Atomistic mechanisms of van der Waals epitaxy and property ...Advanced Review Atomistic mechanisms of van der Waals epitaxy and property optimization of layered materials Jin-Ho Choi,1,2

Advanced Review

Atomistic mechanisms of van derWaals epitaxy and propertyoptimization of layered materialsJin-Ho Choi,1,2 Ping Cui,1 Wei Chen,1,3 Jun-Hyung Cho1,4 and Zhenyu Zhang1*

Since the first isolation of graphene from graphite in 2004, atomically thin orlayered materials have been occupying the central stage of today’s condensedmatter physics and materials sciences because of their rich and exotic propertiesin two dimensions (2D). Many members of the ever-expanding 2D materialsfamily, such as graphene, silicene, phosphorene, borophene, hexagonal boronnitride, transition metal dichalcogenides, and even the strong topological insula-tors, share the distinct commonality of possessing relatively weak van der Waals(vdW) interlayer coupling, whereas each member may invoke its own fabricationapproaches, and is characterized by its unique properties. In this review article,we first discuss the major atomistic processes and related morphological evolu-tion in the epitaxial growth of vdW layered materials, including nucleation, diffu-sion, feedstock dissociation, and grain boundaries. Representative systemscovered include the vdW epitaxy of both monolayered 2D systems and their lat-eral or vdW-stacked heterostructures, emphasizing the vital importance of thevdW interactions in these systems. We also briefly highlight on some of therecent advances in the property optimization and functionalization of the 2Dmaterials, especially in the fields of optics, electronics, and spintronics. © 2017 John

Wiley & Sons, Ltd

How to cite this article:WIREs Comput Mol Sci 2017, 7:e1300. doi: 10.1002/wcms.1300

INTRODUCTION

Graphene, a one-atom-thick crystal of carbonatoms, was first realized by mechanical exfolia-

tion from graphite in 2004,1 and has since served asa significant platform in the fields of condensed

matter physics and interdisciplinary sciences. One ofthe greatest breakthroughs in graphene research isthe discovery of its exotic electronic and transportproperties,2–4 paving a promising way for developingpost-silicon electronics. This technological drive natu-rally leads to intensive efforts for achieving mass pro-duction of high-quality, large-area monolayercrystalline graphene, including the developments ofvarious epitaxial growth techniques on propersubstrates.5–8 Meanwhile, the unprecedented proper-ties of graphene that arise from its two-dimensional(2D) geometry have motivated extensive research ofother 2D layered materials, such as silicene,9–11

phosphorene,12,13 borophene,14,15 hexagonal boronnitride (h-BN),16,17 transition metal dichalcogenides(TMDs),18,19 and even the strong topological insula-tors.20,21 On one hand, each member of the fast-expanding 2D materials family may invoke its ownepitaxial growth mechanisms depending on the con-stituent elements, atomic structure, and growth

*Correspondence to: [email protected] Center for Quantum Design of Functional Materials(ICQD), Hefei National Laboratory for Physical Sciences at theMicroscale, and Synergetic Innovation Center of Quantum Infor-mation and Quantum Physics, University of Science and Technol-ogy of China, Hefei, China2Research Institute of Mechanical Technology, Pusan NationalUniversity, Pusan, Korea3Department of Physics and School of Engineering and AppliedSciences, Harvard University, Cambridge, MA, USA4Department of Physics and Research Institute for NaturalSciences, Hanyang University, Seoul, Korea

Conflict of interest: The authors have declared no conflicts of inter-est for this article.

Volume 7, May/June 2017 © 2017 John Wiley & Sons, Ltd 1 of 21

substrates. On the other hand, the layered nature ofthe 2D family inevitably results in a clear commonal-ity that in most 2D materials the interlayer couplingis relatively weak, which is typically of the van derWaals (vdW) nature.22

The vdW force is ubiquitous,23 and its signifi-cance in nonequilibrium growth of nanostructures oncrystalline substrates has gradually been recognized.A compelling example is the self-assembly of 1Dmolecular wires on semiconductor surfaces,24 wherethe attractive vdW interactions between neighboringadsorbed molecules were first shown to enable theirclose alignment to form a linearly ordered struc-ture.25 The vdW interactions between planar aro-matic molecules and metal surfaces have also beenemphasized to play an important role in the growthof low-D nanostructures.26–28 Similarly, the vdWinteractions were found to be crucial to the epitaxialgrowth of 2D layered materials, such as grapheneand h-BN.29 A substantial advantage of the vdW epi-taxy is the weak interlayer interaction (without spe-cific chemical bonds) that can help to circumvent thelattice mismatch problems commonly encountered inconventional epitaxial growth.30 Such weak vdWinterlayer couplings also allow facile integration ofdifferent 2D materials to form vdW heterostructures,opening the possibility of design of new functionalmaterials.16,19 Quantitative analysis of the vdWeffects in epitaxial materials growth has been enabledby several improvements in first-principlesapproaches.31–33 Although the accuracy of respectivecomputational methods is still an issue in the theorycommunity, to a large extent, the inclusion of thevdW interactions has been found to provide muchmore accurate descriptions of molecular adsorptionand low-D nanostructure growth on varioussubstrates.25,34

The successful fabrications of 2D layered mate-rials have ignited an enormous research interest intheir potential applications to a wide range of devicetechnologies, for example, optics, electronics, spin-tronics, and photovoltaics.8,29 In order to utilize the2D materials family in the practical design ofhigh-performance devices, it is a prerequisite tounderstand and optimize their fundamental physicalproperties, such as their electronic structure, magnet-ism, optical response, excitonic effects, and catalyticreactivity.

This review first focuses on the major atomisticmechanisms in the vdW epitaxial growth of grapheneand other emerging 2D layered materials, includingnucleation, diffusion, feedstock dissociation, andgrain boundaries. The first and second major sectionscover the epitaxial growth of the monolayer 2D

systems, and the third section discusses their lateralor vdW-stacked heterostructures. Then, the finalsection introduces some selective examples from therecent advances in the property optimization andfunctionalization of the 2D materials, especially inthe fields of optics, electronics, and spintronics.

GROWTH OF GRAPHENE

Nucleation of C–C DimersLarge-area and high-quality graphene samples havebeen epitaxially synthesized on several metal sub-strates, including Ru,6,35,36 Ir,6,37,38 Ni,8,39 and Cusurfaces.5,7,8 Such graphene epitaxies are typicallybased on the following approaches: carbon (C) vapordeposition, chemical vapor deposition (CVD) ofhydrocarbons, and C segregation from the bulkmetals.6 Whichever approach is adopted, the growthof graphene requires an intermediate process tonucleate C atoms. For example, graphene flakesnucleate on Ir(111) and Ru(0001) by consuming thesupersaturated 2D gas of C adatoms when either Cor ethylene (C2H4) gas is used.6,35 As a probablenucleation mechanism, the attachment of C clustersincluding around five atoms has been proposed,6,35

with minimal effects of hydrogen on C adsorptionand clustering.6

Island nucleation preference at the step edgeshas been experimentally observed on stepped sur-faces. For example, on stepped Ir(111) andRu(0001), carbon nucleation starts at the lower edgesof the steps, rather than on the terraces.35,37 In fact,similar step-edge preference of C nucleation onstepped (or defective) Ni(111) was reported in earlierstudies of carbon nanotube growth, which was inter-preted by the strong binding between the C adatoms(monomers) and the step edges.40,41 More impor-tantly, it was found that C nucleation in the initialstage of growth can play a decisive role in determin-ing the overall quality and size of epitaxial graph-ene.6 In order to produce sizable and uniformgraphene samples, it is therefore desirable to under-stand the nucleation behavior on different metal sur-faces. In particular, nucleation of graphene islandsover the entire substrate can be more beneficial forfast and mass production of graphene, rather thanonly at the step edges.

In our previous work, we presented thesubstrate-dependent mechanisms of C nucleation inthe epitaxial growth of graphene beyond the hithertobelieved monomer picture, using the examples of rep-resentative stepped metal surfaces: Ir(111), Ru(0001),and Cu(111).42 It was found that a C adatom prefers

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to occupy subsurface sites and also diffuses slowerthan a C–C dimer, which prefers to stay on top ofthe metal substrate for the right metal. UnlikeNi(111), adsorption of C monomers at the steps ofIr(111) and Ru(0001) was found to be no longerenergetically highly favorable, indicating that themonomer-based picture is insufficient to explain theaforementioned step-edge nucleation preferenceobserved experimentally. Instead, it was found thatthe substrate steps prefer the formation of C–Cdimers at their lower edges, invoking a novel dimer-based picture (Figure 1(a)). Furthermore, on flat ter-races, Ir(111) and Ru(0001) show a repulsive interac-tion between C monomers, whereas Cu(111) exhibitsan attractive C–C interatomic interaction; this

eventually leads to widespread nucleation of C–Cdimers over the entire Cu(111) surface, which is ben-eficial for the growth of large-area graphene.

Such contrasting behaviors in the C–C dimerformation can be attributed to the delicate competi-tion between the C–C bonding and C–substratebonding: the weaker the C–substrate interaction is,the more preferred the C–C dimers are.42 Thisgeneric principle can provide a predictive tool for thenucleation mechanism in the initial stage of graphenegrowth on different metal substrates, as shown inFigure 1(b).

Diffusion and GrowthThe novel C–C dimer picture implies that C speciesdiffuse in dimer configurations, whose full effectsneed to be considered in investigations of epitaxialgraphene growth. It is known that graphene growthon Ru(0001) is not limited by the surface C diffusionbut by C attachment (or reaction limited),6 whereasgraphene growth on dimer-dominant Cu(111) is dif-fusion limited.43 In other words, on Cu(111), the dif-fusion of C–C dimers determines the growth rate ofgraphene. First-principles studies reported that thediffusion of C–C dimers on Cu(111) occurs at amuch faster speed than that of C into Cu bulk andthe dissociation of C–C dimers.42,44 It is notable thatthe diffusion-limited growth of graphene is a uniquecharacteristic on Cu(111), which has been proven tobe beneficial for graphene growth.43–45 Indeed,graphene growth even on similar dimer-dominant45

Cu(100) is found to be attachment limited.46 Thegrowth picture with mobile C–C dimers as the domi-nant feeding species is qualitatively different from theconventional monomer picture with immobiledimers, especially with regard to the resulting scalingproperties of the island size distributions.45,47 Unlikethe growth of polycrystalline graphene on Cu(100),graphene islands are dendritic-shaped on Cu(111) attemperatures as low as 690�C, and they can grow asa single crystal.43 At higher temperatures, the growngraphene island becomes more compact, indicatingthat an additional thermally activated diffusion proc-ess smoothens the island edges.

Our previous work demonstrated that onCu(111) and Cu(100), C–C dimers can more readilyattach to the edge of a growing graphene island thanC monomers45 (Figure 2). Moreover, C–C dimersdiffuse at comparable or even faster speeds than Cmonomers on both surfaces. These findings indicatethat C–C dimers are the dominant C sources fed intoa growing graphene island on either substrate.Detailed kinetic analysis further revealed that the

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FIGURE 1 | Binding energies of C–C dimers on flat metal (111)surfaces with respect to the C–C distance (a) and the preferenceof C–C dimer formation on close-packed transition metal surfaces(b). The inset in (a) shows the top view of a C–C dimer on a close-packed metal surface. (Reprinted with permission from Ref 42.Copyright 2010 APS Publishing Group)

WIREs Computational Molecular Science Van der Waals epitaxy and property optimization of layered materials


energy barrier against C–C dimer diffusion onCu(111) is comparable to that of its coalescence intoa graphene island, whereas it is much harder toattach than to diffuse on Cu(100); this accounts forthe contrasting diffusion- versus attachment-limitedgrowth behaviors on Cu(111) and Cu(100), respec-tively. In addition, a dendritic-to-compact transitionis predicted to occur as graphene grows on both sub-strates, but at different transition temperatures.

