English

• 1. INTRODUCTION

  Two-dimensional (2D) materials have stimulated intense research interest due to the extraordinary physical properties that are distinguishably different from their bulk counterparts^[1–4]. Through a decade of research, more and more 2D materials are discovered to be potential candidates for next generation electronic and optical applications such as flexible electronics, spintronics, valley electronics, non-linear optics, etc.^[5–7]. For instance, the direct bandgap transition with suitable exciton energy in several transition metal dichalcogenides (TMDs) monolayers enables the realization of a complete planar field effect transistor^[8–10]; high carrier mobility is obser-ved in layered black phosphor with anisotropic feature^{[11, 12]}; asymmetric electrical conductance is unveiled in monolayer ReS₂^{[13, 14]}; valley splitting is found in monolayer TMDs with spin locking^[15]; and large second harmonic generation (SHG) effect is detected in monolayer InSe^[16]. Besides numerous potential applications in electronics and optics, 2D materials also show great promise in high density energy storage^{[4, 17]}, membrane filtering^{[18, 19]}, biological sequencing^[20], anti-corrosion coating^[21], catalyst and flexible mechanics^{[22, 23]}. Furthermore, due to the vanishing interlayer coupling in the redu-ced dimension, 2D materials also serve as a fruitful reservoir for novel physical phenomenon, such as enhanced superconducting^[24], charge density waves^{[25, 26]} and topological quantum phenomenon^[27].

  Due to the absence of dangling bonds in the layered structure, 2D materials could be considered as material systems with flat surfaces at both sides, while the van der Waals gap is a natural out-of-plane interface between two monolayers. The roadmap of 2D materials points out that various interfacial coupling in the 2D family would eventually be employed to engineer desired physical properties in layered heterostructures with a pre-designed stacked configuration, similar to the scenario of the "Lego" which piles up to build a castle from different building blocks^[1]. Therefore, the interface and surface structures of each 2D material are essential for realizing them as a unique building block. However, defects are inevitably involved in the preparation of 2D materials including bottom-up and top-down approaches, and they are well-known to affect the physical properties of materials substan-tially. Examples include point defects such as vacan-cies and dislocations, which are demonstrated as the scattering centers influencing the transport pro-perties^{[28, 29]}. The defect also causes significant reconstruction in the lattice (in two dimension that is the surface of the materials), and may form complicated defect complex due to reconstruction in the interface, which further modifies their physical properties. Thus, direction visualization of the defect structures and their related reconstructions is essen-tial in understanding their influence to the physical properties of 2D materials.

  In this review article, we review the effort achieved in probing the defect structures and the reconstruction of surface and interface in novel 2D materials through LVSTEM. Aberration corrected STEM imaging is directly interpretable, which correlates the image intensity with the atomic weight of the imaged species in the single atom level^[30], rendering the rapid chemical analysis from quantita-tive intensity statics. Low voltage (below 60kV) is essential for 2D materials due to their atomic thickness, which results in reduced irradiation damage and enhanced the scattering cross section^[31].

  2. PROGRESS

  2.1 Surface and interface reconstruction by defects in 2D materials and their related properties

  The controllable synthesis strategy of high quality and large area 2D materials is the key to realize the industrial production of 2D materials, and also the premise to study the excellent physical properties behind them. In recent years, the synthesis methodology of 2D materials has been widely studied, including top-down exfoliation^{[10, 32]} and bottom-up chemical vapor deposition (CVD) methods^{[4, 33]}. Among them, a synthesis strategy called molten-salt-assisted CVD method has been reported to be universal for most 2D transition-metal chalcogenides (TMCs) that are challenging to be produced by conventional methods^[34]. The growth kinetics showed that adding molten salt can effectively reduce the melting point of the precursor, thus promoting the nucleation and growth of the 2D flakes. Using this method, 47 TMCs compounds have been synthesized systematically, involving 12 transition metals and 3 chalcogenide elements, while the products include not only the common semiconductor MoS₂, but also metal 1T phase MoTe₂, superconducting NbS₂ and NbSe₂ and even quinary layered alloys. Low voltage atomic-resolution STEM images in Fig. 1a~d show several as-synthesized representative 2D crystals, including 1H phase MoS₂, 1T phase PtSe₂, 1T' phase WTe₂, and 1T'' phase ReSe₂. By examining the atomic phases of all as-synthesized 2D materials grown by this method, a structural library of 2D TMC is outlined, which effectively promotes the research progress of 2D materials, and provides a broad research platform for defect engineering, interface engineering, band-structure engineering and other related research fields.

