name=":02" /><ref name=":1" /><ref name=":8" /> These nanotubes can be made into sheets by breaking the bonds connecting the N and B atoms by potassium [[Chemistry:Intercalation|intercalation]]<ref name=":02" /><ref name=":1" /><ref name=":8" /> or by etching by plasma or an inert gas.<ref name=":02" /><ref name=":1" /><ref name=":8" /> The unzipping of boron nitride nanotubes by plasma can be used to control the size of the nanosheets, but it produces semiconducting boron nitride nanosheets.<ref name=":8" /> The potassium intercalation method produces a low yield of nanosheets as boron nitride is resistive to the effects of intercalants.<ref name=":1" />

===== Solvent exfoliation and sonication =====
Solvent exfoliation is often used in tandem with [[Sonication|sonication]] to break the weak Van der Waals interactions present in bulk boron nitride to isolate large quantities of boron nitride nanosheets.<ref name=":02" /><ref name=":1" /><ref name=":8"/> Polar solvents such as [[Chemistry:Isopropyl alcohol|isopropyl alcohol]]<ref name=":1" /> and [[Chemistry:2,5-Dimethylfuran|DMF]]<ref name=":9">{{Cite journal|last1=Zhi|first1=Chunyi|last2=Bando|first2=Yoshio|last3=Tang|first3=Chengchun|last4=Kuwahara|first4=Hiroaki|last5=Golberg|first5=Dimitri|date=2009-07-27|title=Large-Scale Fabrication of Boron Nitride Nanosheets and Their Utilization in Polymeric Composites with Improved Thermal and Mechanical Properties|journal=Advanced Materials|volume=21|issue=28|pages=2889–2893|doi=10.1002/adma.200900323}}</ref> have been found to be more effective in exfoliating boron nitride layers than nonpolar solvents because these solvents possess a similar [[Physics:Surface energy|surface energy]] to the surface energy of boron nitride nanosheets.<ref name=":02" /> Combinations of different solvents also exfoliate boron nitride better than when the solvents were used individually.<ref name=":02" /> However, many solvents that can be used to exfoliate boron nitride are fairly toxic and expensive.<ref name=":8" /> Common solvents such as water and isopropyl alcohol have been determined to be comparable to these toxic polar solvents in exfoliating boron nitride sheets.<ref name=":02" /><ref name=":9" />

===== Chemical functionalization and sonication =====
Chemical functionalization of boron nitride involves attaching molecules onto the outer and inner layers of bulk boron nitride.<ref name=":1" /> There are three types of functionalization that can be done to boron nitride: covalent functionalization, ionic functionalization, or non covalent functionalization.<ref name=":02" /> Layers are then exfoliated by placing the functionalized boron nitride into a solvent and allow the solvation force between the attached groups and the solvent to overcome the Van der Waal forces present in each layer.<ref name=":8" /> This method is slightly different than solvent exfoliation as solvent exfoliation relies on the similarities between the surface energies of the solvent and boron nitride layers to overcome the Van der Waals interactions.

===== Solid state reactions =====
The reaction of a mixture of boron and nitrogen precursors at high temperature can produce boron nitride nanosheets.<ref name=":02" /><ref name=":8" /> In one method, boric acid and urea were reacted together at 900˚C.<ref name=":54"/><ref name=":8" /> The numbers of layers of these nanosheets were controlled by the urea content as well as the temperature<ref name=":8" />

=== Borocarbonitrides ===

==== Properties ====

=====Structural =====
[[File:BCN.jpg|thumb|upright=1.5|A schematic of borocarbonitride (BCN)]]
Borocarbonitrides are two-dimensional compounds that are synthesized such that they contain [[Chemistry:Boron|boron]], [[Chemistry:Nitrogen|nitrogen]], and [[Chemistry:Carbon|carbon]] atoms in a ratio B<small>x</small>C<small>y</small>N<small>z.</small><ref name=":10">{{Cite journal|last1=kumar|first1=Nitesh|last2=Moses|first2=Kota|last3=Pramoda|first3=K.|last4=Shirodkar|first4=Sharmila N.|last5=Mishra|first5=Abhishek Kumar|last6=Waghmare|first6=Umesh V.|last7=Sundaresan|first7=A.|last8=Rao|first8=C. N. R.|date=2013-04-23|title=Borocarbonitrides, BxCyNz|journal=Journal of Materials Chemistry A|volume=1|issue=19|doi=10.1039/c3ta01345f|page=5806}}</ref><ref name=":11">{{Cite journal|last1=Rao|first1=C. N. R.|last2=Gopalakrishnan|first2=K.|date=2016-10-31|title=Borocarbonitrides, BxCyNz: Synthesis, Characterization, and Properties with Potential Applications|journal=ACS Applied Materials & Interfaces|volume=9|issue=23|pages=19478–19494|doi=10.1021/acsami.6b08401|pmid=27797466}}</ref> Borocarbonitrides are distinct from B,N co-doped graphene in that the former contains separate boron nitride and graphene domains as well as rings with B-C, B-N, C-N, and C-C bonds.<ref name=":133">{{Cite journal|date=2015-01-01|title=Inorganic Graphene Analogs|journal=Annual Review of Materials Research|volume=45|issue=1|pages=29–62|doi=10.1146/annurev-matsci-070214-021141|bibcode=2015AnRMS..45...29R|last1=Rao|first1=C. N. R|last2=Maitra|first2=Urmimala}}</ref> These compounds generally have a high surface area, but borocarbonitrides synthesized from a high surface area carbon material, urea, and boric acid tend to have the highest surface areas.<ref name=":10" /><ref name=":12">{{Cite journal|last1=Raidongia|first1=Kalyan|last2=Nag|first2=Angshuman|last3=Hembram|first3=K. P. S. S.|last4=Waghmare|first4=Umesh V.|last5=Datta|first5=Ranjan|last6=Rao|first6=C. N. R.|date=2010-01-04|title=BCN: A Graphene Analogue with Remarkable Adsorptive Properties|journal=Chemistry – A European Journal|volume=16|issue=1|pages=149–157|doi=10.1002/chem.200902478|pmid=19946909}}</ref><ref name=":54"/> This high surface area coupled with the presence of Stone-Wales defects in the structure of borocarbonitrides also allows for high absorption of CO<sub>2</sub> and CH<sub>4,</sub> which may make borocarbonitride compounds a useful material in sequestering these gases<sub>.</sub><ref name=":10" /><ref name=":12" />