Grain BoundariesA grain boundary (GB) refers to the interface betweentwo islands or grains of different orientations in poly-crystalline materials. As an undesirable type of defect,GBs not only appear widespread in epitaxial grapheneon metal substrates, but also severely degrade theoverall quality of graphene.48–50 The size of a graph-ene grain is likely to be dependent on the diffusionrate and nucleation effectiveness of C sources duringgraphene growth. On Cu substrates, the relativelyweak C–substrate interaction enables the fast diffu-sion and efficient nucleation of C sources, eventuallyleading to the formation of large-size grains;42 single-crystalline graphene grains grown on a Cu foil canreach the dimension of 0.5 mm on a side.51 Mean-while, a CVD synthesis scheme has achieved 30-inchmonolayer graphene films on Cu substrates,52 makingthe prevalence of GBs inevitable.

The atomic geometries and related physicalproperties of GBs in epitaxial graphene have beenextensively studied, while experimental attempts tosuppress or control the GB formation have met with

relatively limited success.49–51,53–58 Graphene GBscan be formed in either the initial nucleation stage orthe later growth stage: the former by grain emana-tion from one nucleation site and the latter by thecoalescence of relatively misoriented grains. Our pre-vious first-principles study demonstrated that orienta-tional disorders of graphene flakes are widespread onCu(111) in the early stages of nucleation and growth,and they remain and give rise to the GB formationduring graphene growth.59 Furthermore, we pro-posed a novel approach to suppress these orienta-tional disorders using a functionalized Cu(111)substrate. The proposed approach invokes two steps:(1) carbon clustering is initiated by depositing coro-nene on a Mn-Cu(111) alloyed surface, which effec-tively dictates the orientation of the islands tominimize the orientational disorders and (2) thegraphene island grows from C sources via a conven-tional CVD scheme.

Low-Temperature GrowthLow-temperature production is generally desirable interms of environmental impact, economical effi-ciency, and technical suitability. However, the con-ventional CVD growth of graphene, where methane(CH4) gas is typically used as the C source, requireshigh temperatures (�1000�C), and many efforts havebeen made to lower the growth temperature. Forexample, a modified CVD route using polymers asthe C feedstocks achieved graphene growth on Cusubstrates at about 800�C.60 Strikingly, it was foundthat another CVD approach using a liquid benzene(C6H6) source lowered drastically the growth temper-ature of graphene from 1000 to 300�C.61 A similarCVD method using a pyridine (C5H5N) source wasalso reported to successfully grow N-doped grapheneat temperatures as low as 300�C.62

The drastic reduction in the growth tempera-ture of epitaxial graphene can be attributed to theeffects of vdW interactions between the planar(or aromatic) source molecules and metal sub-strates.63 The vdW interactions can substantiallyenhance the adsorption of aromatic molecules onsubstrates; the planar nature of aromatic moleculesenables their parallel alignment with the substrate,which may maximize the effect of vdW interactions.This prevents easy desorption of the adsorbed mole-cules from the substrate and, therefore, facilitates thedehydrogenation of hydrocarbons. It is noteworthythat the effects of vdW interactions become morepronounced with larger molecules, as demonstratedby comparison of three different C sources, methane,benzene, and p-Terphenyl (C18H14) (Figure 3(a)).

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FIGURE 2 | Minimum energy paths for attachment of a Cmonomer (a) and of a C–C dimer (b) to a zigzag edge on Cu(111).(Reprinted with permission from Ref 45. Copyright 2015 APSPublishing Group)



Detailed kinetic analysis also provided a quantitativedemonstration that graphene growth can be achievedat low temperatures around 300�C using the largerC18H14 source, which is validated by experimentalobservations (Figure 3(b)). It is noticeable that thedomain boundaries are abundant. As the vdW forceis universal in materials, the general trends estab-lished are also expected to be broadly applicable inmolecular assembly and synthesis using other planaror aromatic molecules.26–28,64

Bilayer GrapheneMonolayer graphene is a semimetal and the zeroband gap limits its potential applications in logic

devices.65 In this regard, bilayer graphene has beenviewed as an alternative material because of its tuna-ble band gap,66–68 and the growth of large-scale andhigh-quality bilayer graphene has naturally becomean intriguing topic from the fundamental and techno-logical views.69 It is known that graphene grown onCu substrates is predominantly monolayer, in con-trast to the uncontrollable growth of multilayergraphene on Ni substrates.39 These contrasting beha-viors can be attributed to the different C solubility ofthe substrates and the ensuing growth mechanism:On Cu with low C solubility, graphene growsthrough a surface adsorption process, while on Ni itoccurs through C segregation or precipitation ofhighly soluble C atoms.39

To grow bilayer graphene on Cu substrates,extensive experimental efforts have been made,70–74

but so far only with limited success. Therefore, recentfirst-principles studies proposed new growthapproaches of bilayer graphene on Cu

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FIGURE 4 | Minimum energy paths of C diffusion within andbetween the different regions on (a) Cu(111) and (b) Ni(111). Here,Sub(1) and Sub(2) represent the first and second subsurface sites,respectively. The numbers in the horizontal axes correspond to theroutes shown in the inset of (a). (Reprinted with permission from Ref77. Copyright 2015 APS Publishing Group)



substrates.75,76 For example, effective H passivationof the edge sites of the growing first layer was sug-gested to further expedite the growth of the secondlayer.75 The second graphene layer is found to growbeneath the first layer through the atom-exchangeprocess and the ensuing C penetration onto the Cusurface.76 A comparative first-principles study ofgraphene bilayer growth on Cu(111) and Ni(111)revealed that Cu(111) is a more proper substrate forbilayer growth, because of the weaker interactions atthe graphene–Cu interface than those at thegraphene–Ni interface: The weaker interactionsenhance the lateral diffusion and lead to effective Cnucleation underneath the first layer77 (Figure 4). Itwas also found that there is a critical graphene sizebeyond which nucleation of the second layer isinitiated.77 It should also be cautioned that, thebilayer growth picture developed based on C mono-mer attachment at the island edges may need to befurther developed by considering the relatively easierattachment of C–C dimers revealed recently.45 Fur-thermore, in a very recent experiment, microscopicsteps governing graphene bilayer growth on Cu werediscovered as diffusion of C species into the Cu sub-surface after complete dehydrogenation of hydrocar-bons, offering an intriguing new approach for bilayergraphene growth.78

Graphene Growth on Insulating SubstratesSo far we have focused on epitaxial growth on cat-alytic metal substrates. In these cases, the CVDgrowth of graphene is typically followed by atransfer process to exploit the various physicalproperties of the graphene films, which oftenresults in undesirable degradation of the graphenesamples.79 To circumvent such sample degradationand in many cases also for direct device explora-tions without transferring, graphene growth on tar-geted insulating (or semiconducting) substrates hasbeen intensively studied.80–82 For example, mono-layer or few-layer graphene can be obtained on sili-con carbide (SiC) by the thermal decompositionmethod. This thermal approach evaporates thedecomposed Si atoms from the SiC surface and theremaining C atoms form graphene layers.82 Graph-ene layers could also be achieved on SiC surfacesby molecular beam epitaxy schemes.82,83 BesidesSiC surfaces, a variety of insulating surfaces,including Si3N4, SiO2, and h-BN, have beenexploited for graphene growth, and we refer to afew related reviews for more comprehensiveunderstanding.79–83

GROWTH OF OTHER LAYEREDMATERIALS

SiliceneAs the silicon-based analogue of graphene, silicene hasrecently attracted much attention. Silicene was first pre-dicted by theoretical studies.84,85 Similar to graphene,silicene is a semimetal with high carrier mobility.86–88

Previous theoretical studies demonstrated an electri-cally tunable band gap of silicene,89,90 making it amore promising 2D material for future electronics.Unlike sp2 planar graphene, silicene has a sp3 buckledstructure with height corrugations of �0.4 Å.86,89 Thisis because the sp2 hybridization is more stable than sp3

hybridization in C atoms, whereas Si reverses the rela-tive stabilities. Therefore, silicon has no equivalent of

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FIGURE 5 | Scanning tunneling microscopy (STM) image of(a) silicene grown on Ag(111) and (b) silicene nanowires on Ag(110).(a: Reprinted with permission from Ref 11. Copyright 2012 APSPublishing Group; b: Reprinted with permission from Ref 92. Copyright2005 Elsevier Publishing Group). (c) Top (left panel) and side (rightpanel) views of borophene/Ag(111) structure. (d) Simulated (left panel)and experimental (right panel) STM images of borophene on Ag(111).(Reprinted with permission from Ref 14. Copyright 2015 SciencePublishing Group)



graphite, and exfoliation techniques are no longeravailable for silicene.

Consequently, epitaxial growth has beenregarded as the most probable approach for realizingsilicene samples. Epitaxial silicene was first success-fully grown on Ag(111) by using atomic Si deposi-tion9,11,91 (Figure 5(a)), and then, it has beenachieved on various substrates, such as ZrB2 thinfilms,10 Ir(111),93 and ZrC(111) surfaces.94 On thetheoretical side, several substrates, includinggraphene,95 h-BN,96 and TMDs,97 have also beenproposed for epitaxial silicene growth. On Ag(110),the growth of silicene nanowires was observed92

(Figure 5(b)), implying a surface-orientation depend-ence in the epitaxy mechanism. It was also found thatthe vdW interaction between silicene and the sub-strate helps to stabilize the adsorbate system withoutdestroying the electronic structure of silicene.98 Addi-tionally, H-terminated silicene, called silicane, wasfabricated by exfoliation of layered polysilane, and itwas found to be a direct-gap material.86,99

BoropheneBorophene is a newly discovered monolayer 2D sys-tem consisting of boron atoms, and it shares the samehoneycomb geometry as graphene, except the presenceof extra boron atoms positioned at the center of eachhexagon14 (Figure 5(c)). Borophene growth on Agsurfaces had been achieved under ultrahigh-vacuumconditions by using a solid boron source, as shown inFigure 5(d).14 In this work, scanning tunneling micro-scope (STM) measurements combined with densityfunctional theory (DFT) calculations revealed thatborophene is a highly anisotropic metal. Such an ani-sotropy also results in mechanical stiffness comparableto that of graphene along one axis.14 Epitaxial growthof 2D boron sheets was also demonstrated onAg(111), with two different structural types.15 First-principles investigations further reported on the physi-cal properties of borophene, for example, theoxidation-inert characteristic and vdW-like weakbinding with the substrate. Meanwhile, a DFT studyrevealed that borophene has high electrical conductiv-ity and optical transparency, making it a promisingphotovoltaic material.100