  Figure 1

  

  Figure 1.  Atomic-resolution STEM images of representative monolayer crystalline materials in different phases and representative 0D defects on the surface of 2D materials. Atomic resolved STEM images of MoS₂ in the 1 H phase (a), PtSe₂ in the 1 T phase (b), WTe₂ in the 1 T′ phase (c) and ReSe₂ in the 1 T″ phase (d), with their corresponding FFT patterns and atomic structural models. Reprint from Ref. [34]. (e) STEM image of monolayer V_Re-ReS₂ and corresponding FFT patterns (inset). V_Re was emphasized by the yellow circles. (f) Optimized structural model of the V_Re-ReS₂ defect. Reprint from Ref. [44]. (g) Mo SAs on MoS₂ monolayer and (f) W SAs on WS₂ monolayer, showing high densities of Mo and W SAs as adatoms on the surface. The scale bar is 1 nm for e and f. Reprint from Ref. [23]

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  Due to the process of thermodynamics and dyna-mic equilibrium, all 2D materials, especially CVD grown materials, inevitably have intrinsic defects, such as zero-dimensional (0D) defect like vacan-cies^[28] and adatoms^[35], one-dimensional (1D) defects such as grain boundaries^[36], surface ripples^{[37, 38]}, layer stacking^[39], etc. Rational design, creation and regulation of defects in 2D materials can modulate the band and electronic configuration, thereby optimizing the device performance, or even change the phase structure, realizing the surface and interface reconstruction to create new specific functions^{[40, 41]}. Therefore, a lot of efforts have been put into visualizing the atomic structure of defects in 2D materials and the surface and interface recon-struction caused by these defects^{[42, 43]}. In the following, from 0D to 2D, we will review some recently reported 2D defect structures and their related reconstructions probed by LVSTEM, as well as new properties induced by these new structures.

  Vacancies and adatoms are two common 0D defects in 2D TMDs^[35]. In contrast to other defects, although structural reconstruction is minimal upon forming these two types of defects, they can still modulate the performance of device by tailoring the local electronic structure. Intrinsic vacancies widely exist in 2D TMDs, and its concentration and size can also be artificially controlled by changing experi-mental parameters. For instance, using the molten-salt-assisted CVD strategy, the ReS₂ monolayer with different concentration of Re vacancy was synthe-sized by adjusting the volume ratio of H₂/Ar under the S-rich condition^[44]. From the atomic resolved STEM images in Fig. 1e, the Re vacancies can be clearly identified due to the absence of some atomic sites in Re sublattice, as highlighted by the yellow circles. Because the atomic number of S atom is much smaller than that of Re atom, the intensity of S atom is almost invisible in the Z-contrast STEM image. The FFT pattern in the inset indicates that ReS₂ has a distorted 1T' phase structure. As shown in the model in Fig. 1f, the absence of Re atoms produces 6 kinds of S-dangling bonds around, which substantially increases the S–H binding strength from –0.558 to 0.191 eV, i.e., activates the chemically inert S atoms in hydrogen evolution reaction (HER) activity. More importantly, the existence of transition metal-transition metal (Re-Re) σ bonds can further modulate the electronic state of the activated S atoms, achieving higher catalytic activity. The first principle calculation and HER test demonstrated that in TMDs monolayers with TM-TM bonds, the spontaneous charge compensation activated by metal vacancies can optimize the electronic state, achieving a ΔG_H* close to 0 eV for efficiently electrocatalytic HER.