===== Electrical =====
The band gap of borocarbonitrides range from 1.0–3.9eV<ref name=":10" /> and is dependent on the content of the carbon and boron nitride domains as they have different electrical properties.<ref name=":10" /> Borocarbonitrides with a high carbon content have lower bandgaps<ref name=":11" /> whereas those with higher content of boron nitride domains have higher band gaps.<ref name=":10" /> Borocarbonitrides synthesized in gas or solid reactions also tend to have large bandgaps and are more insulating in character.<ref name=":10" /> The wide range of composition of boronitrides allows for the tuning of the bandgap, which when coupled with its high surface area and Stone-Wales defects may make boronitrides a promising material in electrical devices.<ref name=":11" /><ref name="Gopalakrishnan 43–50"/>

==== Synthesis ====

===== Solid state reaction =====
A high surface area carbon material such as activated charcoal, boric acid, and urea are mixed together and then heated at high temperatures to synthesize borocarbonitride.<ref name=":11" /> The composition of the resulting compounds may be changed by varying the concentration of the reagents as well as the temperature.<ref name=":10" />

====='Gas phase synthesis =====
In chemical vapor deposition, boron, nitrogen, and carbon precursors react at high heat and are deposited onto a metal substrate.<ref name=":10" /> Varying the concentration of precursors and the selection of certain precursors will give different ratios of boron, nitrogen, and carbon in the resulting borocarbonitride compound.<ref name=":11" />

===== Borocarbonitride composites=====
Borocarbonitride can also be synthesized by random stacking of boronitride and graphene domains through covalent interactions<ref name=":11" /> or through liquid interactions.<ref name=":10" /> In the first method, graphene and boron nitride sheets are functionalized and then are reacted to form layers of borocarbonitride.<ref name=":11" /> In the second method, boron nitride and graphite powder are dissolved in isopropanol and dimethylformamide, respectively, and then sonicated.<ref name=":11" /> This is then exfoliated to isolate borocarbonitride layers.

=== Germanane ===

Germanane is a single-layer crystal composed of [[Chemistry:Germanium|germanium]] with one hydrogen bonded in the z-direction for each atom.<ref name=Bianco2013>{{Cite journal | last1 = Bianco | first1 = E. | last2 = Butler | first2 = S. | last3 = Jiang | first3 = S. | last4 = Restrepo | first4 = O. D. | last5 = Windl | first5 = W. | last6 = Goldberger | first6 = J. E. | title = Stability and Exfoliation of Germanane: A Germanium Graphane Analogue | doi = 10.1021/nn4009406 | journal = ACS Nano | pages = 4414–21 | year = 2013 | pmid = 23506286| pmc = | volume=7| issue = 5 | hdl = 1811/54792 | hdl-access = free }}</ref> Germanane’s structure is similar to [[Chemistry:Graphane|graphane]],<ref>{{cite journal|last1=Garcia|first1=J. C.|last2=de Lima|first2=D. B.|last3=Assali|first3=L. V. C.|last4=Justo|first4=J. F.|title=Group IV graphene- and graphane-like nanosheets|journal=J. Phys. Chem. C|year=2011|volume=115|issue=27|pages=13242–13246|doi=10.1021/jp203657w|arxiv=1204.2875}}</ref> Bulk germanium does not adopt this structure. Germanane is produced in a two-step route starting with calcium germanide. From this material, the [[Chemistry:Calcium|calcium]] (Ca) is removed by de-[[Chemistry:Intercalation|intercalation]] with HCl to give a layered solid with the empirical formula GeH.<ref name=kurz>{{cite web|url=http://www.kurzweilai.net/germanane-may-replace-silicon-for-lighter-faster-electronics |title='Germanane' may replace silicon for lighter, faster electronics |publisher=KurzweilAI |date= |accessdate=2013-04-12}}</ref> The Ca sites in Zintyl-phase {{chem|CaGe|2}} interchange with the hydrogen atoms in the HCl solution, producing GeH and CaCl2.

