Experimental formation of monolayer group-IV monochalcogenides
EExperimental formation of monolayer group-IV monochalcogenides
Kai Chang a) and Stuart S. P. Parkin b) Beijing Academy of Quantum Information Sciences, Beijing 100193, China Max Planck Institute of Microstructure Physics, Weinberg 2, Halle 06120, Germany (Dated: 16 June 2020)
Monolayer group-IV monochalcogenides (MX, M = Ge, Sn, Pb; X = S, Se, Te) are a family of novel two-dimensional(2D) materials that have atomic structures closely related to that of the staggered black phosphorus lattice. The structureof most monolayer MX materials exhibits a broken inversion symmetry, and many of them exhibit ferroelectricity witha reversible in-plane electric polarization. A further consequence of the noncentrosymmetric structure is that whencoupled with strong spin-orbit coupling, many MX materials are promising for the future applications in non-linearoptics, photovoltaics, spintronics and valleytronics. Nevertheless, because of the relatively large exfoliation energy, thecreation of monolayer MX materials is not easy, which hinders the integration of these materials into the fast-developingfield of 2D material heterostructures. In this Perspective, we review recent developments in experimental routes to thecreation of monolayer MX, including molecular beam epitaxy and two-step etching methods. Other approaches thatcould be used to prepare monolayer MX are also discussed, such as liquid phase exfoliation and solution phase synthesis.A quantitative comparison between these different methods is also presented.
I. INTRODUCTION
Monolayer (ML) group-IV monochalcogenides, abbreviatedas MX according to their chemical composition, are a series of2D semiconductors with many intriguing structural, optical andelectronic properties, such as in-plane ferroelectricity, fer-roelectric domain wall induced electronic state confinement, second harmonic generation, photostriction, very largeexciton binding energy, photovoltaicity, electronicvalley polarization and valley Hall effect, as well as spinsplitting of the electronic bands and giant spin Halleffect. Many of these phenomena in MX monolayers origi-nate from their noncentrosymmetric lattice structures, as Fig-ure 1 shows. Similar to the monolayers of transition metaldichalcogenides, whose lattice is equivalent to the honeycomblattice of graphene except that the two sublattices are com-posed of different atoms, MX monolayers have a structureanalogus to that of black phosphorus, but with a staggeredlattice. In MX monolayers, the broken structural inversionsymmetry results in a reversible electric polarization that liftsthe valley degeneracy, and spin-orbit coupling simultaneouslylifts the spin degeneracy. These low-symmetry monolayer ma-terials are especially useful for many non-volatile 2D materialheterostructures and devices that have been proposed recently,such as ferroelectric tunneling junctions and correspondingmemory devices.
Recent theoretical developments in thisfield have been summarized in several review articles.
Nev-ertheless, compared with the extensive theoretical advances,the development of experiments concerning the physical prop-erties of MX monolayers has evolved much more slowly.
This can be attributed, in large part, to the difficulty in cre-ating high-quality monolayer flakes or films. Moreover, thelack of controllable and scalable production methods of MXmonolayers also hinders their industrial applications.The difficulty in obtaining MX monolayers is a result of a) Electronic mail: [email protected] b) Electronic mail: [email protected]
TABLE I. Comparison of the exfoliation energies of MX materialswith other 2D materials.Material E exf (meV/Å ) Notes Reference a GeS 36.2 MX [C] GeSe 31.9 MX [C] SnS 34.6 MX [C] SnSe 32.0 MX [C] Black phosphorus 29.9 Isoelectronic of MX [C] Graphite 11.9 2D semimetal [E] hBN 18.2 2D insulator [C] MoS NbSe In Se WTe MoTe a [C] Calculations; [E] Experiments. the relatively high exfoliation energy of bulk MX materials.As shown in Table I, MX materials have significantly higherexfoliation energies than those of other intensively studied2D materials, such as graphene/graphite, hexagonalboron nitride (hBN), transition metal dichalcogenides, and other 2D ferroelectrics such as In Se . Black phosphorus,which is an isoelectronic material to MX, also has a relativelylower exfoliation energy than most of the MX materials. During the past two decades, many approaches have beendeveloped to obtain atomically-thin MX flakes, but only a few a r X i v : . [ c ond - m a t . m t r l - s c i ] J un FIG. 1. Atomic structures of graphene, MoS , monolayer black phosphorus, and monolayer GeSe (as an example of monolayer MX). For MoS ,the cyan balls correspond to Mo atoms and the yellow balls to S atoms. For GeSe, the silver balls are Ge atoms and the green balls are Se atoms.Similar to the relationship between graphene and the MoS lattice, in which the latter can be considered as a staggered graphene lattice withoutinversion symmetry, the lattice of MX is