  Adatom is another abundant 0D defect in 2D TMDs, which can be regulated to reconstruct surface structure, generating highly active catalysts. Recently, a cold H₂ plasma reduction strategy has been reported to synthesize Mo single atoms (SA) residing on the parent MoS₂ monolayer, while this strategy is also applicable to W SA on WS₂ monolayer^[23]. Abundant Mo SAs and W SAs highlighted by cyan cycles can be seen from the HAADF-STEM images, evenly distributing on the surfaces of MoS₂ and WS₂, respectively (Fig. 1g, 1h). The statistic results obtained from HAADF-STEM images show that the densities of Mo and W SAs after cold hydrogen plasma treatment have increased to 4.35 × 10¹³ atoms cm^-2 and 1.76 × 10¹³ atoms cm^-2, respectively, almost 2 orders of magnitude higher than that of their pristine counterparts. Density of states analysis showed that these metal SAs on 2D TMDs are coordinately unsaturated, giving rise to excellent electrocatalytic HER activity, which was also confirmed by electrochemical measurements. Mo SAs as decorating species induced surface recon-struction and activated the basal plane of MoS₂ monolayer, in contrast to previous theoretical and experimental studies which show the basal plane of pristine 2H phase MoS₂ is catalytically inert. DFT calculation explained that the orbital of unsaturated Mo is easy to hybridize with H, which increases the Mo-H bonding strength, promoting rapid hydrogen absorption and desorption, and lowers the Fermi energy compared with the pristine MoS₂ monolayer, leading to a lower ΔG_H*, consequently showing a superior HER activity in basal plane. Therefore, direct visualization of 0D defects and reconstructions on 2D TMDs is essential to understand their contribution in the macroscopic performance of materials.

  Rather than adsorbing to the surface of 2D crystal, foreign atomic species can be substituted into the crystal lattice, forming another type of 0D defect which shows promise in defect engineering^{[35, 45]}. 2D alloy can be formed as the concentration of substitutional atoms increases, and even the phase structure can be changed at a critical alloy con-centration, which exhibits distinguishable properties from the pristine crystal^[46]. Examples include the modulation of the band gap structure in 2D semi-conductor by alloying different atoms in the same phase, showing promise for tunable electronic and optoelectronic devices^[41]. Recent studies advanced the field by alloying 2D materials in different phases, such as 2H phase semiconducting WSe₂ and 1Td semimetallic WTe₂, realizing a structural transition from 1H to 1Td phase controlled by the Te concentration, along with the transition from semiconductor to metal^[47]. The atomic resolved Z-contrast STEM images in Fig. 2a~d show four representative WSe_2(1–x)Te_2x alloy monolayers with different Te concentration, including pristine WSe₂ in 2H phase (x = 0, Fig. 2a), WSe_1.0Te_1.0 in 2H phase (x = 0.5, Fig. 2b), WSe_1.0Te_1.0 in 1Td phase (x = 0.5, Fig. 2c) and pristine WTe₂ in 1Td phase (x = 1, Fig. 2d). Combined with corresponding FFT patterns, the pristine WSe₂ is confirmed to maintain a hexagonal lattice shape, while WTe₂ exists in orthorhombic lattice structure. As the element ratio of Te to Se in ternary alloy reaches 1:1, the monolayer can co-exist in both 2H and 1Td phases, and random substitution of Te atoms in the anion sublattice is seen from the distribution pattern of the bright spots in Fig. 2b and 2c without obvious reconstruction. The strategy of substitutional doping to achieve phase transition has been further developed recently. A consecutive structural phase transition from Td to 1T' to 2H has been realized by elaborately altering the element ratio of Se and Te in Se-substituted MoSe_xTe_2−x (0 ≤x≤2) thin films, and the electrical properties can be finely tuned from superconductor to semicon-ductor along with the increase of Se concentration^[48]. LVSTEM, XPS and Raman spectrum measurements reveal that phase transitions from Td to 1T′ and to 2H phase occur around x ≈ 0.8 and x ≈ 1.1, respec-tively. Additionally, the electrical transport measure-ments indicated that the Se-substitution can enhance the superconductivity of the MoTe₂ thin film.