The formation of striped-phase borophenenanoribbons (BNRs) was also observed experimen-tally.14 The atomic-resolution STM measurementfound that the BNRs prefer the line edge, rather thanthe zigzag edge. A first-principles study confirmedthat the line-edged BNR is energetically more stablethan the zigzag-edged BNR, and further revealed thatthe line-edged BNRs are metallic and rigid.100 The

stabilities and electronic conductivities of BNRsincrease with their width, similar to graphene nanor-ibbons. Furthermore, the line-edged BNRs undergoedge reconstructions to form extra p bonds, resultingin the stabilization of the electron-deficient edge.100

PhosphorenePhosphorene is a single layer of black phosphorus,which is the most stable form among phosphorus(P) allotropes, including white, red, and violet phos-phorus.101 Black P possesses a stacked layered struc-ture weakly bounded by vdW interactions, and hasrecently attracted much attention as a new memberof the 2D materials family101,102 (Figure 6). Bulkblack P is a semiconductor with a direct band gap of0.3 eV, and its band gap increases with decreasingthickness, up to �2 eV for monolayer.102 In 2014,field-effect transistors using the building blocks con-sisting of monolayer or few-layer black P were firstfabricated12,13 (Figure 6(d)), and their outstandingperformances have had stimulated extensive researchin both theory and experiment.101,102 The domainsize is about 200 nm on a side,103 which is muchsmaller than that of graphene. The structure of phos-phorene is anisotropic with a unique puckered con-figuration.102 This naturally results in the distinctiveanisotropic electronic and transport properties of

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phosphorene, making it promising for novel deviceapplications in electronics, optoelectronics, photovol-taics, and spintronics.102–105

Despite the successful fabrication of monolayeror few-layer phosphorene by mechanical exfoliation,mass production of large-area, high-quality phosphor-ene still remains an outstanding challenge. In thisregard, the epitaxial growth of phosphorene is highlydesirable, but related research is still lacking owing tosome practical reasons (e.g., fragile nature of phos-phorene under ambient conditions).102 Moreover,searching for a suitable substrate to grow phosphor-ene is a challenging issue at present. A first-principlesstudy demonstrated that the stability of P islands isstrongly dependent on the P–substrate interactions.106

In that work, it was found that too strong P–substrateinteractions destroy P islands on Cu(111), whereastoo weak interaction with h-BN substrate fails to sta-bilize P islands. Based on these understandings, a sur-face with a moderate interaction (about 0.35 eV/Patom) was proposed as a proper substrate for the epi-taxial growth of phosphorene.

Status of TMD and h-BNTMDs are the most intensively studied class oflayered compound materials with the chemical for-mula of MX2, where M represents a transition metalelement and X is a chalcogen atom (S, Se, orTe).19,107 The band structures of TMDs show drasticthickness dependence, and their monolayer forms areof technical importance owing to the appearance ofdirect band gaps.107–110 In fact, these TMDs werefound to undergo a transition from an indirect todirect band gap as the thickness is reduced down tomonolayer.107–110 Besides the attractive tunable bandstructures of TMD semiconductors, the TMD familyalso exhibits rich physical properties, includingmetallic, half-metallic, superconducting, and chargedensity wave characters.107

Two-dimensional layered TMDs have nearlythe same structures in which three covalently bondedatomic layers form one TMD single layer, and adja-cent TMD layers are bound by relatively weak vdWinteractions. Such weak interlayer interactions enableeasy isolation of a TMD monolayer from its bulkform. Indeed, various monolayer TMDs have beenfabricated by the exfoliation approach.19,109,111 Theepitaxial growth of TMDs has also been achieved onsubstrates by using the CVD approach.19 Most ofthe epitaxially grown TMD samples are based onchalcogen vapor treatment of thermally evaporatedsolid sources, such as MoO3, MoO2, andMoCl5.

18,19,112 It is worth noting that most CVD

syntheses of TMDs have used SiO2 as the growthsubstrate, which inevitably results in a large concen-tration of undesirable grain boundaries.18,19 This islikely due to the random orientation of MoS2domains associated with the high surface roughnessand amorphous nature of SiO2.

113 Moreover, it isusually difficult to precisely control the partial pres-sure and spatial distribution of the evaporatedsources.19 To precisely control the lattice orientationof deposited films, Dumcenco et al. used atomicallysmooth surfaces of sapphire as the growth substrate,and achieved a large-area single-crystalline MoS2film, as shown in Figure 7(a).113

As an analogue of graphene, monolayer h-BNshares the 2D honeycomb lattice and many physicalproperties except for the large band gap.16,114 Similarto the cases of the above 2D materials, mechanical orchemical exfoliation approaches have been used toobtain monolayer or few-layer h-BN, but suchapproaches are ineffective because of relatively stronginterlayer bindings; the resulting products are typi-cally found to be small monolayer flakes of h-BN.16

The epitaxial CVD growth of monolayer h-BN has along history, but most of the early works also yieldedonly small-size monolayer h-BN flakes that are diffi-cult to be transferred onto arbitrary substrates.16

Recently, Song et al. reported on a modified CVDmethod using a Cu substrate, and obtained large-area, transferrable few-layered h-BN sheets17

(Figure 7(b)).Table 1 summarizes the electronic properties

and corresponding epitaxial growth approaches ofthe aforementioned 2D layered materials. It is nota-ble that angle-resolved photoemission spectroscopy(ARPES) is the main experimental technique forunderstanding the unique band structures of 2Dmaterials, supplemented by scanning tunneling spec-troscopy (STS). Furthermore, the electronic proper-ties of some of the latest discovered 2D materialshave not been experimentally confirmed yet.

GROWTH OF HETEROSTRUCTURES

Lateral: Graphene/h-BNand Graphene/TMDTwo-dimensional lateral heterojunctions refer tostructures in which two different layered materialsare stitched together via strong in-plane covalentbonds. Such structures have potential nanoelectronicdevice applications, such as atomically thin circuitsand field-effect transistors. Structurally controlledfabrication of lateral heterostructures in the atomi-cally thin regime has been realized experimentally by



several groups123–129 (Figure 8(a)). Levendorfet al. reported a versatile and scalable process, inwhich they first grew the first layer of graphene onCu, patterned away the unwanted areas by using aprotective layer, and then grew h-BN onto the Cusubstrate to laterally connect with graphene.124 Liuet al. etched the graphene island on Cu(100) toobtain fresh edges and grew h-BN from these siteswith atomic lattice coherence, forming an abrupt 1Dinterface or boundary.128 The h-BN lattice orienta-tion was found to be determined by graphene only,

in contrast to the strict orientational alignment ofh-BN with the Cu(100) surface lattices when sepa-rately grown on Cu(100).130

In addition to the stitching of graphene andh-BN that are both one-atomic-layer thick, recentlyresearchers have successfully fabricated the lateralheterojunctions of graphene/TMD, in which one sideof the junction is one-atomic-thick and the other isthree-atomic-thick.129,131–133 In particular, Linget al. reported a two-step fabrication approach ofsuch heterostructures by mechanical transfer forgraphene deposition and selective sowing of seedmolecules for TMD growth.129 In despite of the largelattice mismatch between graphene and TMD, theatomic force microscopy (AFM) image showed thatthe two layered materials were well connected, withvery narrow overlaps at the interfaces. Furthermore,an Ohmic-type contact with a weak tunneling barrierformed at the junction, making it suitable for usagein electronic devices.129,131,132 Chenet al. demonstrated the direct growth of patternedgraphene/MoS2 lateral heterojunctions for the fabri-cation of transistors, using selective plasma treatmentof a SiO2/Si substrate and CVD growth of MoS2.

133

Beyond these experimental advances, further theoret-ical studies are needed to help unravel the atomicconfigurations of the junctions between the structur-ally dissimilar layered materials.

Vertical: vdW HeterostructuresRecently, there have also been increasing interests instacking two or more atomic layers of different 2Dmaterials into the so-called vdW heterostructures22

(Figure 8(b)). In such compounds, there are manydegrees of freedom in the parameter space of design,which enables the exploration of novel quantum phe-nomena and realization of more practical devices.For example, Gorbachev et al. measured the Cou-lomb drag and symmetry breaking in interacting 2Dlayers using a double-layer graphene heterostructurewith ultrathin h-BN spacers;134 Ponomarenkoet al. showed the tunable metal-insulator transitionin a graphene/h-BN heterostructure with alternatinggraphene and h-BN layers;135 meanwhile, at certaintwisting angles between two 2D materials, due to theperiodic superlattice potential, Hofstadter’s butterflyand superlattice Dirac points were observed in vdWheterojunctions.136–138 It is intriguing to note that,despite the weak vdW nature of the interlayer cou-pling, proper stacking of different 2D materials canalso result in distinctly enhanced target properties. Asan example of vertical stacking, Britnellet al. demonstrated enhanced light-matter interaction

FIGURE 7 | High-resolution transmission electron microscopy(TEM) images of (a) a MoS2 film grown on a sapphire substrate and(b) a hexagonal boron nitride (h-BN) film on a Cu substrate. The scalebars are 0.5 and 2 nm in (a) and (b), respectively. (Reprinted withpermission from Ref 113 and Ref 17. Copyright 2015 and 2010 ACSPublishing Group)



TABLE 1 | Band Gap, Band Dispersion Near the Fermi Level, Growth Substrate, Domain Size, Typical Epitaxy Method, and Known GrowthMechanism of the 2D Layered Materials

Material Band GapBandDispersion Substrates

Domain Size(on a Side)

TypicalEpitaxyMethod Growth Mechanism

Graphene 0 eV2 Linear2 Cu, Ni, Ir, etc.115 0.5 mm51 CVD Precipitation/segregation,surface catalysis116

Silicene 0 eV86 Linear86 Ag, Ir, ZrB2,etc.10,86,93

30 nm117 Atomicdeposition

Si insertion118

Borophene 0 eV14 Parabolic14 Ag14 ~100 nm14 Atomicdeposition

B clustering119

Phosphorene ~2 eV102 Parabolic102 - 200 nm103 - -

MoS2 1.8–2.1 eV19 Parabolic19 SiO2, Au, Al2O3,etc.19,113,120

~10 μm19 CVD Sulfurization of Molayer121,122

h-BN ~6 eV16 Parabolic16,107 Cu, Ni, Pt,etc.16,114

~20 μm114 CVD Surface catalysis17

CVD, chemical vapor deposition; h-BN, hexagonal boron nitride.