a staggered version of black phosphorus. Both monolayer MoS and MX exhibit in-plane electricpolarizations, but in monolayer MX the polarization can be reversed by an external electric field. have reached the limit of a single monolayer. For an overview,Table II summarizes the recent progress in creating ultrathinMX single crystalline flakes using various methods.For experiments concerning 2D materials, mechanical exfo-liation is usually the most straightforward approach to createatomically thin flakes, especially in the early stage of mate-rial characterization. However, simple mechanical exfoliationexperiments using standard scotch tape methods have onlyyielded MX flakes that are tens of nanometers thick, farfrom the 2D limit. Using liquid phase exfoliation, duringwhich bulk MX materials are ultrasonicated in various solvents,several-ML thick MX flakes have been obtained. The lat-eral sizes of these MX flakes are typically several hundreds ofnm, and the lowest thickness so far reached is 2 MLs .Mechanically separating the last two monolayers for MX ma-terials is especially difficult, probably because of the antiferro-electric coupling between the in-plane polarized monolayers:there is a strong attraction between the edges of the two mono-layers that host bound charges with opposite signs. Humiditymay also play an important role in the exfoliation process be-cause MX materials can dissolve water on the time scale ofjust a few nanoseconds even at room temperature. It is very interesting to reduce the thickness from 2 MLs toa single monolayer because many physical properties of MXchange significantly, as these two thicknesses have distinctcrystalline symmetries. A monolayer MX flake has broken inversion symmetry because of the in-plane polarization (spacegroup
Pmn ), while 2-ML thick MX restores inversion sym-metry because of the antiferroelectric coupling (space group Pnma ). For example, spin-splitting of electronic bands existsin monolayer MX flakes, but is absent in the 2-MLthick flakes.Since it is difficult to obtain monolayer MX flakes through“top-down” exfoliation methods, “bottom-up” synthesis meth-ods, including solution-phase and vapor-phase routes, havealso been extensively studied recently. By controlling the con-ditions of chemical reactions in solution, one can generateeither colloidal MX nanoparticles or nanoflakes.
Ap-plying a one-pot solution synthesis method, Li et al. havecreated 2-ML thick SnSe nanosheets with lateral sizes of ∼ µ m wide MX flakes. Compared with PVD and CVD, MBE has a much stricter re-
TABLE II. Experimental approaches to create ultrathin MX crystal flakes. a Method Material Substrate Crystal size Lowest thickness ReferenceMechanical exfoliation (ME) GeSe SiO /Si 4 ∼ µ m 60 ∼
140 nm Mukherjee 2013 SnSe SiO /Si Tens of µ m ∼
100 nm Tayari 2018 SnSe SiO /Si Tens of µ m 70 nm Cho 2017 Liquid phase exfoliation (LPE) GeS - ∼
70 nm 2 MLs Lam 2018 GeSe - Up to 200 nm 4 MLs Ye 2017 GeSe - ∼
300 nm 4.3 nm Ma 2019 SnS - ∼
100 nm 4.1 nm Brent 2015 SnS - Up to 180 nm 2 MLs Sarkar 2020 SnSe - ∼
50 nm 2 MLs Huang 2017 LPE + Li ion intercalation SnSe - ∼
300 nm 6 MLs Ju 2016
SnSe - ∼ µ m 6 nm Ren 2016 SnSe − x S x - Up to 200 nm 6 MLs Ju 2017 Solution phase synthesis GeS - 0.5 ∼ µ m 3 ∼
20 nm Vaughn 2010 GeSe - 0.5 ∼ µ m 5 ∼
100 nm Vaughn 2010 SnSe - 500 nm 10 ∼
40 nm Vaughn 2011 SnSe - ∼
300 nm 2 MLs Li 2013 SnS - 2 ∼ µ m × µ m 10 nm Deng 2012 SnSe - ∼
500 nm 3 nm Zhang 2014 SnS - 0.1 ∼ µ m 60 ∼
80 nm Rath 2015 SnS graphite oxide ∼ µ m 4 ∼ Molecular beam epitaxy (MBE) SnTe Graphene/SiC Up to 1 µ m 1 ML Chang 2016 ; 2019 PbTe Graphene/SiC ∼
300 nm 1 ML Chang 2016 SnSe Graphene/SiC ∼
100 nm 1 ML Chang 2020 Physical vapor deposition (PVD) GeS SiO /Si Tens of µ m 30 nm Li 2012 GeSe Mica ∼ µ m 15 nm Liu 2019 SnS Mica ∼ µ m 5.5 nm Xia 2016 SnSe Mica 1 ∼ µ m 6 nm Zhao 2015 SnSe PDMS b ∼ µ m 9 ∼
20 nm Pei 2016 SnS Au/Si 10 µ m ×
200 nm 15 nm Zhou 2016 Chemical vapor deposition (CVD) GeS SiO /Si ∼ µ m c ∼
50 nm Lan 2015 SnS SiO /Si ∼ µ m 139 nm Yu 2019 SnS SnS ∼ µ m 50 nm Li 2018 SnS SiO /Si 30 ∼ µ m 40 ∼
50 nm Nalin Mehta 2017 CVD + nitrogen etching SnSe SiO /Si Tens of µ m 1 ML Jiang 2017 ME + laser etching GeSe SiO /Si ∼ µ m 1.5 nm (1 ML?) Zhao 2018 ; Mao 2018 a In this table, one ML refers to one van der Waals layer, which is two atomic layers, or the thicness of 0.5 ∼ b Molten polydimethylsiloxane c Width of the GeS nanoribbons. d Reducing SnS flakes into SnS in ethanol vapor. quirement as regards the vacuum environment and a muchlower deposition rate. In 2016, Chang et al. managed to growthe first monolayer MX material — monolayer SnTe nanoplates— on graphitized SiC substrates, and demonstrated their ferro-electric properties by scanning tunneling microscopy (STM).The lateral size of the monolayer SnTe nanoplates can reach 1 µ m. The experimental growth of monolayer PbTe and SnSenanoplates was also reported later. It can be seen from Table II that, in general, there is astrong dependence of the lateral crystal size on the thicknessof the MX flakes: reducing