  Figure 2

  

  Figure 2.  Atomic resolution STEM images of WSe_2(1–x)Te_2x (x = 0~1) and ReS_1.4Se_0.6 alloyed monolayers. (a) Pristine WSe₂ monolayer in 2H phase, (b) Alloyed WSe_1.0Te_1.0 monolayer in 2H phase, (c) Alloyed monolayer in 1Td phase, and (d) Monolayer WTe₂ in 1Td phase. Corresponding FFT patterns and atomic structure models are shown in the inset. Scale bar: 0.5 nm. Reprint from Ref. [47]. STEM images of the representative WSe_2xTe_2(1–x) alloyed monolayer in the 2H phase (e) and 1T' phase (g), and corresponding atom-by-atom mapping in the 2H phase (f) and 1T' phase (h). Te concentration in this region is ∼24%. Reprint from Ref. [49]. (i) STEM image of ReS_1.4Se_0.6 monolayer. (j) Intensity profiles along the lines marked in (i). (k) Filtered STEM image in (i), illustrating the spatial distribution of Re, S and Se atoms. (l) Scheme of atomic structure corresponding to the STEM image in (k). Reprint from Ref. [50]

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  Quantitative statistical intensity analysis of the Se- and S-substituted WTe₂ monolayers STEM images further unveils that under similar alloy concentration, substituted Se and S atoms maintain a random distribution in 2H phase, while highly anisotropic arrangement of the Se and S atoms is observed in the 1T' phase^[49]. Z-contrast STEM image in Fig. 2e and corresponding atomic model in Fig. 2f show that WSe_2xTe_2(1–x) alloy maintains a hexagonal lattice structure, and Te and Se atoms are randomly distributed in the anion lattice. In contrast, more than 90% of Se atoms in 1T' phase WSe_2xTe_2(1–x) alloy tends to occupy the anion sites near the cation chain, showing obvious anisotropic arrangement (Fig. 2g, 2h). The same phenomenon has also been observed in the 2H phase MoSe_2xTe_2(1–x) and 1T' phase MoS_2xTe_2(1–x) alloyed monolayer with different Te concentration, showing that such anisotropic arran-gement is solely determined by the phase structure of the 2D alloy regardless of the ternary ratio. DFT calculation explained that the anisotropic arrange-ment in 1T' phase is due to the bonding stability. Similar anisotropic ordering was also observed in the ReS_2xTe_2(1–x) monolayer, as shown in Fig. 2i~l, in which Se atoms preferentially occupy the sites between two Re atomic chains^[50]. The highly selec-tive occupation in specific atomic sites provides a possibility to reconstruct the surface of 2D materials by regulating the concentration of substitutional atoms and experimental conditions.

  Edges are the most prominent 1D defects in 2D materials, especially in 2D TMDs flakes, and are demonstrated to be the active sites for HER^{[51, 52]}. Therefore, reconstructing surface structure, increa-sing edge concentration and engineering microscopic structure of the active edge sties have become one of the main topics to improve the performance of HER.

  Oxygen plasma exposure and H₂ annealing on the as-grown MoS₂ flakes have been reported to create more electrocatalytic active sites via introducing a high density of exposed edges, which can signifi-cantly improve the HER performance^[53]. A large number of continuous mesh-like cracks on the surface of MoS₂ after oxygen plasma treatment introduced are seen, which substantially increased the density of exposed edges (Fig. 3a). The atomic structures of these newly generated edges as shown in Fig. 3b confirmed the existence of both Mo and S terminated edge structure along the cracks. Hydrogen annealing showed stronger electrocatalytic activity enhancement, which is attributed by the generation of high density of small triangle holes around 10~20 nm size, thus introducing more active edges compared to oxygen plasma treatment (Fig. 3c, d).