(a)Graphene-MoS2

(M-S)

Au/Pd contact layer

hBN substrate

Vacuum

MoS2

Graphene

500 nm

(b)

FIGURE 8 | Lateral graphene-MoS2 heterostructure (a): (left panel) TEM image and (right panel) schematic illustration, and Illustration ofstate-of-the-art van der Waals (vdW) heterostructures and devices (b), highlighting a graphene–hexagonal boron nitride (h-BN) superlatticeconsisting of six stacked bilayers. (a: Reprinted with permission from Ref 129. Copyright 2016 WILEY Publishing Group; b: Reprinted withpermission from Ref 22. Copyright 2016 Nature Publishing Group)



using TMD/graphene heterostructures, salient naturefor enhanced solar energy conversion.139 Later wewill also present an example of lateral zipping ofgraphene and h-BN for substantially enhanced half-metallicity.140

The properties of vdW heterostructures are gen-erally sensitive to their structural parameters, such asstacking sequences and twisting angles. Precise con-trol of these parameters in the fabrication process ofvdW heterostructures thus becomes very significant.A widely used fabrication method is to deposit 2Dcrystals sequentially using transfer techniques;22

however, it requires layer-by-layer deposition withmanual operations, which makes the fabrication inef-ficient and lack of precise structural control. A morepractical approach would be to epitaxially grow 2Dcrystals on top of each other with proper sequentialorder. As the interlayer interaction is only of theweak vdW force, the Volmer–Weber (island) growthmode is usually preferred. To suppress the islandgrowth, fine-tuning of the growth conditions, such astemperature, precursors, and flux rates, is criticallyneeded. Recently, Yang et al. reported a plasma-assisted CVD growth of single-domain graphene onthe h-BN substrate;141 Kim et al. identified the multi-ple possibilities of the stacking features in the CVD-grown graphene/h-BN heterostructures on Cu;127

furthermore, Gong et al. reported the epitaxialgrowth of vertically stacked WS2/MoS2 bilayers athigh temperatures.142

PROPERTY OPTIMIZATION

OpticalThe exotic electronic and optical properties of graph-ene and other emerging 2D materials have opened upnew possibilities in optoelectronics and photon-ics.29,143 For example, graphene has drawn tremen-dous attention in the field of high-speed opticalcommunications owing to its strong light-matterinteraction and high carrier mobility.143–145 The gap-less nature and reduced dimensionality of graphenegive rise to novel physical phenomena, such as theultrafast relaxation of excited quasiparticles (QPs)and the photo-thermoelectric effects.143,146,147 Unlikesemi-metallic graphene, some TMD members includ-ing MoS2 and WS2 are semiconductors with opticalgaps greater than 1 eV, making them promising fornear-infrared absorbing/emitting technologies.143

Moreover, the valley degree of freedom in 2D TMDsoffers new opportunities for controlling the valleyindex, thereby enabling various device applicationsin optoelectronics.148 Another new member of the

2D family, black P, has unique anisotropic opticalproperties that can be utilized for light-polarization-based devices,102,143 and its narrow band gap isexpected to be useful for bridging 2D metals andwide-gap semiconductors and eventually help theoptimal design of optical devices.101

A series of many-body perturbation theory cal-culations have demonstrated that the excitonic effectsin 2D semiconductors (or insulators) are critical totheir optical properties.149–151 This is somewhat sur-prising at first because the excitonic electron–holeinteraction is typically negligible in bulk semiconduc-tors: The electrostatic interaction in 3D is effectivelyscreened by surrounding electrons. Nevertheless, acloser view suggests that, in 2D systems, the electronicscreening is expected to be much less effective owingto the reduced dimensionality, potentially resulting inmore strongly bound excitons. Indeed, the excitonbinding energies of several 2D materials were pre-dicted to be comparable to or even larger than1 eV,149–151 at least an order of magnitude largerthan their bulk counterparts. On the experimentalside, the exciton binding energies of monolayeredMoSe2 and WS2 were reported to be 0.55 and�0.7 eV, respectively,152,153 confirming stronglybound excitons. Our recent GW–Bethe–Salpeterequation calculations further revealed that phosphor-ene and graphene fluoride also possess strongly boundexcitons: the exciton binding energies for phosphor-ene and graphene fluoride are 0.85 and 2.03 eV,respectively.154 Furthermore, we discovered a strikinglinear scaling law of the exciton binding energy versusthe QP band gap in different classes of 2D materialswith widely ranged band gaps (Figure 9), and itsunderlying physical reason was explained by a simplehydrogenic model and k∙p perturbation theory. Weexpect that the linear scaling law is broadly applicableto many existing and future 2D materials, as shownin a more recent study that generalized the linear scal-ing law to 51 TMD layered systems.155

FerroelectricFerroelectric materials are of great interest owing totheir potential applications in nanoscale devices, suchas sensors and nonvolatile memories.156 However,ferroelectricity vanishes when the material thicknessis below a certain critical value, hindering furtherminiaturization of devices: for example, the criticalthickness of perovskite BaTio3 is �24 Å.157 Junqueraand Ghosez revealed that a depolarization field at theferroelectric–electrode interface destroys the ferroe-lectricity.157 Extensive investigations have revealedthat the depolarization issue is closely related to



several factors including the charge screening, straineffect, and interfacial interactions.156,157

The vdW epitaxy of 2D layered materials mayeliminate such depolarization issues arising from thelattice misfit and interfacial dipoles. In fact, mono-layer 1T-MoS2 has been predicted to be a ferroelec-tric material, although it is energetically unfavorableagainst the nonferroelectric 2H configuration.158 Ithas also been predicted that monolayer group IVmonochalcogenides (GeS, GeSe, SnS, and SnSe) areanother class of robust 2D ferroelectrics.159 Recently,Chang et al. prepared 2D ferroelectric SnTe films ona graphitized 6H-SiC(0001) substrate with stable in-plane spontaneous polarization.160 The ferroelectrictransition temperature of 1-unit cell SnTe film(Figure 10) is 270 K, much higher than that (98 K)of bulk SnTe. As the used substrate is mostly coveredby graphene layers, the epitaxial SnTe films areweakly bound to the substrate, thereby reducing theeffect of interfacial strain; the weak nature of interfa-cial binding helps ultrathin SnTe films to sustain thein-plane ferroelectricity even at room temperature.160

Based on these findings, a novel design of nonvolatileferroelectric random access memory was proposed,operated by an in-plane electric field.

Magnetic/SpintronicExtensive research has also been conducted into theapplications of 2D layered materials in spintronics,where the spin degree of freedom is utilized for novelinformation storage and logic devices.18,161,162

Typically, a spin-polarized current is generated byusing ferromagnetic materials, and, therefore, the fer-romagnetically ordered edge states of zigzag graph-ene nanoribbons (ZGNRs) have drawn greatinterest.163 In fact, several first-principles studies havepredicted that ZGNRs possess two ferromagneticspin channels at either edge with opposite spin orien-tations, resulting in an overall antiferromagneticground state.164–166 A recent experiment suggestedthat the magnetic orderings of ZGNRs could be sta-ble at room temperature.167 Additionally, there havebeen a few attempts to precisely control the spin con-duction channels of ZGNRs.168,169 A new mechan-ism for generating spin-polarized current is to inducethe spin Hall effect in a nonmagnetic material.161 Intheir pioneering work, Kane and Mele proposed that

FIGURE 9 | The exciton binding energy (Eb) versus thequasiparticle (QP) band gap (Eg) for various representative 2Dmaterials. The dashed line represents the fitted linear relation in theform of Eb = αEg + β, with α = 0.21 and β = 0.40. (Reprinted withpermission from Ref 154. Copyright 2015 APS Publishing Group)

(6√3×6√3)R30°

(001)Sn

Te

1 UC SnTe

Graphene

6H-SiC

1 UC

2.5

(a)

(b)

2

1.5

1

0.5

0

2 UC

3 UC

4 UC

0.8

0.6

0.4

Δα

(°)

Δα

(°)

0.2

0

240 250 260Temperature (K)

Temperature (K)

270 280

0 50 100 150 200 250 300

(100) (010)

FIGURE 10 | Atomic geometries (a) of the SnTe crystal (upperpanel) and the SnTe/graphen/6H-SiC system (lower panel), andtemperature dependence of the distortion angle for the 1- to 4-unitcell SnTe films (b). (Reprinted with permission from Ref 160. Copyright2016 Science Publishing Group)



spin-orbit coupling converts graphene from 2D semi-metal to a quantum spin Hall (QSH) insulator.170

However, the effect of spin-orbit coupling is toosmall to be observed experimentally in graphene.171

The realization of elemental spin-logic buildingblocks is a prerequisite in development of future spin-tronic devices. In this regard, theoretical studies havedemonstrated a range of elemental spin-logic devicesbased on the manipulation of the magnetic propertiesof graphene nanoflakes or nanoribbons.172–174 Ourprevious work also demonstrated that C tetragonscan serve as spin switches in properly connectedZGNRs, where the switching effect is observed witha large variety of nanoribbon segments175

(Figure 11). Furthermore, it was found that such spinswitches can lift the degeneracy between the twochannels, rendering tunable magnetic states uponcharge doping.

Besides graphene, semiconducting 2D TMDskeep attracting much attention owing to their strongspin-orbit coupling (inherently much stronger than ingraphene) and spin and valley degrees of freedom,offering new opportunities toward valley spintron-ics.18 Recent first-principles studies also reported onthe magnetic ordering of phosphorene nanoribbonsand the QSH state of bilayer phosphorene,176,177

stimulating further experimental and theoreticalresearch.

Half-MetallicA half-metal is a material that is metallic to electronsfor one spin orientation, but simultaneously semicon-ducting or insulating to those of the opposite orienta-tion, an important spin degree of freedom that canbe useful for spintronics.178 A pioneering work bySon et al. predicted theoretically that the half-metallicity can be induced in freestanding ZGNRs byapplying a strong transverse electric field,165 but therequired external fields are hard to be achieved inpractice at such nanoscales. There have been many

attempts to search for novel 2D half-metals, forexample, a series of theoretical studies have proposedthe half-metallicity in low-D materials.179–181 Edgeengineering and functionalization have also been pro-posed as alternative routes for achieving half-metallicity in such 2D systems.182,183

The lateral heterostructures of graphene andh-BN (G-BN) have been successfully synthesized,opening a new route toward the realization of 2Dhalf-metallicity.123,124 On the theoretical side, a first-principles study predicted a size-dependent half-metallicity of h-BN triangular clusters embedded ingraphene.184 In the lateral G-BN heterostructures,the spin-polarized edge states of graphene nanorib-bons are found to be well preserved in the hybrid sys-tems.182 However, the corresponding half-metallicityis severely suppressed from that of freestandinggraphene.183 Recently, we have revealed that orienta-tionally misaligned lateral G-BN heterojunctions(Figure 12) can drastically enhance the half-metallicity from the orientationally aligned cases.Such enhanced half-metallicity can be attributed tothe restored strength of the superexchange interac-tion between the electrons located at the two oppo-site interfaces.140 These central predictive findingscall for future experimental validation.

CatalyticConfined catalysis keeps drawing increasing attentionas a promising approach for catalyst design.185–189

Here, confined catalysis refers to the increase in therate of a chemical process in a spatially confinedenvironment. The emergence of novel 2D materialsand their intrinsic spatial restriction naturally lead toincreasing attempts for optimizing confined catalysisbased on the 2D materials family, for example, COoxidation in the confined nanostructures formed bygraphene or h-BN on Pt(111).188,190 One expectedadvantage of 2D materials in confined catalysis is itshighly selective permeability, ruling out interventionof undesirable species.