the thickness typically makes thelateral size smaller. In order to solve this dilemma, some post-etching methods have been developed recently. Jiang et al. created 30 ∼ µ m large monolayer SnSe flakes by etchingCVD grown flakes with nitrogen at elevated temperatures. Lasers have also been applied to etch mechanically exfoliatedGeSe flakes, yielding 1.5 nm thick, µ m-sized patches. In this focused Perspective, we will give an insight intoexperimental approaches for creating MX monolayers. Asgeneral review articles of the synthesis of MX thin flakes/filmshave been published elsewhere, the emphasis of this Per-spective will be on currently available techniques that cancontrollably generate MX monolayers. At the same time, wewill also briefly review the routes that can create several-MLthick MX flakes, bearing in mind that improved techniques inthe future might lead to the successful creation of monolayers.In Section II, we will review the two existing experimental ap-proaches for obtaining MX monolayers — MBE and nitrogenpost-etching. There are claims that the laser etching method iscapable of creating MX monolayers, but some issues remain tobe clarified: these will also be discussed in this section. In Sec-tion III, we will review other routes that can possibly achievethe monolayer limit in the near future, including liquid phaseexfoliation and solution-phase synthesis. We will discuss theadvantages and disadvantages of these various approaches, andpresent an outlook for the future development of this topic inSection IV, and finally conclude the article in Section V.Before we begin detailed discussions, it is very importantto clearly define the notations that we use here since differentnotations have been used in the literature. In this Perspective,the orthogonal vectors a and a are the crystalline basis alongthe armchair and zigzag directions of a MX monolayer, asshown in Fig 1. Even the definition of a “monolayer” or “asingle layer” varies in the literature. Here, we shall use thedescription of “a monolayer” (1 ML) to refer to a van der Waalslayer, or two atomic layers (AL), which is the smallest possiblethickness of MX materials. One “unit cell” contains two MLs,or four ALs. II. CURRENTLY AVAILABLE ROUTES OF CREATING MXMONOLAYERSA. Molecular beam epitaxy
In the forty years’ history of the MBE growth of MX mate-rials, studies mainly focused on the (001)- and (111)-orientedlead monochalcogenides (PbS, PbSe and PbTe) with rock saltstructures, because of their importance as narrow-bandgapsemiconductors for mid-infrared lasers and sensors. Mostof the films grown for these purposes are thicker than 100nm. Since the discovery of the topological crystalline insulator(TCI) phase in MX materials such as SnTe, Pb − x Sn x Te andPb − x Sn x Se, and the prediction of a low-dimensional TCIphase in their atomic-thin films, multiple MBE growthstudies of ultrathin MX films have also been reported.
However, none of these studies have reached the thicknessof a single monolayer. Furthermore, all of these films are inrock salt structures, while the MBE growth of staggered blackphosphorus structured MX is still rare.
The first MBE experiment that unambiguously reached themonolayer limit of MX was the growth of ferroelectric mono-layer SnTe nanosheets on graphitized 6H-SiC(0001) substrates,reported by Chang et al. in 2016 . (It should be noted that amonolayer of SnTe was referred to as “one unit cell” in thatarticle, because a rhombic distorted rock salt lattice, whoseunit cell contains two atomic layers, was presumed at thattime.) In the Supporting Material of this article, the authorsalso reported the growth of monolayer PbTe nanosheets asan example of a paraelectric MX material for comparison,which has a similar growth mode as SnTe monolayers. Inthese experiments, SnTe/PbTe nanosheets were deposited frommolecular fluxes generated by heating up hBN crucibles filledwith high-purity SnTe/PbTe granules in ultra-high vacuum, asillustrated in Fig. 2(a). Variable-temperature STM was uti-lized to characterize the in-plane ferroelectricity in monolayerSnTe nanosheets from the perspectives of lattice distortion,nanosheet edge bound charge induced electronic band bend- ing, and the domain wall motion induced by applying electricfield pulses between the STM tip and the sample. Surprisingly,the ferroelectric transition temperature T c of SnTe monolayersreaches 270 K, much higher than that of bulk SnTe. Subsequentstudies revealed the mechanism behind this counterintuitiveenhancement phenomenon, which is related to a thickness de-pendent structural phase transition . We will further discussthe growth mechanism of SnTe nanosheets in the first sectionbelow.Very recently, monolayer SnSe nanoplates have also beensuccessfully prepared through a two-step MBE growth recipe,also on graphene/SiC substrates. Controlled room tempera-ture ferroelectric switching and a T c close to 400 K have beendemonstrated in these monolayer nanoplates. In the secondsection below, we will discuss the growth of monolayer SnSenanoplates.