  Figure 3

  

  Figure 3.  SEM image (a) and STEM image (b) of CVD grown MoS₂ with 30s oxygen plasma treatment. (c, d) STEM images of CVD grown MoS₂ with hydrogen annealing. Reprint from Ref. [53]. Z-contrast STEM images of the atomically sharp lateral interfaces along the zigzag (e) and armchair (f) directions. The atomic models on the right correspond to the structure in the highlighted regions. Scale bars: 0.5 nm. (g) Z-contrast STEM image of the step edge of the WS₂/MoS₂ bilayer, with a FFT pattern in the inset. (h) Schematic of the 2H stacking in the stacked WS₂/MoS₂ heterostructure. Reprint from Ref. [39]. (i) Optimized structural model of strained ripples in bilayer graphene. (j) Top view of the optimized structural model. (k) Simulated STEM-ADF images based on the structural model in panel (i). (l) Experimental STEM image (filtered), Scale bars: 1 nm. Reprint from Ref. [54]. (m) HRTEM image of MAC, 10 × 10 nm², and the corresponding FFT pattern of the selected region (inset). (n) Large-scale atom-by-atom mapping of the selected region, 5 × 5 nm², in (m). (o) Theoretical model of MAC, replicating the MAC features in (n). Reprint from Ref. [55]

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  Lateral hetero-interfaces compose another form of 1D defect in 2D materials and may induce significant interfacial reconstruction due to the lattice mis-match^{[56, 57]}. A one-step growth strategy has been reported to synthesize WS₂/MoS₂ lateral hetero-monolayer with atomically clean and sharp interfaces^[39]. Both zigzag and armchair hetero-interface have been observed in high-magnification Z-contrast STEM images (Fig. 3e, f), clearly indicating the atomically sharp and seamless connection between the MoS₂ and WS₂ lattice with slight inter-diffusion within 1~3 unit cells. By altering the growth temperature, this one-step growth strategy can also synthesize stacked MoS₂/WS₂ vertical hetero-bilayer (Fig. 3g, h), in which the interface between vertically stacked bilayer can be regarded as a 1D defect, which induces new physical properties. More importantly, vertical hetero-bilayer generates novel van der Waals interface, which can be viewed as a 2D defect, having strong impact on the electronic and optical properties of 2D material devices. Rather than the interface of hetero-bilayer, ripples are the most abundant 2D defects in 2D materials. Ripples introduce strain into the 2D materials, which significantly affects their electronic configuration, and in turn, ripples can also be generated by artificially producing strain to achieve the surface or interface reconstruction in 2D system, and thus modulate their physical properties. Atomic resolved STEM images combined with molecular dynamics simulations have directly visualized the local ripple structures in bilayer graphene, in the form of a gradual changing Moire pattern transiting between the different bilayer AB and AC stacking (Fig. 3i~l)^[54]. Such relative fluctuation can be regarded as a strained channel between the AB and AC stacking boundaries, and the DFT calculation shows that such continuous reconstructed stacking boundary structure may significantly change the electron transport characteristics.

  At present, the research on defects and surface structure in 2D materials mainly focuses on those single crystals with high lattice symmetry, and the relationship between atomic structure and per-formance of these 2D crystals is being gradually established. In contrast, there are few reports on 2D amorphous materials due to the unknowability of their atomic structures. In fact, 2D amorphous materials can be regarded as a special form of collective defects, i.e., the assembly of high con-centration defects which destroy the lattice symmetry of 2D crystal, thereby reconstructing a completely disordered atomic arrangement. Recently, a laser-assisted CVD low temperature growth method has been reported to successfully synthesize monolayer amorphous carbon (MAC) films, and the atomic structure o MAC has been accurately measured by using the monochromated and aberration-corrected low voltage high-resolution transmission electron microscopy (HRLVTEM)^[55]. Furthermore, the radial distribution function of the long-range disorder was calculated in real space, and the atomic structure of the 2D MAC was interpreted successfully for the first time. A large scale HRTEM image as shown in Fig. 3m unfold that MAC is composed of multiple disordered connections of five-, six-, seven-, and eight-membered rings. From the enlarged image in Fig. 3n, nanocrystallite (~1 nm) is clearly observed to consist of seriously distorted six-membered rings embedded in random irregular member ring structure with random orientations. Fig. 3o shows a theoretical model of MAC based on the result of atom-by-atom mapping from HRTEM image. sp²-bonded freestanding and stable 2D MAC exhibits unique physical properties different from graphene, and has potential applications in flexible electronics and magnetic recording devices. Benefiting from the powerful LVSTEM technique, many reconstructed structures in 2D materials and even amorphous 2D materials can now be investigated down to the single atom level.