===Transition metal dichalcogenides (TMDs)===
{{main|Chemistry:Transition metal dichalcogenide monolayers}}

====Molybdenum disulfide====
{{main|Chemistry:Molybdenum disulfide}}

[[File:Molybdenite-3D-balls.png|thumb|Layered structure of {{chem|MoS|2}}, with Mo in blue and S in yellow]]

===== Properties =====

====== Structural ======
[[File:Balls_and_sticks_model_of_a_single_layer_1H_and_1T_molybdenum_disulfide.svg|alt=Balls and sticks model of a single layer 1H and 1T molybdenum disulfide|frameless|right|upright=1.5|Balls and sticks model of a single layer 1H and 1T molybdenum disulfide]]
Molybdenum disulfide monolayers consist of a unit of one layer of molybdenum atoms covalently bonded to two layers of sulfur atoms. While bulk molybdenum sulfide exists as 1T, 2H, or 3R polymorphs, molybdenum disulfide monolayers are found only in the 1T or 2H form.<ref name=":133"/> The 2H form adopts a trigonal prismatic geometry<ref name=":15">{{Cite journal|last1=Li|first1=Xiao|last2=Zhu|first2=Hongwei|date=2015-03-01|title=Two-dimensional MoS2: Properties, preparation, and applications|journal=Journal of Materiomics|volume=1|issue=1|pages=33–44|doi=10.1016/j.jmat.2015.03.003|doi-access=free}}</ref> while the 1T form adopts an octahedral or trigonal antiprismatic geometry.<ref name=":133"/> Molybdenum monolayers can also be stacked due to Van der Waals interactions between each layer.

====== Electrical ======
The electrical properties of molybdenum sulfide in electrical devices depends on factors such as the number of layers,<ref name=":17">{{Cite journal|last1=Mak|first1=Kin Fai|last2=Lee|first2=Changgu|last3=Hone|first3=James|last4=Shan|first4=Jie|last5=Heinz|first5=Tony F.|title=Atomically ThinMoS2: A New Direct-Gap Semiconductor|journal=Physical Review Letters|volume=105|issue=13|pages=136805|doi=10.1103/physrevlett.105.136805|pmid=21230799|bibcode=2010PhRvL.105m6805M|arxiv=1004.0546|year=2010}}</ref> the synthesis method,<ref name=":133"/> the nature of the substrate on which the monolayers are placed on,<ref>{{Cite journal|last1=Najmaei|first1=Sina|last2=Zou|first2=Xiaolong|last3=Er|first3=Dequan|last4=Li|first4=Junwen|last5=Jin|first5=Zehua|last6=Gao|first6=Weilu|last7=Zhang|first7=Qi|last8=Park|first8=Sooyoun|last9=Ge|first9=Liehui|date=2014-03-12|title=Tailoring the Physical Properties of Molybdenum Disulfide Monolayers by Control of Interfacial Chemistry|journal=Nano Letters|volume=14|issue=3|pages=1354–1361|doi=10.1021/nl404396p|pmid=24517325|bibcode=2014NanoL..14.1354N|citeseerx=10.1.1.642.1938}}</ref> and mechanical strain.<ref name=":16">{{Cite journal|last1=Conley|first1=Hiram J.|last2=Wang|first2=Bin|last3=Ziegler|first3=Jed I.|last4=Haglund|first4=Richard F.|last5=Pantelides|first5=Sokrates T.|last6=Bolotin|first6=Kirill I.|year=2013|title=Bandgap Engineering of Strained Monolayer and Bilayer MoS2|journal=Nano Letters|volume=13|issue=8|pages=3626–3630|doi=10.1021/nl4014748|pmid=23819588|arxiv=1305.3880|bibcode=2013NanoL..13.3626C}}</ref>

As the number of layers decrease, the band gap begins to increase from 1.2eV in the bulk material up to a value of 1.9eV for a monolayer.<ref name=":54"/> Odd number of molybdenum sulfide layers also produce different electrical properties than even numbers of molybednum sulfide layers due to cyclic stretching and releasing present in the odd number of layers.<ref>{{Cite journal|last1=Wu|first1=Wenzhuo|last2=Wang|first2=Lei|last3=Li|first3=Yilei|last4=Zhang|first4=Fan|last5=Lin|first5=Long|last6=Niu|first6=Simiao|last7=Chenet|first7=Daniel|last8=Zhang|first8=Xian|last9=Hao|first9=Yufeng|date=2014-10-23|title=Piezoelectricity of single-atomic-layer MoS2 for energy conversion and piezotronics|journal=Nature|volume=514|issue=7523|pages=470–474|doi=10.1038/nature13792|pmid=25317560|bibcode=2014Natur.514..470W}}</ref> Molybdenum sulfide is a p-type material, but it shows [[Physics:Ambipolar diffusion|ambipolar]] behavior when molybdenum sulfide monolayers that were 15&nbsp;nm thick were used in transistors.<ref name=":54"/> However, most electrical devices containing molybdenum sulfide monolayers tend to show n-type behavior.<ref name=":15" /><ref name="Lee 4178–4182"/>

The band gap of molybdenum disulfide monolayers can also be adjusted by applying mechanical strain<ref name=":16" /> or an electrical field.<ref name=":54"/> Increasing mechanical strain shifts the phonon modes of the molybdenum sulfide layers.<ref name=":16" /> This results in a decrease of the band gap and metal-to-insulator transition.<ref name=":133"/> Applying an electric field of 2-3Vnm<sup>−1</sup> also decreases the indirect bandgap of molybdenum sulfide bilayers to zero.<ref name=":133"/>

Solution phase lithium intercalation and exfolation of bulk molybdenum sulfide produces molybdenum sulfide layers with metallic and semiconducting character due to the distribution of 1T and 2H geometries within the material.<ref name=":54"/><ref name=":133"/> This is due to the two forms of molybdenum sulfide monolayers having different electrical properties. The 1T polymorph of molybednum sulfide is metallic in character while the 2H form is more semiconducting.<ref name=":15" /> However, molybdenum disulfide layers produced by electrochemical lithium intercalation are predominantly 1T and thus metallic in character as there is no conversion to the 2H form from the 1T form.<ref name=":133"/>