1. MBE growth of SnTe and PbTe monolayers
The choice and treatment of substrates is highly importantfor the MBE growth of atomically-thin materials. Comparedwith other substrates, the graphene surface is extremely smoothand has no dangling bonds, which guarantees a van der Waalsepitaxial growth. Since the graphene substrates used for MBEexperiments should be uniform and single-crystalline acrossthe surface, monolayer or bilayer graphene, epitaxially grownon the surface of Si-terminated 6H-SiC(0001) substrates wereused. There are three approaches to prepare the graphene/SiCsubstrates: (i) annealing SiC in an H /Ar gas mixture; (ii)annealing doped SiC in an ultra-high vacuum environment bypassing direct current through the substrate in a Si molecularflux; (iii) flash annealing SiC in ultra-high vacuum by directcurrent heating. A full coverage of graphene on the sub-strate surface is essential for obtaining monolayer nanosheets.As Fig. 2(c) shows, on insufficiently graphitized substrates,there are some exposed SiC patches with ( √ × √ ) R30 ◦ reconstruction (sometimes also termed a 6 × Compared with graphene surfaces, these patches have signif-icantly higher surface energy, and as a result, the nucleationrate of SnTe is much higher on these patches, leading to thickclusters hindering the subsequent STM studies.On properly graphitized SiC substrates, monolayer SnTenanosheets as large as ∼ µ m and monolayer PbTe nanosheetsof ∼
300 nm in size have been grown, as shown in Fig. 2(e) and(f). Specifically, when a monolayer SnTe nanosheet is quicklycooled down below T c , regular parallel or needle-shaped 90 ◦ ferroelectric domains are observed by STM. By displaying thehole-like electronic standing waves between parallel domainwalls, a space-resolved scanning tunneling spectroscopy studyat liquid helium temperature reveals the strong confinementeffect of these electrically neutral domain walls to the holestates at the valence band maximum of a SnTe monolayer. The origin of such a strong electronic state modulation effectis attributed to the mismatch of hole valleys in k space across a90 ◦ domain wall, which is a novel type of “electronic valleyquantum well”.Neither SnTe nor PbTe has a preferred crystalline orienta- FIG. 2. (a) Schematic diagram of the MBE growth of monolayer SnTe nanoplates on graphene/SiC substrates. (b)-(f) are STM topographyimages. (b) A graphene/SiC substrate flash-annealed in ultra-high vacuum. The solid and dashed arrows indicate the crystal basis of SiC andgraphene, respectively. (c) Small SiC patches on insufficiently graphitized substrates. (d) The nucleation sites of SnTe at room temperature. Thesubstrate was kept at room temperature during the deposition. All the irregular SnTe islands are one monolayer thick. The cyan arrows labelthe islands nucleating at the SiC patches, and the white arrows indicate those that nucleate at the other sites of the surface. Copyright, 2019,authors. Reproduced under CC BY 4.0. (e) Part of a ∼ µ m wide monolayer SnTe nanoplate with 90 ◦ domains resolved. Setpoint: sample biasvoltage V s = − . I t =
30 pA. Measured at T = . − . − . Reproduced with permission. tion on graphene, which is another strong indication of vander Waals bonding. At the substrate temperature of 200 ◦ C, themonolayer nanosheets have nearly rectangular shapes, withatomically smooth edges along the (cid:104) (cid:105) directions. The nucle-ation rate increases exponentially as the substrate temperaturedecreases. However, lowering the substrate temperature alsomakes the second monolayer easier to form, thus hindering thegrowth of a fully-capped uniform monolayer film.It is usually impossible to mechanically exfoliate ultra-thin SnTe flakes from bulk materials, because it has a three-dimensional (3D) rock salt structure (space group Fm m ) atroom temperature. However, in the aforementioned MBE ex-periments, the growth of ultrathin SnTe and PbTe nanosheetsexhibit strong 2D character. This is the result of a thickness-dependent structural phase transition, as shown in Fig. 3(a)and (b). Bulk SnTe has a rock salt structured β phase (or aslightly distorted rhombic α phase with the space group of R m when the temperature is lower than its ferroelectric transitiontemperature, typically below 100 K), while as the thicknessdecreases, the layered γ phase (space group Pnma for even- ML thick plates and