  2.2 Dynamical defect reconstruction in 2D materials

  Defect structures and their induced reconstruc-tions in 2D materials can not only be unveiled in the single atom level by LVSTEM technique, the atom-by-atom dynamical evolution process of various defects can also be monitored by sequential LVSTEM imaging, where the electron beam simultaneously acts as the excitation source to impart energy to the defects^[58–60]. The scattering interaction could transfer momentum and energy from electrons to the observed materials, which in some extent mimics the dynamical reconstruction process of the targeting materials under extreme experimental conditions, such as phase transformation under heating process, grain boundary migration, and creation of new nanostructures^[61–63], providing an atomic view in understanding the change of materials properties and also useful guidance for materials design. In this part, we reviewed several cases of dynamical defect reconstructions probed by the sequential LVSTEM imaging.

  Previous study shows that triangular 1T phase can be formed inside the 2H MoS₂ network under synergetic effect of electron irradiation and in situ heating^[63]. Similar to the 1T-2H phase boundaries, Se-deficiency 60° grain boundaries (GBs) between domains with opposite orientations, the so called inversion domain boundaries, are also observed in 2H MoSe₂ after thermal annealing process. To examine the reconstruction process of the grain boundary and inversion domain, high electron dose was applied to irradiate pristine MoSe₂ monolayer in order to simulate the annealing process. Simul-taneous STEM imaging shows that Se vacancies are generated by ionization damage under the electron beam and tend to aggregate into line defect as the accumulated electron dose increases, as shown in Fig. 4a. When Se vacancy reached a specific con-centration, subsequently, the line defect recon-structed into irregular strings of four-fold rings (4|4E GB-like structure) due to its lower formation energy. In the 4|4E GB-like structure, Mo atoms are greatly com-pressed since the reconstruction of the local lattice shrinkage. The instability of the line defect, espe-cially its distorted corner which con-tains huge strain, then drives the Mo atoms nearby to shift half a lattice (blue circles in Fig. 4a), which nucleated as an inversion domain with two 4|4P 60 GBs embedded in the parent hexagonal pattern^[62]. The nucleation and growth processes reveal that chalcogen vacancies play a vital role in dynamical reconstruction and 2D materials with specific electronic properties could be regulated by this kind of atomic-level defects engineering.

  Figure 4

  

  Figure 4.  Dynamical defect reconstruction in 2D materials. (a) Nucleation process of the inversion domain from 4|4E GB-like structure and the typical triangular inversion domain embedded within the MoSe₂ monolayer. Reprinted from Ref. [62]. (b) Sequential STEM images of the nucleation and growth of a hexagonal 1H-MoSe₂ flake out of the square β-FeSe lattice decorated with dispersed Mo atoms. Reprinted from Ref. [60] (c) High resolution ADF STEM image of layered PdSe₂ sample and in situ observation of the reconstruction process process from PdSe₂ bilayer to Pd₂Se₃ monolayer. Reprinted from Ref.[61]

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  Defect reconstruction could not only induce evolu-tion in crystal structure, but also form new materials with different crystal lattice structures from matrix materials^{[57, 65]}. For example, by choosing proper material precursor, hexagonal lattice 1H-MoSe₂ was grown through substantial reconstruction from the β-FeSe with square lattice inside a microscope chamber, where the electron beam transfers energy to excite the reconstruction. As shown in Fig. 4b, the atomic structure of square β-FeSe decorated by randomly dispersed Mo atom can be confirmed by the fast Fourier transformation (FFT) pattern, electron energy loss spectrum (EELS) and image contrast. Under irradiation of electron beam, the lattice of precursor changed to amorphous cluster. Then, nucleation of several hexagonal cells of MoSe₂ monolayer is observed, followed by the epitaxial growth from its edges to a larger size until surrounding Se or Mo is depleted^[60]. This work opens up new possibilities for the synthesis of in-plane heterojunctions between different types of crystal lattice through electron irradiation induced dynamical structural recons-truction.