Confined catalysis using 2D materials also mayoffer a new route for converting a nonprecious metalinto an effective catalyst for important chemical reac-tions, such as hydrogen evolution reaction (HER) orCO oxidation; use of a nonprecious metal is desira-ble for low-cost production. In fact, a related first-principles study demonstrated that the introductionof a metal substrate can substantially alter the Hbinding energy on the MoS2 overlayer.191 Nonpre-cious metal candidates for HER catalysts can beselected as those located close to the peak of the vol-cano curve.192 Recently, we performed first-

FIGURE 11 | Schematic spin configurations of different zigzaggraphene nanoribbons (ZGNRs) (a) without and (b) with a C tetragonas a definitive spin switch. (Reprinted with permission from Ref 175.Copyright 2016 APS Publishing Group)



principles DFT calculations to investigate the HERcatalysis on graphene-covered metals, with the inclu-sion of vdW corrections. In the graphene-modifiedvolcano curve, Ni(111) surface, which exhibits amoderate HER rate without graphene overlayer,shifts to be close to the volcano peak. Moreover, thehigh mobility of H atom on bare Ni(111) is largelypreserved in the graphene-covered system, a benefi-cial aspect for HER catalysis. Therefore, graphene-covered Ni is a promising catalytic platform forHER.193

CONCLUSION

In this article, we have briefly summarized the recentprogresses in the vdW epitaxy and property optimi-zation of graphene and other emerging 2D layeredmaterials, focusing on their potential applications inoptics, electronics, and spintronics. The vdW epitaxyof the 2D materials family takes advantage of the rel-atively weak vdW interlayer coupling to circumventthe lattice-matching requirements in traditional epi-taxial growth, allowing the formation of variouscombinations of vdW heterostructures. Extensive

research has demonstrated that the epitaxial growthof graphene is governed by the major chemical pro-cesses and structure formation, such as the nuclea-tion, diffusion, and dissociation of C sources as wellas the GB formation. Especially, the newly proposeddimer-based picture, instead of traditional monomer-based picture, accounts for various experimentalobservations in epitaxial graphene growth. The other2D materials, such as silicene, borophene, phosphor-ene, TMDs, and h-BN, invoke their own epitaxymechanisms depending on the constituent elementsand atomic structures. The heterostructures with thevarious combinations of 2D materials have been pro-posed or achieved by vdW epitaxy, and their promis-ing perspectives have also been reviewed. Manyexperimental and theoretical efforts have been madeto utilize the 2D materials family in importantdevice applications, such as optics, electronics, andspintronics. The 2D layered members show rich elec-tronic properties, ranging from semi-metals to wide-gap insulators. In wide-gap 2D semiconductors, anexciton is strongly bound, and the excitonic effectsare critical to the optical properties of the materials.Recent studies of 2D ferroelectrics have demon-strated that epitaxially grown 2D films enable therealization of robust monolayer ferroelectricity atroom temperature. The members of the 2D materialsfamily and their heterostructures have also shown avariety of spin degree of freedom, offering newopportunities in spintronics or valley-spintronics.Proper use of the 2D materials family can also befound in confined catalysis to enhance importantchemical reactions, such as HER and CO oxidation.Overall, discoveries of the ever-expanding family oflayered materials can benefit critically from precisecontrol and elegant exploitation of the seeminglyweak vdW interfacial forces, and the availabilities ofsuch 2D materials and their heterostructures offerunprecedented opportunities for emergent functionalproperties and device application potentials.

ACKNOWLEDGMENTS

During the course of this line of research, we have benefited tremendously from many collaborators, includ-ing (but not limited to) Hua Chen, Valentino R. Cooper, Hong-Jun Gao, Yanfei Gao, Gong Gu, EfthimiosKaxiras, Haiping Lan, Guo Li, Zhancheng Li, Zhenyu Li, Elton J. G. Santos, Tim P. Schulze, Ping Wu, Jin-long Yang, Changgan Zeng, Jiang Zeng, Dongbo Zhang, Yinong Zhou, and Wenguang Zhu. The comple-tion of this review has been partially supported by the National Natural Science Foundation of China(Grant Nos. 11634011, 61434002, and 11504357), the National Key Basic Research Program of China(Grant No. 2014CB921103), the 111 Project (No. B13027), and ARO MURI Award No. W911NF-14-0247.

a1

(a) (b)b1 b2

T1

hBN Graphene GraphenehBN hBN hBN

T2 T2

T1 10T2

a2a1

b1 b2a2

FIGURE 12 | Lateral heterojunctions of zigzag graphene andhexagonal boron nitride (h-BN) nanoribbons with (a) orientationalalignment and (b) orientational misalignment.



REFERENCES1. Novoselov KS, Geim AK, Morozov SV, Jiang D,

Zhang Y, Dubonos SV, Grigorieva IV, Firsov AA.Electric field effect in atomically thin carbon films.Science 2004, 306:666–669.

2. Geim AK, Novoselov KS. The rise of graphene. NatMater 2007, 6:183–191.

3. Ando T. Exotic electronic and transport properties ofgraphene. Phys E 2007, 40:213–227.

4. Geim AK. Graphene: status and prospects. Science2009, 324:1530–1534.

5. Li X, Cai W, An J, Kim S, Nah J, Yang D, Piner R,Velamakanni A, Jung I, Tutuc E, et al. Large-areasynthesis of high-quality and uniform graphene filmson copper foils. Science 2009, 324:1312–1314.

6. Loginova E, Bartelt NC, Feibelman PJ, McCarty KF.Factors influencing graphene growth on metal sur-faces. New J Phys 2009, 11:063046.

7. Gao L, Guest JR, Guisinger NP. Epitaxial grapheneon Cu(111). Nano Lett 2010, 10:3512–3516.

8. Zhang Y, Zhang L, Zhou C. Review of chemicalvapor deposition of graphene and related applica-tions. Acc Chem Res 2013, 46:2329–2339.

9. Feng B, Ding Z, Meng S, Yao Y, He X, Cheng P,Chen L, Wu K. Evidence of silicene in honeycombstructures of silicon on Ag(111). Nano Lett 2012,12:3507–3511.

10. Fleurence A, Friedlein R, Ozaki T, Kawai H,Wang Y, Yamada-Takamura Y. Experimental evi-dence for epitaxial silicene on diboride thin films.Phys Rev Lett 2012, 108:245501.

11. Vogt P, Padova DP, Quaresima C, Avila J,Frantzeskakis E, Asensio MC, Resta A, Ealet B,Lay GL. Silicene: compelling experimental evidencefor graphenelike two-dimensional silicon. Phys RevLett 2012, 108:155501.

12. Li L, Yu Y, Ye GJ, Ge Q, Ou X, Wu H, Feng D,Chen XH, Zhang Y. Black phosphorus field-effecttransistors. Nat Nanotechnol 2014, 9:372–377.

13. Liu H, Neal AT, Zhu Z, Luo Z, Xu X, Tománek D,Ye PD. Phosphorene: an unexplored 2D semiconduc-tor with a high hole mobility. ACS Nano 2014,8:4033–4041.

14. Mannix AJ, Zhou XF, Kiraly B, Wood JD,Alducin D, Myers BD, Liu X, Fisher BL, Santiago U,Guest JR, et al. Synthesis of borophenes: anisotropic,two-dimensional boron polymorphs. Science 2015,350:1513–1516.

15. Feng B, Zhang J, Zhong Q, Li W, Li S, Li H,Cheng P, Meng S, Chen L, Wu K. Experimental reali-zation of two-dimensional boron sheets. Nat Chem2016, 8:563–568.

16. Lin Y, Connell JW. Advances in 2D boron nitridenanostructures: nanosheets, nanoribbons,

nanomeshes, and hybrids with graphene. Nanoscale2012, 4:6908–6939.

17. Song L, Ci L, Lu H, Sorokin PB, Jin C, Ni J,Kvashnin AG, Kvashnin DG, Lou J, Yakobson BI,et al. Large scale growth and characterization ofatomic hexagonal boron nitride layers. Nano Lett2010, 10:3209–3215.

18. Wang QH, Kalantar-Zadeh K, Kis A, Coleman JN,Strano MS. Electronics and optoelectronics of two-dimensional transition metal dichalcogenides. NatNanotechnol 2012, 7:699–712.

19. Duan X, Wang C, Pan A, Yu R, Duan X. Two-dimensional transition metal dichalcogenides as atom-ically thin semiconductors: opportunities and chal-lenges. Chem Soc Rev 2015, 44:8859–8876.

20. Chen YL, Analytis JG, Chu JH, Liu ZK, Mo SK,Qi XL, Zhang HJ, Lu DH, Dai X, Fang Z,et al. Experimental realization of a three-dimensionaltopological insulator, Bi2Te3. Science 2009,325:178–181.

21. Zhang H, Liu CX, Qi XL, Dai X, Fang Z, Zhang SC.Topological insulators in Bi2Se3, Bi2Te3 and Sb2Te3with a single Dirac cone on the surface. Nat Phys2009, 5:438–442.

22. Geim AK, Grigorieva IV. Van der Waals heterostruc-tures. Nature 2013, 499:419–425.

23. Van der Waals JD. The thermodynamic theory capil-larity under the hypothesis of a continuous variationof density. Verhandel Konink Akad Weten Amster-dam 1893, 1:8; translation by Rowlinson JS. J StatPhys 1979, 20:197–200.

24. Lopinski GP, Wayner DDM, Wolkow RA. Self-directed growth of molecular nanostructures on sili-con. Nature 2000, 406:48–51.

25. Li G, Cooper VR, Cho JH, Du S, Gao HJ, Zhang Z.Self-assembly of molecular wires on H-terminatedSi(100) surfaces driven by London dispersion forces.Phys Rev B 2011, 84:241406.

26. Gourdon A. On-surface covalent coupling in ultra-high vacuum. Angew Chem Int Ed 2008,47:6950–6953.

27. Otero G, Biddau G, Sánchez-Sánchez C, Caillard R,López F, Rogero C, Palomares FJ, Cabello N,Basanta MA, Ortega J, et al. Fullerenes from aro-matic precursors by surface-catalysed cyclodehydro-genation. Nature 2008, 454:865–868.

28. Bartels L. Tailoring molecular layers at metal sur-faces. Nat Chem 2010, 2:87–95.

29. Butler SZ, Hollen SM, Cao L, Cui Y, Gupta JA,Gutiérrez HR, Heinz TF, Hong SS, Huang J,Ismach AF, et al. Progress, challenges, and opportu-nities in two-dimensional materials beyond graphene.ACS Nano 2013, 7:2898–2926.



30. Utama MIB, Zhang Q, Zhang J, Yuan Y, Belarre J,Arbiol J, Xiong Q. Recent developments and futuredirections in the growth of nanostructures by van derWaals epitaxy. Nanoscale 2013, 5:3570–3588.

31. Dion M, Rydberg H, Schröder E, Langreth DC,Lundqvist BI. Van der Waals density functional forgeneral geometries. Phys Rev Lett 2004, 92:246401.

32. Román-Pérez G, Soler JM. Efficient implementationof a van der Waals density functional: application todouble-wall carbon nanotubes. Phys Rev Lett 2009,103:096102.

33. Tkatchenko A, Scheffler M. Accurate molecular vander Waals interactions from ground-state electrondensity and free-atom reference data. Phys Rev Lett2009, 102:073005.

34. Ruiz VG, Liu W, Zojer E, Scheffler M,Tkatchenko A. Density-functional theory withscreened van der Waals interactions for the modelingof hybrid inorganic-organic systems. Phys Rev Lett2012, 108:146103.

35. Loginova E, Bartelt NC, Feibelman PJ, McCarty KF.Evidence for graphene growth by C cluster attach-ment. New J Phys 2008, 10:093026.

36. Marchini S, Günther S, Wintterlin J. Scanning tunnel-ing microscopy of graphene on Ru(0001). Phys Rev B2007, 76:075429.

37. Coraux J, N’Diaye AT, Engler M, Busse C, Wall D,Buckanie N, Heringdorf FJMZ, Gastel RV,Poelsema B, Michely T. Growth of graphene onIr(111). New J Phys 2009, 11:039801.