Pmn for odd-ML thick plates), whichis isostructural to GeS, GeSe, SnS and SnSe, becomes themost stable structure. Besides the lattice structures, γ -SnTeand α / β -SnTe are also different in many other respects, asshown in Fig 3(c)-(g). The width of atomic terraces can easilyreach hundreds of nanometers in γ -SnTe, while in α / β -SnTe,this number is only in the tens of nmnanometers. Besides, theheight of atomic steps is always 1 ML at the surface of γ -SnTe,while 0.5-ML (1 atomic layer) atomic steps dominate at thesurface of α / β -SnTe. Last but not least, the concentrationof Sn vacancies in α / β -SnTe (10 ∼ cm − ) is severalorders of magnitude higher than that in γ -SnTe (10 ∼ cm − ), which introduces a large number of p-type carriers andresults in a deterioration of ferroelectricity in bulk SnTe. Therobust in-plane ferroelectricity in monolayer SnTe nanoplatesis found in γ -SnTe.All the 1-ML, 2-ML and nearly all the 3-ML thick SnTenanosheets take up the γ phase, while above 4-ML, the ratioof the α / β phase gradually increases. There is not a certaincritical thickness for the structural phase transition, because FIG. 3. (a)-(b) Atomic structures of SnTe in the layered γ phase and the 3D α / β phase. The α phase has a slight rhombic distortion along(111) from the rock salt structure, and the β phase has a undistorted rock salt structure. (c)-(d) STM topography images of the areas in α phase(c) and γ phase (d). (e)-(f) Height profiles along the line segments indicated in (c) and (d), respectively. “AL” refers to “atomic layer”. 1 MLcontains 2 ALs. (g) Atom-resolved topography image on the boarder of two phases on the same terrace. Setpoint: V s = − . I t =
30 pA.All the images were acquired at 77 K. Copyright, 2018, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, reproduced with permission. (h) The percentage of γ -SnTe as a function of the average thickness of the SnTe film and the substrate temperature. Copyright, 2019, authors,reproduced under CC BY 4.0. the γ phase is still metastable above the thickness of 4-ML.Nevertheless, γ -SnTe nanosheets thicker than 10 MLs are veryrare, as shown in Fig 3(h).The discovery of a structural phase transition in SnTenanosheets unambiguously bridges monolayer SnTe withthe monolayers of other ferroelectric group-IV monochalco-genides, which have been theoretically predicted since2013 . Although detailed thickness-dependent studyof PbTe nanosheets is still absent, it is very likely that a similar3D-to-2D structural phase transition also takes place, whichaccounts for the large monolayer PbTe nanosheets in Fig 2(f).
2. Two-step MBE growth of SnSe monolayers
Although bulk SnSe itself has a layered structure, it is never-theless difficult to grow large-area monolayer SnSe nanoplateson graphene/SiC substrates with a single deposition process,probably because of a higher surface energy in SnSe, inducedby a larger lattice buckling compared with that of γ -SnTe. The dilemma in the one-step MBE growth of SnSe is that, atrelatively low substrate temperatures (70 ◦ C, for instance), thenuclei are 1-ML thick, but the second monolayer starts to growwhen the nanoplates are only tens of nanometers wide; while athigher substrate temperatures, the nuclei are thicker than 1 ML.A temperature window that can balance the nuclei thicknessand nanoplate size has not been found yet.Very recently, a two-step MBE growth recipe has been de- veloped to prepare monolayer SnSe nanoplates with widthsabove 100 nm. The procedure is demonstrated in Fig. 4. First,the substrate temperature is stabilized at 40 ∼ ◦ C during afirst deposition that lasts for only 30 seconds, so that the sec-ond monolayer of SnSe does not grow. Then the sample isannealed at 210 ◦ C for 1 hour, time during which the irregularSnSe islands become rectangular. Finally, a second depositionis carried out using the same parameters as the first one, ex-cept that the substrate temperature is kept at 210 ◦ C. The smallmonolayer SnSe islands grow into larger nanoplates whichretain a rectangular shape.Interestingly, because of the coincidental lattice matchingbetween graphene ( a g = .
46 Å) and the a of monolayer SnSe(4.26 Å, which is √ a g ), the monolayer SnSenanoplates follow a highly oriented growth mode, which is verydifferent from monolayer SnTe and PbTe. According to bothreflection high energy electron diffraction (RHEED) patternsand atom resolved topography images recorded by STM, the a direction of SnSe is always parallel to the zigzag direction ofgraphene. The oriented growth of SnSe nanoplates is greatlybeneficial for anisotropic characterization experiments, suchas angle-resolved photoemission spectroscopy (ARPES) andsecond-harmonic generation (SHG) measurements.It should be noted that the phase diagram of SnSe has asignificant difference compared with those of either SnTe orPbTe.