  Dynamical defect evolution can also induce inter-face reconstruction. For example, the vacancy can give rise to a significant interface reconstruction which leads to interlayer fusion in 2D materials. Recently, a novel monolayer structure with different stoichiometry was exfoliated from bulk PdSe₂^[61]. To examine the formation mechanism of this new structure, electron beam was applied to irradiate bilayer PdSe₂ and in-situ observe dynamical growth process of Pd₂Se₃ monolayer. From the sequential STEM images (Fig. 4c), bilayer PdSe₂ disappears gradually at the edge of the two phases as Pd₂Se₃ monolayer grows under continuous electron beam irradiation, indicating the growth was led by interlayer fusion from bilayer PdSe₂. Another case exhibits similar result except with different orienta-tions between monolayer Pd₂Se₃ and bilayer PdSe₂. It is worth noting that the reconstruction at the interface between monolayer Pd₂Se₃ and bilayer PdSe₂ could result in lateral junction. DFT calcula-tion results predict that it can serve as 1D channel of 2D materials by doping other elements, such as Cl, Br and other donor elements^[66]. Overall, the dynamical defect evolution process, induced by electron irradiation, could be directly captured with single atom accuracy. Combined with DFT calcula-tion, special physical properties of novel structures or materials could be predicted to guide future work.

  2.3 Creating novel nanostructures by LVSTEM

  Dynamical evolution of the defect structures in 2D materials can be controlled by manipulating the electron beam to scan at designated areas, inducing intentional surface and interface reconstruction to form exotic nanostructures. Previous work showed that using parallel TEM electron illumination, complex defects cluster could be created within a MoS₂ monolayer, and subsequently form nanorib-bons but with poor spatial precision^[59]. However, the use of focused electron beam in STEM, especially in low voltage, makes it possible to realize precise defect creation in pre-designed locations of 2D materials. Recent work has adapted such strategy to pattern defects and sculpted TMC nanowires inside the TMDC monolayer, forming seamless metal-semiconductor junction owing to the inherent me-tallic nature of the nanowire^[67–71]. In detail, by steering the focused electron beam at selected re-gions of MoSe monolayer, two adjacent holes were formed, and the middle region thereafter evolved into a nanoribbon at the designated locations. Such nanoribbons are thinned into nanowires under further electron irradiation. Fig. 5a shows the sequential STEM Z-contrast images during the dynamical reconstruction of the nanowire, demonstrating the atomic-scale thinning process and the final stable structure^[58].

  Figure 5

  

  Figure 5.  Creating MoSe nanowires by precise STEM pattering and the corresponding electronic properties. (a) Serial STEM Z-contrast snapshots of the sculpting process of MoSe nanowires and the corresponding atomic structure. Dashed red triangles indicate the orientation of each layer in the nanowire. Reprintd from Ref. [58]. (b) Experimental Z-contrast STEM images of MoSe nanowires with structural flexibility: rotational twisting, axial kinking, branching of an individual nanowire and X-junctions (from left to right). (c) Z-contrast STEM image of an alloyed MoS_0.79Se_0.21 and its atomic structure. (d) Total density of states (DOS) near the Fermi energy in the MoS_xSe_1–x with different alloying concentrations. Reprintd from Ref. [64]

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  By precisely controlling the creation of defects and the surface reconstruction, different fine structures of nanowire clusters can be obtained, as shown in Fig. 5b. With the defects created at designed locations, an individual MoSe nanowire can rotate axially to form different twists, produce an off-axis kink, split into two branching nanowires, and generate a well-ordered X-junction structure. The formed MoSe nanowires are proved to be metallic by DFT calcula-tions which show diversified appli-cations due to its intrinsic structural flexibility. Furthermore, alloyed MoS_xSe_1−x nanowires were also successfully fabricated by irradiating and creating defect clusters within the alloyed monolayer counterpart, as shown in Fig. 5c. DFT calculations illustrate that the electrical conductivity of the alloy nanowire is sensitive to the alloying concentrations, thus their electrical performance can be tuned by altering the element ratio of S and Se (Fig. 5d), which can be realized by regulating the acceleration voltage in STEM during the fabrication^[64]. Thus, defect reconstruction induced by electron irradiation can be well designed for creating novel nano-structures with tunable properties, providing a useful guideline for defect engineering.