38. N’Diaye AT, Bleikamp S, Feibelman PJ, Michely T.Two-dimensional Ir cluster lattice on a graphenemoiré on Ir(111). Phys Rev Lett 2006, 97:215501.

39. Li X, Cai W, Colombo L, Ruoff RS. Evolution ofgraphene growth on Ni and Cu by carbon isotopelabeling. Nano Lett 2009, 9:4268–4272.

40. Helveg S, López-Cartes C, Sehested J, Hansen PL,Clausen BS, Rostrup-Nielsen JR, Abild-Pedersen F,Nørskov JK. Atomic-scale imaging of carbon nanofi-bre growth. Nature 2004, 427:426–429.

41. Abild-Pedersen F, Nørskov JK, Rostrup-Nielsen JR,Sehested J, Helveg S. Mechanisms for catalytic carbonnanofiber growth studied by ab initio density func-tional theory calculations. Phys Rev B 2006,73:115419.

42. Chen H, Zhu W, Zhang Z. Contrasting behavior ofcarbon nucleation in the initial stages of graphene epi-taxial growth on stepped metal surfaces. Phys RevLett 2010, 104:186101.

43. Nie S, Wofford JM, Bartelt NC, Dubon OD,McCarty KF. Origin of the mosaicity in graphenegrown on Cu(111). Phys Rev B 2011, 84:155425.

44. Riikonen S, Krasheninnikov AV, Halonen L,Nieminen RM. The role of stable and mobile carbon

adspecies in copper-promoted graphene growth.J Phys Chem C 2012, 116:5802–5809.

45. Wu P, Zhang Y, Cui P, Li Z, Yang J, Zhang Z. Car-bon dimers as the dominant feeding species in epitax-ial growth and morphological phase transition ofgraphene on different Cu substrates. Phys Rev Lett2015, 114:216102.

46. Wofford JM, Nie S, McCarty KF, Bartelt NC,Dubon OD. Graphene islands on Cu foils: the inter-play between shape, orientation, and defects. NanoLett 2010, 10:4890–4896.

47. Amar JG, Family F. Critical cluster size: island mor-phology and size distribution in submonolayer epitax-ial growth. Phys Rev Lett 1995, 74:2066–2069.

48. Ajayan PM, Yakobson BI. Graphene: pushing theboundaries. Nat Mater 2011, 10:415–417.

49. Yu Q, Jauregui LA, Wu W, Colby R, Tian J, Su Z,Cao H, Liu Z, Pandey D, Wei D, et al. Control andcharacterization of individual grains and grainboundaries in graphene grown by chemical vapourdeposition. Nat Mater 2011, 10:443–449.

50. Huang PY, Ruiz-Vargas CS, van der Zande AM,Whitney WS, Levendorf MP, Kevek JW, Garg S,Alden JS, Hustedt CJ, Zhu Y, et al. Grains and grainboundaries in single-layer graphene atomic patch-work quilts. Nature 2011, 469:389–392.

51. Li X, Magnuson CW, Venugopal A, Tromp RM,Hannon JB, Vogel EM, Colombo L, Ruoff RS. Large-area graphene single crystals grown by low-pressurechemical vapor deposition of methane on copper.J Am Chem Soc 2011, 133:2816–2819.

52. Bae S, Kim H, Lee Y, Xu X, Park JS, Zheng Y,Balakrishnan J, Lei T, Kim HR, Song YI. Roll-to-rollproduction of 30-inch graphene films for transparentelectrodes. Nat Nanotechnol 2010, 5:574–578.

53. Yazyev OV, Louie SG. Electronic transport in poly-crystalline graphene. Nat Mater 2010, 9:806–809.

54. Yazyev OV, Louie SG. Topological defects in graph-ene: dislocations and grain boundaries. Phys Rev B2010, 81:195420.

55. Grantab R, Shenoy VB, Ruoff RS. Anomalousstrength characteristics of tilt grain boundaries ingraphene. Science 2010, 330:946–948.

56. Kim K, Lee Z, Regan W, Kisielowski C,Crommie MF, Zettl A. Grain boundary mapping inpolycrystalline graphene. ACS Nano 2011,5:2142–2146.

57. An J, Voelkl E, Suk JW, Li X, Magnuson CW, Fu L,Tiemeijer P, Bischoff M, Freitag B, Popova E,et al. Domain (grain) boundaries and evidence of‘twinlike’ structures in chemically vapor depositedgrown graphene. ACS Nano 2011, 5:2433–2439.

58. Li X, Magnuson CW, Venugopal A, An J, Suk JW,Han B, Borysiak M, Cai W, Velamakanni A, Zhu Y,et al. Graphene films with large domain size by a



two-step chemical vapor deposition process. NanoLett 2010, 10:4328–4334.

59. Chen W, Chen H, Lan H, Cui P, Schulze TP, Zhu W,Zhang Z. Suppression of grain boundaries in graph-ene growth on superstructured Mn-Cu(111) surface.Phys Rev Lett 2012, 109:265507.

60. Sun Z, Yan Z, Yao J, Beitler E, Zhu Y, Tour JM.Growth of graphene from solid carbon sources.Nature 2010, 468:549–552.

61. Li Z, Wu P, Wang C, Fan X, Zhang W, Zhai X,Zeng C, Li Z, Yang J, Hou J. Low-temperaturegrowth of graphene by chemical vapor depositionusing solid and liquid carbon sources. ACS Nano2011, 5:3385–3390.

62. Xue Y, Wu B, Jiang L, Guo Y, Huang L, Chen J,Tan J, Geng D, Luo B, Hu W, et al. Low temperaturegrowth of highly nitrogen-doped single crystal graph-ene arrays by chemical vapor deposition. J Am ChemSoc 2012, 134:11060–11063.

63. Choi JH, Li Z, Cui P, Fan X, Zhang H, Zeng C,Zhang Z. Drastic reduction in the growth tempera-ture of graphene on copper via enhanced London dis-persion force. Sci Rep 2013, 3:1925.

64. Cai J, Ruffieux P, Jaafar R, Bieri M, Braun T,Blankenburg S, Muoth M, Seitsonen AP, Saleh M,Feng X, et al. Atomically precise bottom-up fabrica-tion of graphene nanoribbons. Nature 2010,466:470–473.

65. Castro Neto AH, Peres NMR, Novoselov KS,Geim AK, Guinea F. The electronic properties ofgraphene. Rev Mod Phys 2009, 81:109–162.

66. Zhang Y, Tang TT, Girit C, Hao Z, Martin MC,Zettl A, Crommie MF, Shen YR, Wang F. Directobservation of a widely tunable bandgap in bilayergraphene. Nature 2009, 459:820–823.

67. Pal AN, Ghosh A. Ultralow noise field-effect transis-tor from multilayer graphene. Appl Phys Lett 2009,95:082105.

68. Samuels AJ, Carey JD. Molecular doping and band-gap opening of bilayer graphene. ACS Nano 2013,7:2790–2799.

69. Nie S, Wu W, Xing S, Yu Q, Bao J, Pei SS,McCarty KF. Growth from below: bilayer grapheneon copper by chemical vapor deposition. New J Phys2012, 14:093028.

70. Lee S, Lee K, Zhong Z. Wafer scale homogeneousbilayer graphene films by chemical vapor deposition.Nano Lett 2010, 10:4702–4707.

71. Yan K, Peng H, Zhou Y, Li H, Liu Z. Formation ofbilayer bernal graphene: layer-by-layer epitaxy viachemical vapor deposition. Nano Lett 2011,11:1106–1110.

72. Kalbac M, Frank O, Kavan L. The control of graph-ene double-layer formation in copper-catalyzed chem-ical vapor deposition. Carbon 2012, 50:3682–3687.

73. Li Q, Chou H, Zhong JH, Liu JY, Dolocan A,Zhang J, Zhou Y, Ruoff RS, Chen S, Cai W. Growthof adlayer graphene on Cu studied by carbon isotopelabeling. Nano Lett 2013, 13:486–490.

74. Song W, Jeon C, Kim SY, Kim Y, Kim SH, Lee SI,Jung DS, Jung MW, An KS, Park CY. Two selectivegrowth modes for graphene on a Cu substrate usingthermal chemical vapor deposition. Carbon 2014,68:87–94.

75. Zhang X, Wang L, Xin J, Yakobson BI, Ding F. Roleof hydrogen in graphene chemical vapor depositiongrowth on a copper surface. J Am Chem Soc 2014,136:3040–3047.

76. Wu P, Zhai X, Li Z, Yang J. Bilayer graphene growthvia a penetration mechanism. J Phys Chem C 2014,118:6201–6206.

77. Chen W, Cui P, Zhu W, Kaxiras E, Gao Y, Zhang Z.Atomistic mechanisms for bilayer growth of grapheneon metal substrates. Phys Rev B 2015, 91:045408.

78. Hao Y, Wang L, Liu Y, Chen H, Wang X, Tan C,Nie S, Suk JW, Jiang T, Liang T, et al. Oxygen-activated growth and bandgap tunability of largesingle-crystal bilayer graphene. Nat Nanotechnol2016, 11:426–431.

79. Wang H, Yu G. Direct CVD graphene growth onsemiconductors and dielectrics for transfer-free devicefabrication. Adv Mater 2016, 28:4956–4975.

80. Berger C, Song Z, Li T, Li X, Ogbazghi AY, Feng R,Dai Z, Marchenkov AN, Conrad EH, First PN,et al. Ultrathin epitaxial graphite: 2D electron gasproperties and a route toward graphene-based nanoe-lectronics. J Phys Chem B 2004, 108:19912–19916.

81. Ohta T, Gabaly FE, Bostwick A, McChesney JL,Emtsev KV, Schmid AK, Seyller T, Horn K,Rotenberg E. Morphology of graphene thin filmgrowth on SiC(0001). New J Phys 2008, 10:023034.

82. Norimatsu W, Kusunoki M. Epitaxial graphene onSiC{0001}: advances and perspectives. Phys ChemChem Phys 2014, 16:3501–3511.

83. Riedl C, Coletti C, Starke U. Structural and electronicproperties of epitaxial graphene on SiC(0001): areview of growth, characterization, transfer dopingand hydrogen intercalation. J Phys D 2010,43:374009.

84. Takeda K, Shiraishi K. Theoretical possibility of stagecorrugation in Si and Ge analogs of graphite. PhysRev B 1994, 50:14916–14922.

85. Guzmán-Verri GG, Voon LYLC. Electronic structureof silicon-based nanostructures. Phys Rev B 2007,76:075131.

86. Kara A, Enriquez H, Seitsonen AP, Voon LCLY,Vizzini S, Aufray B, Oughaddou H. A review onsilicene—new candidate for electronics. Surf Sci Rep2012, 67:1–18.



87. Shao ZG, Ye XS, Yang L, Wang CL. First-principlescalculation of intrinsic carrier mobility of silicene.J Appl Phys 2013, 114:093712.

88. Li X, Mullen JT, Jin Z, Borysenko KM, Nardelli MB,Kim KW. Intrinsic electrical transport properties ofmonolayer silicene and MoS2 from first principles.Phys Rev B 2013, 87:115418.