Sn (Pb) and Te form only one stable binary compound,which is Sn(Pb)Te; however, Sn and Se can form both SnSe andSnSe . Therefore, when growing SnTe (PbTe) from separate FIG. 4. (a) STM topography image of the nuclei of monolayer SnSedeposited on a graphene substrate. The substrate was kept at 40 ◦ Cduring the 30s long deposition. (b) The same sample as in (a) afterannealing at 210 ◦ C for 1h. (c) The topography after a second depo-sition lasting for 180s, with the substrate temperature set to 210 ◦ C.Setpoint: V s = − . I t = ∼ Sn (Pb) and Te evaporation sources, it is safe to apply extraTe flux, as the excessive Te molecules will re-evaporate atsufficiently high substrate temperatures. Nevertheless, if SnSeis grown in this way, the Se flux must be carefully controlledto prevent the formation of SnSe . Another simpler solution isto directly evaporate SnSe molecules from high-purity SnSegranules, which is the method adopted in the two-step growthprocess described above.In principle, other MX monolayers, such as GeS, GeSe andSnS, can also be grown in a similar way as SnSe, if their nucleiare also 1-ML thick at certain temperatures. However, giventhat these materials have larger lattice buckling than SnSe, thesize of nuclei islands in the first step probably needs to befurther reduced. Currently, the lateral size of SnSe nanoplatesprepared by this method is mainly limited by the distancebetween neighboring nuclei. Controlling the density of nucle-ation centers on graphene substrates is an important topic forfurther studies. B. Nitrogen post-etching method
In 2017, Jiang et al. reported a two-step growth-etchingmethod of obtaining monolayer SnSe flakes that are tens of µ m wide, as shown in Fig. 5. The first step is the growth ofrectangular SnSe flakes by vapor transport deposition, whichis relatively routine. The SnSe powders, placed within a ce-ramic boat, are evaporated at 700 ◦ C in a tube furnace in anAr/H flow: this results in ∼
50 nm thick rectangularly shapedSnSe flakes to be deposited onto a SiO /Si substrate that isupside down, facing the boat. In a second step, a nitrogenflow is introduced into the tube furnace, and the SnSe flakes,thereby, are etched at 700 ◦ C, during which the rectangularshape of the flakes are retained but their thickness is reduced toa single monolayer. Electrical transport experiments on thesemonolayer SnSe flakes show that they are p-type intrinsicallydoped, which is consistent with the MBE grown monolayerSnSe nanoplates that was described in the previous section.The mechanism behind this etching process is intriguing.The authors have tried different types of etching gases, includ-ing pure Ar, an Ar/H mixture, and pure H . The etching effectof pure Ar is very weak, while pure H causes severe deteri-oration at the surface of the flakes. An Ar/H mixture has asimilar etching effect as nitrogen, but the latter is safer since itis not flammable. Furthermore, according to the experimentsin which the etching times were varied from 1 to 20 min, thereis, likely, a self-limiting mechanism: the etching halts oncethe whole flake is uniformly 1-ML thick. This mechanism isespecially ideal for the fabrication of devices based on largemonolayer SnSe flakes. The reason for this self-limiting etch-ing process is still not clear. Nevertheless, the etching processis not layer-by-layer, but rather starts from the edges of a flake,which hinders precise thickness control for creating uniformseveral-ML thick flakes. C. Laser post-etching method
Etching using a high-power laser is another approach thatcan etch the mechanically exfoliated MX flakes down to atomicthicknesses. In 2018, Zhao et al. and Mao et al. reported thelaser etching of GeSe flakes, from over 100 nm to a lowestthickness of 1.5 nm, as illustrated in Fig. 6(a) and (b).
Thelateral size of the thinned areas can reach several µ m in ex-tent. Similar to the nitrogen etching discussed above, this laseretching procedure also shows a self-limiting behavior. As thelaser power density is gradually increased to 9 . × W/cm (with an etching time fixed at 5 min), the minimum thicknessof the thinned area decreases monotonically to 1.5 nm; whilewhen the laser power density is further increased, the minimumthickness stops decreasing. This property makes such a laserthinning method a robust approach for fabricating ultrathinMX flakes with a certain thickness. The authors reported anoptimal thinning laser power density of 36 . × W/cm , atwhich the resulting thinned GeSe exhibits maximum photo-luminescence intensity. Furthermore, as compared with bulkGeSe, the 1.5-nm thick GeSe exhibits a faster response andhigher sensitivity in photocurrent measurements [see Fig. 6(c)and (d)], implying an indirect-to-direct band gap transition asthe thickness is decreased, and suggesting that atomically-thinGeSe is a promising photodetection and photovoltaic material.However, it should be noted that, although the authors as- FIG. 5. (a) Schematic diagrams of the growth of bulk rectangular SnSe flakes by vapor transport deposition (upper panels) and monolayerrectangular SnSe flakes resulting from nitrogen etching (lower panels). (b) Optical image of an as-synthesized bulk rectangularly shaped SnSeflake. (c) A typical atomic force microscopy (AFM) image at the edge of a bulk flake. (d) Optical image of an as-synthesized monolayer SnSeflake. (e) A typical AFM image at the edge of a monolayer SnSe flake. Copyright, 2017, IOP Publishing Ltd. Reproduced with permission.FIG. 6. (a) AFM topography image of a mechanically exfoliated GeSenanosheet, with the central area laser etched down to a thickness of1.5 nm. The sample is subsequently annealed at 200 ◦ C to furtherdecrease the roughness of the etched area. Copyright, 2018, authors. Reproduced under CC BY 4.0. (b) Thinning laser power densitiesdependence of the average minimum layer thickness. (c) and (d)Photoresponse characteristics of the devices made from a pristineGeSe nanosheet (c) and a second one that is laser thinned to 1.5 nm(d). Copyright, 2018, WILEY-VCH Verlag GmbH & Co. KGaA,Weinheim. Reproduced with permission. cribe the thinnest GeSe (1.5 nm) to a single monolayer, thisthickness is not consistent with other reports: both the MBEgrown and nitrogen etched monolayer MX flakes reviewedabove show thicknesses of 0 . ∼ ∼ III. OTHER POSSIBLE ROUTES TO CREATEMONOLAYER MXA. Liquid phase exfoliation
Since a simple scotch tape exfoliation method has not yetbeen shown to generate atomically-thin MX flakes, liquidphase exfoliation recipes have been developed, which includethree main steps, as illustrated in Fig 7 (a). In a first step, bulkMX crystalline granules or powders are ultrasonicated in a sol-vent, during which the cavity bubbles generated by the intensesound waves will collapse to form a high energy jet, whichbreaks the bulk layered compounds into thin sheets. Varioussolvents have been used in different studies, including water,hexane, ethanol, acetone, chloroform, N-methylpyrrolidone(NMP), dimethylformamide (DMF), and isopropyl alcohol(IPA), etc.