====== Thermal ======
The thermal conductivity of molybdenum disulfide monolayers at room temperature is 34.5W/mK<ref name=":14">{{Cite journal|last1=Yan|first1=Rusen|last2=Simpson|first2=Jeffrey R.|last3=Bertolazzi|first3=Simone|last4=Brivio|first4=Jacopo|last5=Watson|first5=Michael|last6=Wu|first6=Xufei|last7=Kis|first7=Andras|last8=Luo|first8=Tengfei|last9=Walker|first9=Angela R. Hight|title=Thermal Conductivity of Monolayer Molybdenum Disulfide Obtained from Temperature-Dependent Raman Spectroscopy|journal=ACS Nano|volume=8|issue=1|pages=986–993|doi=10.1021/nn405826k|pmid=24377295|year=2014|url=http://infoscience.epfl.ch/record/197223}}</ref> while the thermal conductivity of few-layer molybdenum disulfide is 52W/mK.<ref name=":14" /> The thermal conductivity of graphene, on the other hand, is 5300W/mK.<ref name=":14" /> Due to the rather low thermal conductivity of molybdenum disulfide nanomaterials, it is not as promising material for high thermal applications as some other 2D materials.

===== Synthesis =====
<ref>{{cite journal|display-authors=et al|last1=Backes|first1=Claudia|title=Production and processing of graphene and related materials|journal=2D Materials|volume=7|pages= 022001|date=2020|issue=2|doi=10.1088/2053-1583/ab1e0a|doi-access=free|bibcode=2020TDM.....7b2001B}}</ref>

====== Exfoliation ======
Exfoliation techniques for the isolating of molybdenum disulfide monolayers include mechanical exfoliation,<ref name=":133"/> solvent assisted exfoliation,<ref name=":15" /> and chemical exfolation.<ref name=":54"/>

Solvent assisted exfoliation is done by sonicating bulk molybdenum disulfide in an organic solvent such as isopropanol and N-methyl-2-pyrrolidone, which disperses the bulk material into nanosheets as the Van der Waals interactions between the layers in the bulk materaial are broken.<ref name=":133"/> The amount of nanosheets produced is controlled by the sonication time,<ref name=":15" /> the solvent-molybdenum disulfide interactions,<ref name=":133"/> and the centrifuge speed.<ref name=":133"/> Compared to other exfoliation techniques, solvent assisted exfoliation is the simplest method for large scale production of molybdenum disulfide nanosheets.<ref name=":18">{{Cite journal|last1=Kannan|first1=Padmanathan Karthick|last2=Late|first2=Dattatray J.|last3=Morgan|first3=Hywel|last4=Rout|first4=Chandra Sekhar|date=2015-08-06|title=Recent developments in 2D layered inorganic nanomaterials for sensing|journal=Nanoscale|volume=7|issue=32|pages=13293–13312|doi=10.1039/c5nr03633j|pmid=26204797|bibcode=2015Nanos...713293K}}</ref>

The micromechanical exfoliation of molybdenum disulfide was inspired by the same technique used in the isolation of graphene nanosheets.<ref name=":18" /> Micromechanical exfoliation allows for low defect molybdenum disulfide nanosheets but is not suitable for large scale production due to low yield.<ref name=":15" />

Chemical exfoliation involves functionalizing molybdenum difsulfide and then sonicating to disperse the nanosheets.<ref name=":18" /> The most notable chemical exfoliation technique is lithium intercalation in which lithium is intercalated into bulk molybdenum disulfide and then dispersed into nanosheets by the addition of water.<ref name=":54"/>

====== Chemical vapor deposition ======
Chemical vapor deposition of molybdenum disulfide nanosheets involves reacting molybdenum and sulfur precursors on a substrate at high temperatures.<ref name=":18" /> This technique is often used in the preparing electrical devices with molybdenum disulfide components because the nanosheets are applied directly on the substrate; unfavorable interactions between the substrate and the nanosheets that would have occurred had they been separately synthesized are decreased.<ref name=":15" /> In addition, since the thickness and area of the molybdenum disulfide nanosheets can be controlled by the selection of specific precursors, the electrical properties of the nanosheets can be tuned.<ref name=":15" />

====== Laser ablation ======
Pulsed laser deposition involves the thinning of bulk molybdenum disulfide by laser to produce single or multi-layer molybdenum disulfide nanosheets.<ref name=":133"/> This allows for synthesis of molybdenum disulfide nanosheets with a defined shape and size.<ref name=":54"/> The quality of the nanosheets are determined by the energy of the laser and the irradation angle.<ref name=":18" />

Lasers can also be used to form molybdenum disulfide nanosheets from molybdenum disulfide [[Chemistry:Fullerene|fullerene]]-like molecules.<ref name="Wu 1276–1281">{{Cite journal|last1=Wu|first1=Haihua|last2=Yang|first2=Rong|last3=Song|first3=Baomin|last4=Han|first4=Qiusen|last5=Li|first5=Jingying|last6=Zhang|first6=Ying|last7=Fang|first7=Yan|last8=Tenne|first8=Reshef|last9=Wang|first9=Chen|title=Biocompatible Inorganic Fullerene-Like Molybdenum Disulfide Nanoparticles Produced by Pulsed Laser Ablation in Water|journal=ACS Nano|volume=5|issue=2|pages=1276–1281|doi=10.1021/nn102941b|pmid=21230008|year=2011}}</ref>