89. Drummond ND, Zólyomi V, Fal’ko VI. Electricallytunable band gap in silicene. Phys Rev B 2012,85:075423.

90. Ni Z, Liu Q, Tang Q, Zheng J, Zhou J, Qin R,Gao Z, Yu D, Lu J. Tunable bandgap in silicene andgermanene. Nano Lett 2012, 12:113–118.

91. Lalmi B, Oughaddou H, Enriquez H, Kara A,Vizzini S, Ealet B, Aufray B. Epitaxial growth of asilicene sheet. Appl Phys Lett 2010, 97:223109.

92. Leandri C, Lay GL, Aufray B, Girardeaux C, Avila J,Dávila ME, Asensio MC, Ottaviani C, Cricenti A.Self-aligned silicon quantum wires on Ag(110). SurfSci 2005, 574:L9–L15.

93. Meng L, Wang Y, Zhang L, Du S, Wu R, Li L,Zhang Y, Li G, Zhou H, Hofer WA, et al. Buckledsilicene formation on Ir(111). Nano Lett 2013,13:685–690.

94. Aizawa T, Suehara S, Otani S. Silicene on zirconiumcarbide (111). J Phys Chem C 2014,118:23049–23057.

95. Cai Y, Chuu CP, Wei CM, Chou MY. Stability andelectronic properties of two-dimensional silicene andgermanene on graphene. Phys Rev B 2013,88:245408.

96. Liu H, Gao J, Zhao J. Silicene on substrates: a way topreserve or tune its electronic properties. J Phys Chem C2013, 117:10353–10359.

97. Zhu J, Schwingenschlögl U. Stability and electronicproperties of silicene on WSe2. J Mater Chem C2015, 3:3946–3953.

98. Kokott S, Pflugradt P, Matthes L, Bechstedt F. Non-metallic substrates for growth of silicene: an ab initioprediction. J Phys Condens Matter 2014, 26:185002.

99. Okamoto H, Kumai Y, Sugiyama Y, Mitsuoka T,Nakanishi K, Ohta T, Nozaki H, Yamaguchi S,Shirai S, Nakano H. Silicon nanosheets and their self-assembled regular stacking structure. J Am Chem Soc2010, 132:2710–2718.

100. Liu Y, Dong YJ, Tang Z, Wang XF, Wang L, Hou T,Lin H, Li Y. Stable and metallic borophene nanorib-bons from first-principles calculations. J Mater Chem C2016, 4:6380–6385.

101. Ling X, Wang H, Huang S, Xia F, Dresselhaus MS.The renaissance of black phosphorus. Proc Natl AcadSci 2015, 112:4523–4530.

102. Kou L, Chen C, Smith SC. Phosphorene: fabrication,properties, and applications. J Phys Chem Lett 2015,6:2794–2805.

103. Brent JR, Savjani N, Lewis EA, Haigh SJ, Lewis DJ,O’Brien P. Production of few-layer phosphorene byliquid exfoliation of black phosphorus. Chem Com-mun 2014, 50:13338–13341.

104. Fei R, Yang L. Strain-engineering the anisotropic elec-trical conductance of few-layer black phosphorus.Nano Lett 2014, 14:2884–2889.

105. Xu Y, Dai J, Zeng XC. Electron-transport propertiesof few-layer black phosphorus. J Phys Chem Lett2015, 6:1996–2002.

106. Gao J, Zhang G, Zhang YW. The critical role of sub-strate in stabilizing phosphorene nanoflake: a theoret-ical exploration. J Am Chem Soc 2016,138:4763–4771.

107. Jariwala D, Sangwan VK, Lauhon LJ, Marks TJ,Hersam MC. Emerging device applications for semi-conducting two-dimensional transition metal dichal-cogenides. ACS Nano 2014, 8:1102–1120.

108. Mak KF, Lee C, Hone J, Shan J, Heinz TF. Atomi-cally thin MoS2: a new direct-gap semiconductor.Phys Rev Lett 2010, 105:136805.

109. Lee HS, Min SW, Chang YG, Park MK, Nam T,Kim H, Kim JH, Ryu S, Im S. MoS2 nanosheet photo-transistors with thickness-modulated optical energygap. Nano Lett 2012, 12:3695–3700.

110. Mak KF, He K, Shan J, Heinz TF. Control of valleypolarization in monolayer MoS2 by optical helicity.Nat Nanotechnol 2012, 7:494–498.

111. Zheng J, Zhang H, Dong S, Liu Y, Nai CT, Shin HS,Jeong HY, Liu B, Loh KP. High yield exfoliation oftwo-dimensional chalcogenides using sodiumnaphthalenide. Nat Commun 2014, 5:2995.

112. Wu S, Huang C, Aivazian G, Ross JS, Cobden DG,Xu X. Vapor–solid growth of high optical qualityMoS2 monolayers with near-unity valley polarization.ACS Nano 2013, 7:2768–2772.

113. Dumcenco D, Ovchinnikov D, Marinov K, Lazic P,Gibertini M, Marzari N, Sanchez OL, Kung YC,Krasnozhon D, Chen MW, et al. Large-area epitaxialmonolayer MoS2. ACS Nano 2015, 9:4611–4620.

114. Pakdel A, Bando Y, Golberg D. Nano boron nitrideflatland. Chem Soc Rev 2014, 43:934–959.

115. Tetlow H, de Boer JP, Ford IJ, Vvedensky DD,Coraux J, Kantorovich L. Growth of epitaxial graph-ene: theory and experiment. Phys Rep 2014,542:195–295.

116. Seah CM, Chai SP, Rahman Mohamed A. Mechan-isms of graphene growth by chemical vapour deposi-tion on transition metals. Carbon 2014, 70:1–21.

117. Sone J, Yamagami T, Aoki Y, Nakatsuji K,Hirayama H. Epitaxial growth of silicene on ultra-thin Ag(111) films. New J Phys 2014, 16:095004.

118. Bernard R, Borensztein Y, Cruguel H, Lazzeri M,Prévot G. Growth mechanism of silicene on Ag(111)determined by scanning tunneling microscopy



measurements and ab initio calculations. Phys Rev B2015, 92:045415.

119. Liu H, Gao J, Zhao J. From boron cluster to two-dimensional boron sheet on Cu(111) surface: growthmechanism and hole formation. Sci Rep 2013,3:3238.

120. Grønborg SS, Ulstrup S, Bianchi M, Dendzik M,Sanders CE, Lauritsen JV, Hofmann P, Miwa JA.Synthesis of epitaxial single-layer MoS2 on Au(111).Langmuir 2015, 31:9700–9706.

121. Zhan Y, Liu Z, Najmaei S, Ajayan PM, Lou J. Large-area vapor-phase growth and characterization ofMoS2 atomic layers on a SiO2 substrate. Small 2012,8:966–971.

122. Orofeo CM, Suzuki S, Sekine Y, Hibino H. Scalablesynthesis of layer-controlled WS2 and MoS2 sheets bysulfurization of thin metal films. Appl Phys Lett2014, 105:083112.

123. Ci L, Song L, Jin C, Jariwala D, Wu D, Li Y,Srivastava A, Wang ZF, Storr K, Balicas L,et al. Atomic layers of hybridized boron nitride andgraphene domains. Nat Mater 2010, 9:430–435.

124. Levendorf MP, Kim CJ, Brown L, Huang PY,Havener RW, Muller DA, Park J. Graphene andboron nitride lateral heterostructures for atomicallythin circuitry. Nature 2012, 488:627–632.

125. Fiori G, Betti A, Bruzzone S, Iannaccone G. Lateralgraphene–hBCN heterostructures as a platform forfully two-dimensional transistors. ACS Nano 2012,6:2642–2648.

126. Gao Y, Zhang Y, Chen P, Li Y, Liu M, Gao T,Ma D, Chen Y, Cheng Z, Qiu X. Toward single-layeruniform hexagonal boron nitride–graphene patch-works with zigzag linking edges. Nano Lett 2013,13:3439–3443.

127. Kim SM, Hsu A, Araujo PT, Lee YH, Palacios T,Dresselhaus M, Idrobo JC, Kim KK, Kong J. Synthe-sis of patched or stacked graphene and hBN flakes: aroute to hybrid structure discovery. Nano Lett 2013,13:933–941.

128. Liu L, Park J, Siegel DA, McCarty KF, Clark KW,Deng W, Basile L, Idrobo JC, Li AP, Gu G. Heteroe-pitaxial growth of two-dimensional hexagonal boronnitride templated by graphene edges. Science 2014,343:163–167.

129. Ling X, Lin Y, Ma Q, Wang Z, Song Y, Yu L,Huang S, Fang W, Zhang X, Hsu AL, et al. Parallelstitching of 2D materials. Adv Mater 2016,28:2322–2329.

130. Liu L, Siegel DA, Chen W, Liu P, Guo J, Duscher G,Zhao C, Wang H, Wang W, Bai X, et al. Unusualrole of epilayer–substrate interactions in determiningorientational relations in van der Waals epitaxy. ProcNatl Acad Sci 2014, 111:16670–16675.

131. Zhao M, Ye Y, Han Y, Xia Y, Zhu H, Wang S,Wang Y, Muller DA, Zhang X. Large-scale chemicalassembly of atomically thin transistors and circuits.Nat Nanotechnol 2016, 11:954–959.

132. Guimarães MH, Gao H, Han Y, Kang K, Xie S,Kim CJ, Muller DA, Ralph DC, Park J. Atomicallythin Ohmic edge contacts between two-dimensionalmaterials. ACS Nano 2016, 10:6392–6399.

133. Chen X, Park YJ, Das T, Jang H, Lee JB, Ahn JH.Lithography-free plasma-induced patterned growth ofMoS2 and its heterojunction with graphene. Nano-scale 2016, 8:15181–15188.

134. Gorbachev RV, Geim AK, Katsnelson MI,Novoselov KS, Tudorovskiy T, Grigorieva IV,MacDonald AH, Morozov SV, Watanabe K,Taniguchi T, et al. Strong Coulomb drag and brokensymmetry in double-layer graphene. Nat Phys 2012,8:896–901.

135. Ponomarenko LA, Geim AK, Zhukov AA, Jalil R,Morozov SV, Novoselov KS, Grigorieva IV, Hill EH,Cheianov VV, Fal’ko VI, et al. Tunable metal–insulator transition in double-layer graphene hetero-structures. Nat Phys 2011, 7:958–961.

136. Yang W, Lu X, Chen G, Wu S, Xie G, Cheng M,Wang D, Yang R, Shi D, Watanabe K,et al. Emergence of superlattice Dirac points in graph-ene on hexagonal boron nitride. Nat Phys 2012,8:382–386.

137. Dean CR, Wang L, Maher P, Forsythe C, Ghahari F,Gao Y, Katoch J, Ishigami M, Moon P, Koshino M,et al. Hofstadter’s butterfly and the fractal quantumHall effect in moiré superlattices. Nature 2013,497:598–602.

138. Hunt B, Sanchez-Yamagishi JD, Young AF,Yankowitz M, LeRoy BJ, Watanabe K, Taniguchi T,Moon P, Koshino M, Jarillo-Herrero P,et al. Massive Dirac fermions and Hofstadter butter-fly in a van der Waals heterostructure. Science 2013,340:1427–1430.