Then, the solvent is centrifuged to sepa-rate out the larger particles, leaving only nanosheets. Finally,the resulting nanosheets can be redistributed in another solventto form a stable colloidal dispersion. Fig. 7(b)-(d) show typicalTEM images of the nanosheets obtained via such a liquid phaseexfoliation approach. The lowest thickness of MX nanosheetsachieved in this approach is 2 MLs, with lateral sizes rangingfrom several tens of nanometers to ∼
200 nm.Several factors are important in determining the yield fromliquid phase exfoliation procedures. First, the surface ten-sion and polarity of the solvents, which influence the rate ofre-stacking of the nanosheets, play essential roles in the exfo-liation processes.
A comparative study suggests that NMP
FIG. 7. (a) Schematic illustration of liquid phase exfoliation of MX materials. (b) Transmission electron micrography (TEM) image of exfoliatedGeSe sheets. Inset, selected area electron diffraction (SAED) pattern of the nanosheet inside the red rectangle. (c) TEM image of a single GeSenanosheet. (d) High-resolution TEM image of GeSe nanosheet with lattice fringes. (e)-(g) Statistics of the thicknesses of GeSe nanosheetscollected at different centrifugation speeds. Copyright, 2017, American Chemical Society. Reproduced with permission. (h) AFM image ofexfoliated SnS nanosheets. The height profile is from a 2-ML thick nanosheet as indicated by the horizontal line. Copyright, 2020, authors. Reproduced under CC BY 4.0. yields the darkest dispersion of GeS, implying a high exfo-liation efficiency and stable dispersion. Second, a highercentrifugation speed tends to yield thinner nanosheets, as il-lustrated in Fig 7(e)-(g). With a centrifugation speed of 9000rpm, the peak of the resulting thickness distribution curve canbe pushed to 2 nm, corresponding to 4 MLs.Furthermore, given that MX materials are promising forlithium-ion batteries because of their high energy capacity, high Li + diffusion coefficient and low diffusion barrier, there have been a series of studies concerning liquid phase ex-foliation after Li ion intercalation into bulk MX materials, which are also very useful for photovoltaic and thermoelectricdevices. B. Solution phase synthesis
As a fast and low-cost method for producing nanomaterialsfor large scale industrial applications, the solution phase synthe-sis of MX nanosheets has drawn significant attention. Becauseof the highly anisotropic crystalline structure of MX materi-als, colloidal thin flakes can be generated through chemicalreactions that take place in organic solvents . All of thesestudies use inorganic halides as the source of the group-IVelements, such as SnCl , SnCl , GeI , SnI , etc. The source of the group-VI elements is usually organic mate-rial, such as dodecanethiol, trioctylphosphine selenide (TOP-Se), and thioacetamide, while recipes with cheaperand/or less toxic inorganic materials such as NaHS, SeO and (NH ) S have also been developed. These reactants are FIG. 8. SnSe nanosheets synthesized with colloidal one-pot reactionmethods. (a) TEM image of rectangularly shaped SnSe nanosheetswith thicknesses ranging from 10 to 40 nm. Copyright, 2011, Ameri-can Chemical Society. Reproduced with permission. (b) TEM and(c) AFM images of SnSe nanosheets with thickness of 2 MLs (oneunit cell, or 4 atomic layers). (d) HRTEM image and SAED pattern ofa SnSe nanosheet. Copyright, 2013, American Chemical Society. Reproduced with permission. in a flask, heated upto reaction temperatures ranging from 180 ◦ C to 320 ◦ C, thenthe products are dispersed into new organic solvents, and cen-trifuged to separate away large particles. The last two steps arerepeated until stable colloidal nanosheets are obtained.With this procedure, one can usually obtain several-nm-thick nanosheets with lateral sizes of several hundred nanome-ters. Specifically, in 2013, Li et al. reported the synthesis of ∼
300 nm-wide and 2-ML thick SnSe nanosheets (referred toas “single-layer” in the article, which actually means a singleunit cell, or 4 atomic layers) using a one-pot recipe, which wasthe thinnest MX nanosheet that had been formed at that time[Fig. 8(b)-(d)]. Substrates can also be applied in solution-phase synthe-sis. For example, Li et al. reported the synthesis of SnSenanosheets on graphite oxide surfaces in 2015. IV. DISCUSSION