==== Hafnium disulfide ====
[[File:HfS2 structure.png|thumb|{{chem|HfS|2}} structure]] 
[[Chemistry:Hafnium disulfide|Hafnium disulfide]] ({{chem|HfS|2}}) has a layered structure with strong covalent bonding between the Hf and S atoms in a layer and weak van der Waals forces between layers. The compound has {{chem|CdI|2}} type structure and is an indirect band gap semiconducting material. The interlayer spacing between the layers is 0.56&nbsp;nm, which is small compared to group VIB TMDs like {{chem|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.<ref>{{Cite journal|last1=Kaur|first1=Harneet|title=High Yield Synthesis and Chemical Exfoliation of Two-Dimensional Layered Hafnium Disulphide|journal=Nano Research|volume=11|pages=343–353|arxiv=1611.00895|year=2016|doi=10.1007/s12274-017-1636-x}}</ref> 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.

==== Tungsten diselenide ====
[[File:WSe2 TEM.jpg|thumb|upright|Atomic image of a WSe<sub>2</sub> monolayer showing hexagonal symmetry and three-fold defects. Scale bar: 2 nm (0.5 nm in the inset).]]
[[Chemistry:Tungsten diselenide|Tungsten diselenide]] is an [[Chemistry:Inorganic compound|inorganic compound]] with the formula {{chem|WSe|2}}. The compound adopts a hexagonal crystalline structure similar to [[Chemistry:Molybdenum disulfide|molybdenum disulfide]]. Every [[Chemistry:Tungsten|tungsten]] atom is covalently bonded to six [[Chemistry:Selenium|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.<ref>{{cite journal | last1 = Schutte | first1 = W.J. | last2 = De Boer | first2 = J.L. | last3 = Jellinek | first3 = F. | title = Crystal Structures of Tungsten Disulfide and Diselenide | journal = Journal of Solid State Chemistry | year = 1986 | volume = 70 | issue = 2 | pages = 207–209 | doi = 10.1016/0022-4596(87)90057-0|bibcode = 1987JSSCh..70..207S }}</ref> Layers stack together via [[Physics:Van der Waals force|van der Waals interactions]]. {{chem|WSe|2}} is a stable [[Physics:Semiconductor|semiconductor]] in the group-VI transition metal dichalcogenides. The electronic bandgap of {{chem|WSe|2}} can be tuned by mechanical strain<ref>{{cite journal|doi=10.1088/2053-1583/3/2/021011|title=Reversible Uniaxial Strain Tuning in Atomically thin WSe2|journal=2D Materials|volume=3|issue=2|pages=021011|year=2016|last1=Schmidt|first1=Robert|last2=Niehues|first2=Iris|last3=Schneider|first3=Robert|last4=Drüppel|first4=Matthias|last5=Deilmann|first5=Thorsten|last6=Rohlfing|first6=Michael|last7=Michaelis de Vasconcellos|first7=Steffen|last8=Castellanos-Gomez|first8=Andres|last9=Bratschitsch|first9=Rudolf|bibcode=2016TDM.....3b1011S}}</ref> which can also allow for conversion of the band type from indirect-to-direct in a {{chem|WSe|2}} bilayer.<ref>{{cite journal|doi=10.1021/acs.nanolett.7b05229|pmid=29558623|title=Giant Mechano-Optoelectronic Effect in an Atomically Thin Semiconductor|journal=Nano Letters|volume=18|issue=4|pages=2351–2357|year=2018|last1=Wu|first1=Wei|last2=Wang|first2=Jin|last3=Ercius|first3=Peter|last4=Wright|first4=Nicomario|last5=Leppert-Simenauer|first5=Danielle|last6=Burke|first6=Robert|last7=Dubey|first7=Madan|last8=Dongare|first8=Avinash|last9=Pettes|first9=Michael|bibcode=2018NanoL..18.2351W|url=http://www.escholarship.org/uc/item/979683v3}}</ref>