139. Britnell L, Ribeiro RM, Eckmann A, Jalil R,Belle BD, Mishchenko A, Kim YJ, Gorbachev RV,Georgiou T, Morozov SV, et al. Strong light-matterinteractions in heterostructures of atomically thinfilms. Science 2013, 340:1311–1314.

140. Zeng J, Chen W, Cui P, Zhang DB, Zhang Z.Enhanced half-metallicity in orientationally misa-ligned graphene/hexagonal boron nitride lateral het-erojunctions. Phys Rev B 2016, 94:235425.

141. Yang W, Chen G, Shi Z, Liu CC, Zhang L, Xie G,Cheng M, Wang D, Yang R, Shi D, et al. Epitaxialgrowth of single-domain graphene on hexagonalboron nitride. Nat Mater 2013, 12:792–797.

142. Gong Y, Lin J, Wang X, Shi G, Lei S, Lin Z, Zou X,Ye G, Vajtai R, Yakobson BI, et al. Vertical and in-plane heterostructures from WS2/MoS2 monolayers.Nat Mater 2014, 13:1135–1142.



143. Xia F, Wang H, Xiao D, Dubey M,Ramasubramaniam A. Two-dimensional materialnanophotonics. Nat Photonics 2014, 8:899–907.

144. Pospischil A, Humer M, Furchi MM, Bachmann D,Guider R, Fromherz T, Mueller T. CMOS-compatiblegraphene photodetector covering all optical commu-nication bands. Nat Photonics 2013, 7:892–896.

145. Gan X, Shiue RJ, Gao Y, Meric I, Heinz TF,Shepard K, Hone J, Assefa S, Englund D. Chip-integrated ultrafast graphene photodetector with highresponsivity. Nat Photonics 2013, 7:883–887.

146. Xu X, Gabor NM, Alden JS, Van Der Zande AM,McEuen PL. Photo-thermoelectric effect at a graph-ene interface junction. Nano Lett 2010, 10:562–566.

147. Sun D, Wu ZK, Divin C, Li X, Berger C, deHeer WA, First PN, Norris TB. Ultrafast relaxationof excited dirac fermions in epitaxial graphene usingoptical differential transmission spectroscopy. PhysRev Lett 2008, 101:157402.

148. Xu X, Yao W, Xiao D, Heinz TF. Spin and pseudos-pins in layered transition metal dichalcogenides. NatPhys 2014, 10:343–350.

149. Wirtz L, Marini A, Rubio A. Excitons in boronnitride nanotubes: dimensionality effects. Phys RevLett 2006, 96:126104.

150. Cudazzo P, Attaccalite C, Tokatly IV, Rubio A.Strong charge-transfer excitonic effects and the Bose-Einstein exciton condensate in graphane. Phys RevLett 2010, 104:226804.

151. Qiu DY, da Jornada FH, Louie SG. Optical spectrumof MoS2: many-body effects and diversity of excitonstates. Phys Rev Lett 2013, 111:216805.

152. Ugeda MM, Bradley AJ, Shi SF, da Jornada FH,Zhang Y, Qiu DY, Ruan W, Mo SK, Hussain Z,Shen ZX, et al. Giant bandgap renormalization andexcitonic effects in a monolayer transition metaldichalcogenide semiconductor. Nat Mater 2014,13:1091–1095.

153. Ye Z, Cao T, O’Brien K, Zhu H, Yin X, Wang Y,Louie SG, Zhang X. Probing excitonic dark states insingle-layer tungsten disulphide. Nature 2014,513:214–218.

154. Choi JH, Cui P, Lan H, Zhang Z. Linear scaling ofthe exciton binding energy versus the band gap oftwo-dimensional materials. Phys Rev Lett 2015,115:066403.

155. Olsen T, Latini S, Rasmussen F, Thygesen KS. Simplescreened hydrogen model of excitons in two-dimensional materials. Phys Rev Lett 2016,116:056401.

156. Setter N, Damjanovic D, Eng L, Fox G, Gevorgian S,Hong S, Kingon A, Kohlstedt H, Park NY,Stephenson GB, et al. Ferroelectric thin films: reviewof materials, properties, and applications. J Appl Phys2006, 100:051606.

157. Junquera J, Ghosez P. Critical thickness for ferroelec-tricity in perovskite ultrathin films. Nature 2003,422:506–509.

158. Shirodkar SN, Waghmare UV. Emergence of ferroe-lectricity at a metal-semiconductor transition in a 1Tmonolayer of MoS2. Phys Rev Lett 2014,112:157601.

159. Fei R, Kang W, Yang L. Ferroelectricity and phasetransitions in monolayer group-IV monochalcogen-ides. Phys Rev Lett 2016, 117:097601.

160. Chang K, Liu J, Lin H, Wang N, Zhao K, Zhang A,Jin F, Zhong Y, Hu X, Duan W, et al. Discovery ofrobust in-plane ferroelectricity in atomic-thick SnTe.Science 2016, 353:274–278.

161. Han W. Perspectives for spintronics in 2D materials.APL Mater 2016, 4:032401.

162. Han W, Kawakami RK, Gmitra M, Fabian J. Graph-ene spintronics. Nat Nanotechnol 2014, 9:794–807.

163. Kan E, Li Z, Yang J. Magnetism in graphene systems.Nano 2008, 3:433–442.

164. Lee H, Son YW, Park N, Han S, Yu J. Magneticordering at the edges of graphitic fragments: magnetictail interactions between the edge-localized states.Phys Rev B 2005, 72:174431.

165. Son YW, Cohen ML, Louie SG. Half-metallic graph-ene nanoribbons. Nature 2006, 444:347–349.

166. Jung J, Pereg-Barnea T, MacDonald AH. Theory ofinteredge superexchange in zigzag edge magnetism.Phys Rev Lett 2009, 102:227205.

167. Magda GZ, Jin X, Hagymási I, Vancsó P, Osváth Z,Nemes-Incze P, Hwang C, Biró LP, Tapasztó L.Room-temperature magnetic order on zigzag edges ofnarrow graphene nanoribbons. Nature 2014,514:608–611.

168. Wimmer M, Adagideli I, Berber S, Tománek D,Richter K. Spin currents in rough graphene nanorib-bons: universal fluctuations and spin injection. PhysRev Lett 2008, 100:177207.

169. Cocchi C, Prezzi D, Calzolari A, Molinari E. Spin-transport selectivity upon co adsorption on antiferro-magnetic graphene nanoribbons. J Chem Phys 2010,133:124703.

170. Kane CL, Mele EJ. Quantum spin hall effect ingraphene. Phys Rev Lett 2005, 95:226801.

171. Min H, Hill JE, Sinitsyn NA, Sahu BR, Kleinman L,MacDonald AH. Intrinsic and Rashba spin-orbitinteractions in graphene sheets. Phys Rev B 2006,74:165310.

172. Wang WL, Yazyev OV, Meng S, Kaxiras E. Topolog-ical frustration in graphene nanoflakes: magneticorder and spin logic devices. Phys Rev Lett 2009,102:157201.

173. Bullard Z, Girão EC, Owens JR, Shelton W,Meunier V. Improved all-carbon spintronic devicedesign. Sci Rep 2015, 5:7634.



174. Zeng M, Shen L, Su H, Zhang C, Feng Y. Graphene-based spin logic gates. Appl Phys Lett 2011,98:092110.

175. Cui P, Zhang Q, Zhu H, Li X, Wang W, Li Q,Zeng C, Zhang Z. Carbon tetragons as definitive spinswitches in narrow zigzag graphene nanoribbons.Phys Rev Lett 2016, 116:026802.

176. Farooq MU, Hashmi A, Hong J. Ferromagnetismcontrolled by electric field in tilted phosphorenenanoribbon. Sci Rep 2016, 6:26300.

177. Zhang T, Lin JH, Yu YM, Chen XR, Liu WM.Stacked bilayer phosphorene: strain-induced quantumspin Hall state and optical measurement. Sci Rep2015, 5:13927.

178. Katsnelson MI, Irkhin VY, Chioncel L,Lichtenstein AI, de Groot RA. Half-metallic ferro-magnets: from band structure to many-body effects.Rev Mod Phys 2008, 80:315–378.

179. Kan EJ, Xiang HJ, Wu F, Tian C, Lee C, Yang JL,Whangbo MH. Prediction for room-temperature half-metallic ferromagnetism in the half-fluorinated singlelayers of BN and ZnO. Appl Phys Lett 2010,97:122503.

180. Li X, Wu X, Yang J. Half-metallicity in MnPSe3 exfo-liated nanosheet with carrier doping. J Am Chem Soc2014, 136:11065–11069.

181. Wu F, Huang C, Wu H, Lee C, Deng K, Kan E,Jena P. Atomically thin transition-metal dinitrides:high-temperature ferromagnetism and half-metallic-ity. Nano Lett 2015, 15:8277–8281.

182. Pruneda JM. Origin of half-semimetallicity induced atinterfaces of C-BN heterostructures. Phys Rev B2010, 81:161409.

183. Kim SW, Kim HJ, Choi JH, Scheicher RH, Cho JH.Contrasting interedge superexchange interactions ofgraphene nanoribbons embedded in h-BN and gra-phane. Phys Rev B 2015, 92:035443.

184. Menezes MG, Capaz RB. Half-metallicity induced bycharge injection in hexagonal boron nitride clustersembedded in graphene. Phys Rev B 2012,86:195413.

185. Pan X, Fan Z, Chen W, Ding Y, Luo H, Bao X.Enhanced ethanol production inside carbon-nanotubereactors containing catalytic particles. Nat Mater2007, 6:507–511.

186. Pan X, Bao X. The effects of confinement inside car-bon nanotubes on catalysis. Acc Chem Res 2011,44:553–562.

187. Xiao J, Pan X, Guo S, Ren P, Bao X. Toward funda-mentals of confined catalysis in carbon nanotubes.J Am Chem Soc 2015, 137:477–482.

188. Zhang Y, Weng X, Li H, Li H, Wei M, Xiao J,Liu Z, Chen M, Fu Q, Bao X. Hexagonal boronnitride cover on Pt(111): a new route to tunemolecule–metal interaction and metal-catalyzed reac-tions. Nano Lett 2015, 15:3616–3623.

189. Jiao F, Li J, Pan X, Xiao J, Li H, Ma H, Wei M,Pan Y, Zhou Z, Li M, et al. Selective conversion ofsyngas to light olefins. Science 2016, 351:1065–1068.

190. Yao Y, Fu Q, Zhang YY, Weng X, Li H, Chen M,Jin L, Dong A, Mu R, Jiang P. Graphene cover-promoted metal-catalyzed reactions. Proc Natl AcadSci 2014, 111:17023–17028.

191. Chen W, Santos EJG, Zhu W, Kaxiras E, Zhang Z.Tuning the electronic and chemical properties ofmonolayer MoS2 adsorbed on transition metal sub-strates. Nano Lett 2013, 13:509–514.

192. Rothenberg G. Catalysis. Weinheim: Wiley-VCHVerlag GmbH & Co. KGaA; 2008. doi:10.1002/9783527621866.

193. Zhou Y, Chen W, Cui P, Zeng J, Lin Z, Kaxiras E,Zhang Z. Enhancing the hydrogen activation reactiv-ity of nonprecious metal substrates via confined catal-ysis underneath graphene. Nano Lett 2016,16:6058–6063.



http://dx.doi.org/10.1002/9783527621866

http://dx.doi.org/10.1002/9783527621866

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