In Fig. 9, we summarize the thickness and lateral size ofMX flakes created by different methods listed in Table II. Ingeneral, methods that can generate thin and large flakes arefavored, which corresponds to the upper-left corner of Fig. 9(a).However, it is clearly shown that, except from flakes preparedby etching methods, the dimensions of all the as-preparedflakes, no matter whether exfoliated or synthesized, followa simple rule: the thickness and lateral size are positivelycorrelated. On the one hand, mechanical exfoliation, PVD andCVD methods can produce flakes as large as tens of µ m inextent, but it is difficult to reduce the thickness below 5 nm.On the other hand, MBE, liquid phase exfoliation and solutionphase synthesis methods can create atomically-thin, or evenmonolayer thick flakes, but the average lateral sizes of all theflakes thinner than 4 MLs are smaller than 1 µ m. Again, therelatively large inter-layer coupling energy in MX materials isprobably the reason for this dependence.Based on current studies, there are two possible routes tosolve this dilemma. First, since this limit originates from theintrinsic properties of MX, one can introduce extrinsic factorsto enhance the anisotropy of MX flakes. Proper choice ofsubstrates and synthesis recipes could make the tendency forlateral growth stronger. For example, monolayer nanosheetsof SnTe, over 1 µ m in extent, have been found using MBEgrowth. Second, chemical and physical etching methods canovercome the intrinsic limit due to the material’s anisotropy,and thus are promising for creating large monolayer MX flakesthat are suitable for device fabrication. Especially, the self-limiting etching mechanism, which has been observed both innitrogen and laser etching, guarantees a uniform final thickness.Such a self-limiting mechanism is likely to be a result of thestrong interaction at the interface between the substrate andthe MX flakes, which, in turn, leads to new questions: how thesubstrate influences the atomic and electronic structures of MXflakes, and whether the monolayer MX flakes strongly bond tothe substrate while still maintaining their intrinsic properties,are important topics that should be explored in future studies. −1 Thickness (nm) La t e r a l s i z e ( n m ) MELPESPSPVDCVDMBECVD+EME+L10 ME LPE SPS MBE PVD CVDCVD+E M E + L S i z e / t h i ck ne ss r a t i o M L 2 M L (a)(b) FIG. 9. (a) Plot of lateral size against the lowest thickness of MXflakes fabricated using various methods, from the references given inTable II. Abbreviations, SPS: solution phase synthesis; CVD+E: CVD+ nitrogen etching; ME+L: mechanical exfoliation + laser etching. (b)Comparison of the ratio between the lateral size and thickness of theMX flakes in (a).
These points above are better supported by exploring theanisotropy ratio, which is the ratio between the lateral size andthe thickness of the MX flakes created by different methods,in Fig. 9(b). For the methods that do not introduce any extraanisotropy, such as exfoliation and solution phase synthesis,the anisotropy ratios are generally around 10 . For the methodsthat require substrates, which introduces extra anisotropy to thesynthesis process, such as MBE, PVD and CVD, the anisotropyratio can be close to 10 . With etching methods, the anisotropicratio can be further pushed above 10 , where the lateral size isonly limited by that of the pristine thicker flakes. V. CONCLUSIONS
In this Perspective, we have reviewed experimental ap-proaches that have successfully created monolayer MX, aswell as those that could be used to generate monolayer MXin the near future. Currently, clear reports of the creation ofMX monolayers include MBE grown SnTe, PbTe and SnSe1(among which the growth of monolayer SnSe adopts a two-steprecipe), and SnSe flakes that are synthesized by CVD and thenetched in a nitrogen atmosphere. Laser etching of mechan-ically exfoliated GeSe nanoflakes can also achieve a stablethickness of 1.5 nm. Despite them being claimed as monolay-ers, the actual thickness of the laser thinned samples needs tobe cross-checked by further experiments. Both the chemicalnitrogen etching and physical laser etching processes exhibitself-limiting mechanisms, which allow large windows for theetching parameters. Furthermore, 2-ML thick MX flakes canbe created through liquid phase exfoliation and solution phasesynthesis. We have quantitatively compared the dimensions ofthe MX flakes fabricated in various approaches, and suggestthat the introduction of extra anisotropy during synthesis or byusing post-etching techniques are the keys to the creation oflarge-size monolayer MX flakes.Note added: We noticed that a study of the PVD growth ofmonolayer SnS and the demonstration of its in-plane ferroelec-tricity was published during peer review.
ACKNOWLEDGMENTS
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