===MXenes===

[[Chemistry:MXenes|MXenes]] are layered transition metal carbides and carbonitrides with general formula of M<sub>n+1</sub>X<sub>n</sub>T<sub>x</sub>, where M stands for early transition metal, X stands for carbon and/or nitrogen and T<sub>x</sub> stands for surface terminations (mostly =O, -OH or -F), and n=1-4.<ref>{{cite journal |last1=Deysher |first1=Grayson |last2=Shuck |first2=Christopher Eugene |last3=Hantanasirisakul |first3=Kanit |last4=Frey |first4=Nathan C. |last5=Foucher |first5=Alexandre C. |last6=Maleski |first6=Kathleen |last7=Sarycheva |first7=Asia |last8=Shenoy |first8=Vivek B. |last9=Stach |first9=Eric A. |last10=Anasori |first10=Babak |last11=Gogotsi |first11=Yury |title=Synthesis of Mo<sub>4</sub>VAlC<sub>4</sub> MAX Phase and Two-Dimensional Mo<sub>4</sub>VC<sub>4</sub> MXene with Five Atomic Layers of Transition Metals |journal=ACS Nano |date=5 December 2019 |volume=14 |issue=1 |pages=204–217 |doi=10.1021/acsnano.9b07708|pmid=31804797 }}</ref> MXenes have high electric conductivity (10000-1500 Scm<sup>−1</sup>) combined with hydrophilic surfaces that can be tuned with solvents. It was shown that MXene synthesis is readily scalable, with large (>50 g) batch sizes produced with no loss or change in properties as the size is increased.<ref>{{cite journal |last1=Shuck |first1=Christopher E. |last2=Sarycheva |first2=Asia |last3=Anayee |first3=Mark |last4=Levitt |first4=Ariana |last5=Zhu |first5=Yuanzhe |last6=Uzun |first6=Simge |last7=Balitskiy |first7=Vitaliy |last8=Zahorodna |first8=Veronika |last9=Gogotsi |first9=Oleksiy |last10=Gogotsi |first10=Yury |title=Scalable Synthesis of Ti<sub>3</sub>C<sub>2</sub>T<sub>x</sub> MXene |journal=Advanced Engineering Materials |date=March 2020 |volume=22 |issue=3 |pages=1901241 |doi=10.1002/adem.201901241}}</ref> This materials show promise in energy storage applications, gas sensing, and composites.<ref>{{Cite journal|last1=Anasori|first1=Babak|last2=Lukatskaya|first2=Maria R.|last3=Gogotsi|first3=Yury|date=2017-01-17|title=2D metal carbides and nitrides (MXenes) for energy storage|journal=Nature Reviews Materials|language=en|volume=2|issue=2|pages=1–17|doi=10.1038/natrevmats.2016.98|issn=2058-8437|osti=1399374|url=https://www.osti.gov/biblio/1399374}}</ref><ref>{{Cite journal|last1=Khakbaz|first1=Pedram|last2=Moshayedi|first2=Milad|last3=Hajian|first3=Sajjad|last4=Soleimani|first4=Maryam|last5=Narakathu|first5=Binu B.|last6=Bazuin|first6=Bradley J.|last7=Pourfath|first7=Mahdi|last8=Atashbar|first8=Massood Z.|date=2019-12-12|title=Titanium Carbide MXene as NH3 Sensor: Realistic First-Principles Study|journal=The Journal of Physical Chemistry C|volume=123|issue=49|pages=29794–29803|doi=10.1021/acs.jpcc.9b09823|issn=1932-7447}}</ref><ref>{{Cite journal|last1=Shahzad|first1=F.|last2=Alhabeb|first2=M.|last3=Hatter|first3=C. B.|last4=Anasori|first4=B.|last5=Man Hong|first5=S.|last6=Koo|first6=C. M.|last7=Gogotsi|first7=Y.|date=2016-09-09|title=Electromagnetic interference shielding with 2D transition metal carbides (MXenes)|journal=Science|language=en|volume=353|issue=6304|pages=1137–1140|doi=10.1126/science.aag2421|pmid=27609888|issn=0036-8075|doi-access=free|bibcode=2016Sci...353.1137S}}</ref> They are synthesized from ceramic precursor MAX phases by removing the single atomic layer "A" where M stands for Ti, Mo, W, Nb, Zr, Hf, V, Cr, Ta, Sc, A stands for Al, Si, and X stands for C, N. Millions of predicted solid solutions of these materials have been identified and 30+ MXenes have been synthesized.

== Organic ==
Ni<sub>3</sub>(HITP)<sub>2</sub> is an organic, crystalline, structurally tunable electrical conductor with a high surface area. HITP is an organic chemical (2,3,6,7,10,11-hexaamino[[Chemistry:Triphenylene|triphenylene]]). 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&nbsp;[[Siemens (unit)|S]]&nbsp;cm<sup>−1</sup>, 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&nbsp;K and 500&nbsp;K, suggesting an unusual charge transport mechanism that has not been previously observed in organic semiconductors.<ref>{{Cite journal|title = High Electrical Conductivity in Ni<sub>3</sub>(2,3,6,7,10,11-hexaiminotriphenylene)<sub>2</sub>, a Semiconducting Metal–Organic Graphene Analogue|doi = 10.1021/ja502765n|journal = [[Organization:Journal of the American Chemical Society|Journal of the American Chemical Society]]|year = 2014|pages = 8859–8862|volume = 136|issue = 25|pmid = 24750124|first1 = Dennis|last1 = Sheberla|first2 = Lei|last2 = Sun|first3 = Martin A.|last3 = Blood-Forsythe|first4 = Süleyman|last4 = Er|first5 = Casey R.|last5 = Wade|first6 = Carl K.|last6 = Brozek|first7 = Alán|last7 = Aspuru-Guzik|first8 = Mircea|last8 = Dincă|url = http://nrs.harvard.edu/urn-3:HUL.InstRepos:23597721}}</ref>

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&nbsp;S&nbsp;cm<sup>−1</sup>, respectively.<ref>{{cite web|url=http://www.kurzweilai.net/a-new-self-assembling-graphene-like-material-for-flat-semiconductors |title=A new self-assembling graphene-like material for flat semiconductors |publisher=KurzweilAI |date=2014-05-01 |accessdate=2014-08-24}}</ref>

==Combinations==
Single layers of 2D materials can be combined into layered assemblies. For example, [[Physics:Bilayer graphene|bilayer graphene]] is a material consisting of two layers of [[Physics:Graphene|graphene]]. One of the first reports of bilayer graphene was in the seminal 2004 ''Science'' paper by [[Biography:Andre Geim|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. [[Physics:Twistronics|Twistronics]] is the study of how the angle (the twist) between layers of two-dimensional materials can change their electrical properties.

== Characterization of 2D materials ==
Microscopy techniques such as [[Engineering:Transmission electron microscopy|transmission electron microscopy]],<ref name=":19">{{Cite journal|last1=Butler|first1=Sheneve Z.|last2=Hollen|first2=Shawna M.|last3=Cao|first3=Linyou|last4=Cui|first4=Yi|last5=Gupta|first5=Jay A.|last6=Gutiérrez|first6=Humberto R.|last7=Heinz|first7=Tony F.|last8=Hong|first8=Seung Sae|last9=Huang|first9=Jiaxing|title=Progress, Challenges, and Opportunities in Two-Dimensional Materials Beyond Graphene|journal=ACS Nano|volume=7|issue=4|pages=2898–2926|doi=10.1021/nn400280c|pmid=23464873|year=2013}}</ref><ref name="Bhimanapati 11509–11539">{{Cite journal|last1=Bhimanapati|first1=Ganesh R.|last2=Lin|first2=Zhong|last3=Meunier|first3=Vincent|last4=Jung|first4=Yeonwoong|last5=Cha|first5=Judy|last6=Das|first6=Saptarshi|last7=Xiao|first7=Di|last8=Son|first8=Youngwoo|last9=Strano|first9=Michael S.|title=Recent Advances in Two-Dimensional Materials beyond Graphene|journal=ACS Nano|volume=9|issue=12|pages=11509–11539|doi=10.1021/acsnano.5b05556|pmid=26544756|year=2015}}</ref><ref name=":20">{{Cite journal|last1=Rao|first1=C. N. R.|last2=Nag|first2=Angshuman|date=2010-09-01|title=Inorganic Analogues of Graphene|journal=European Journal of Inorganic Chemistry|volume=2010|issue=27|pages=4244–4250|doi=10.1002/ejic.201000408}}</ref> 3D [[Physics:Electron diffraction|electron diffraction]],<ref name="hovden2019">{{Cite journal|last1=Sung|first1=S.H.|last2=Schnitzer|first2=N.|last3=Brown|first3=L.|last4=Park|first4=J.|last5=Hovden|first5=R.|date=2019-06-25|title=Stacking, strain, and twist in 2D materials quantified by 3D electron diffraction|journal=Physical Review Materials|volume=3|issue=6|pages=064003|doi=10.1103/PhysRevMaterials.3.064003|bibcode=2019PhRvM...3f4003S|arxiv=1905.11354}}</ref> [[Physics:Scanning probe microscopy|scanning probe microscopy]],<ref name=":54"/> [[Engineering:Scanning tunneling microscope|scanning tunneling microscope]],<ref name=":19" /> and atomic-force microscopy<ref name=":19" /><ref name=":20" /><ref name=":54" /> 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 [[Physics:Raman spectroscopy|Raman spectroscopy]],<ref name=":19" /><ref name=":20" /><ref name=":54" /> [[Physics:X-ray crystallography|X-ray diffraction]],<ref name=":19" /><ref name=":20" /> and [[Physics:X-ray photoelectron spectroscopy|X-ray photoelectron spectroscopy]].<ref name=":133"/>

== Applications ==
{{main|Chemistry:Potential applications of graphene}}

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<ref>{{Cite journal|last1=Briggs|first1=Natalie|last2=Subramanian|first2=Shruti|last3=Lin|first3=Zhong|last4=Li|first4=Xufan|last5=Zhang|first5=Xiaotian|last6=Zhang|first6=Kehao|last7=Xiao|first7=Kai|last8=Geohegan|first8=David|last9=Wallace|first9=Robert|last10=Chen|first10=Long-Qing|last11=Terrones|first11=Mauricio|last12=Ebrahimi|first12=Aida|last13=Das|first13=Saptarshi|last14=Redwing|first14=Joan|last15=Hinkle|first15=Christopher|last16=Momeni|first16=Kasra|last17=van Duin|first17=Adri|last18=Crespi|first18=Vin|last19=Kar|first19=Swastik|last20=Robinson|first20=Joshua A.|date=2019|title=A roadmap for electronic grade 2D materials|journal=2D Materials|volume=6|issue=2|pages=022001|doi=10.1088/2053-1583/aaf836|bibcode=2019TDM.....6b2001B|osti=1503991|url=https://www.osti.gov/biblio/1503991}}</ref> and optoelectronics, sensors, [[Biology:Biological engineering|biological engineering]], [[Chemistry:Filtration|filtration]], lightweight/strong composite materials, photovoltaics, medicine, quantum dots, thermal management, ethanol distillation, [[Physics:Electromagnetic shielding|electromagnetic shielding]]<ref>{{cite journal |title=Electromagnetic interference shielding with 2D transition metal carbides (MXenes) |journal=Science |volume=353 |issue=6304 |pages=1137–1140 |doi=10.1126/science.aag2421 |pmid=27609888 |year=2016 |last1=Shahzad |first1=F. |last2=Alhabeb |first2=M. |last3=Hatter |first3=C. B. |last4=Anasori |first4=B. |last5=Man Hong |first5=S. |last6=Koo |first6=C. M. |last7=Gogotsi |first7=Y. |bibcode=2016Sci...353.1137S |doi-access=free }}</ref> and [[Engineering:Energy storage|energy storage]],<ref>{{cite web |url=http://www.graphenea.com/pages/graphene-uses-applications |title=Graphene Uses & Applications |publisher=Graphenea |date= |accessdate=2014-04-13 }}</ref> [[Physical unclonable function|cryptography]]<ref>{{Cite journal|last1=cao|first1=yameng|last2=Robson|first2=Alexander J.|last3=Alharbi|first3=Abdullah|last4=Roberts|first4=Jonathan|last5=Woodhead|first5=Christopher Stephen|last6=Noori|first6=Yasir Jamal|last7=Gavito|first7=Ramon Bernardo|last8=Shahrjerdi|first8=Davood|last9=Roedig|first9=Utz|date=2017|title=Optical identification using imperfections in 2D materials|journal=2D Materials|volume=4|issue=4|pages=045021|doi=10.1088/2053-1583/aa8b4d|arxiv=1706.07949|bibcode=2017TDM.....4d5021C}}</ref> 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.<ref>{{cite web|url=http://www.appliedgraphenematerials.com/products/graphene-dispersions/|title=Applied Graphene Materials plc :: Graphene dispersions|website=appliedgraphenematerials.com}}</ref><ref>{{Cite journal|last1=Hu|first1=Guohua|last2=Kang|first2=Joohoon|last3=Ng|first3=Leonard W. T.|last4=Zhu|first4=Xiaoxi|last5=Howe|first5=Richard C. T.|last6=Jones|first6=Christopher G.|last7=Hersam|first7=Mark C.|last8=Hasan|first8=Tawfique|date=2018|title=Functional inks and printing of two-dimensional materials|journal=Chemical Society Reviews|volume=47|issue=9|pages=3265–3300|doi=10.1039/c8cs00084k|pmid=29667676|url=https://www.repository.cam.ac.uk/handle/1810/277530}}</ref>

===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 [[Physics:Nanomaterials|nanomaterials]].<ref>{{cite journal|last1=Kerativitayanan|first1=P|last2=Carrow|first2=JK|last3=Gaharwar|first3=AK|title=Nanomaterials for Engineering Stem Cell Responses|journal=Advanced Healthcare Materials|date=26 May 2015|doi=10.1002/adhm.201500272|pmid=26010739|volume=4|issue=11|pages=1600–27}}</ref> 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 [[Physics:Nanomaterials|nanomaterials]] with a high degree of [[Anisotropy|anisotropy]] and chemical functionality.<ref>{{cite journal|last1=Huang|first1=X|last2=Tan|first2=C|last3=Yin|first3=Z|last4=Zhang|first4=H|title=25th anniversary article: hybrid nanostructures based on two-dimensional nanomaterials|journal=Advanced Materials & Processes|date=9 April 2014|volume=26|issue=14|pages=2185–204|pmid=24615947|doi=10.1002/adma.201304964}}</ref> 2D nanomaterials are highly diverse in terms of their [[Engineering:Mechanical engineering|mechanical]], chemical, and optical properties, as well as in size, shape, biocompatibility, and degradability.<ref>{{cite journal|last1=Carrow|first1=James K.|last2=Gaharwar|first2=Akhilesh K.|title=Bioinspired Polymeric Nanocomposites for Regenerative Medicine|journal=Macromolecular Chemistry and Physics|date=February 2015|volume=216|issue=3|pages=248–264|doi=10.1002/macp.201400427}}</ref><ref>{{cite journal|last1=Nandwana|first1=Dinkar|last2=Ertekin|first2=Elif|title=Lattice mismatch induced ripples and wrinkles in planar graphene/boron nitride superlattices|journal=Journal of Applied Physics|date=21 June 2015|volume=117|issue=234304|pages=234304|doi=10.1063/1.4922504|arxiv=1504.02929|bibcode=2015JAP...117w4304N}}</ref> These diverse properties make 2D nanomaterials suitable for a wide range of applications, including [[Medicine:Drug delivery|drug delivery]], [[Medicine:Medical imaging|imaging]], [[Engineering:Tissue engineering|tissue engineering]], and biosensors, among others.<ref>{{cite journal|last1=Gaharwar|first1=AK|last2=Peppas|first2=NA|last3=Khademhosseini|first3=A|title=Nanocomposite hydrogels for biomedical applications|journal=Biotechnology and Bioengineering|date=March 2014|volume=111|issue=3|pages=441–53|pmid=24264728|doi=10.1002/bit.25160|pmc=3924876}}</ref> 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 [[Medicine:Drug delivery|drug delivery]] systems, where they can adsorb large numbers of drug molecules and enable superior control over release kinetics.<ref>{{cite journal|last1=Goenka|first1=S|last2=Sant|first2=V|last3=Sant|first3=S|title=Graphene-based nanomaterials for drug delivery and tissue engineering|journal=Journal of Controlled Release|date=10 January 2014|volume=173|pages=75–88|pmid=24161530|doi=10.1016/j.jconrel.2013.10.017}}</ref> 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 [[Physics:Nanocomposite hydrogels|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, [[Physics:Photothermal therapy|photothermal therapy]] (PTT), and [[Physics:Photodynamic therapy|photodynamic therapy]] (PDT).

Despite the rapid pace of development in the field of 2D nanomaterials, these materials must be carefully evaluated for [[Chemistry:Biocompatibility|biocompatibility]] in order to be relevant for biomedical applications.<ref>{{cite book|last1=Gaharwar|first1=A.K.|title=Nanomaterials in tissue engineering : fabrication and applications|date=2013|publisher=Woodhead Publishing|location=Oxford|isbn=978-0-85709-596-1|display-authors=0}}</ref> The newness of this class of materials means that even the relatively well-established 2D materials like [[Physics:Graphene|graphene]] are poorly understood in terms of their physiological interactions with living [[Biology:Tissue|tissue]]s. Additionally, the complexities of variable particle size and shape, impurities from manufacturing, and [[Biology:Protein|protein]] and immune interactions have resulted in a patchwork of knowledge on the biocompatibility of these materials.

== References ==
{{reflist}}

[[Category:Condensed matter physics]]
[[Category:Semiconductors]]
[[Category:Monolayers]]

{{Sourceattribution|Two-dimensional materials|1}}