  3. CONCLUSION AND PERSPECTIVE

  The structures of surface and interface are deter-minant components affecting the physical properties of materials, especially for 2D materials which maintain huge surface and omnipresent interface. The above work that has been reviewed shows that, apart from the conventional defect structures in three dimensions, various new types of defects could exist in the 2D framework, which forms complicated defect complex or even induces substantial recon-structions at the interface. Through the assistance from DFT calculations, the accurate atomic structure of the reconstructed defects can be modelled and their influence to the physical properties is predicted, which is subsequently verified by other independent researches. The understanding of the correlation between structures and properties of defect-induced reconstructions at the surface and interface of 2D materials not only greatly propels the insight of several novel physical phenomena like enhanced 2D superconducting^{[48, 72]}, but also advances the study in intentional defect engineering for better performance in planar devices. However, in our perspective there is still plenty room for researcher to investigate in this field and many open questions remain elusive and call for solutions. For instance, we are just thoroughly examining tens of 2D materials right now, nevertheless thousands of potential candidates in the 2D family remain unexplored, most of which are air-sensitive so specific techniques are necessary. Moreover, artificial 2D materials are now being screened out by high throughput calculations, topping up more space for novel 2D materials that wait for experimental realization. In the ocean of 2D materials, we are just seeing a tiny iceberg on top, but a huge frozen glacier still sinks underneath the water and remains invisible.

  It is worthy to note that magnetism is becoming a popular topic in the research community of 2D materials^{[73, 74]} because either Heisenberg or Ising spin model rules out intrinsic magnetism in two dimensions due to the thermal fluctuation which breaks the weak ferromagnetic or antiferromagnetic exchange. But several intrinsic 2D ferromagnetic monolayers have been discovered recently^{[74, 75]}, a breakthrough yet needing further investigation to elaborate the origin of the magnetism in 2D systems in both theory and experiment. Atomic structures and direct visualization of the atomic magnetic confi-gurations in these materials are the keys to solve the ongoing debates, however, it is still challenging since magnetic materials are not suitable for TEM/STEM study due to the strong interaction inside the micro-scope chamber full of electromagnetic field. Fortunately, magnetic-free chamber is being inve-stigated and its prototype has been demonstrated to be promising for intrinsic magnetic materials. Combined with the low voltage and cryo-TEM technique, we may be able to directly see the atomic structure and the corresponding magnetic configura-tions in 2D magnetic materials during magnetic phase transition.

  
  
• 1. [1]

     Geim, A. K.; Grigorieva, I. V. Van der Waals heterostructures, Nature 2013, 499, 419–425. doi: 10.1038/nature12385

  2. [2]

     Xu, M.; Liang, T.; Shi, M.; Chen, H. Graphene-like two-dimensional materials, Chem. Rev. 2013, 113, 3766–3798. doi: 10.1021/cr300263a

  3. [3]

     Zhang, H. Ultrathin two-dimensional nanomaterials, ACS Nano 2015, 9, 9451–9469. doi: 10.1021/acsnano.5b05040

  4. [4]

     Shi, Y.; Li, H.; Li, L. J. Recent advances in controlled synthesis of two-dimensional transition metal dichalcogenides via vapour deposition techniques, Chem. Soc. Rev. 2015, 44, 2744–2756. doi: 10.1039/C4CS00256C

  5. [5]

     Geng, D.; Yang, H. Y. Recent advances in growth of novel 2D materials: beyond graphene and transition metal dichalcogenides, Adv. Mater. 2018, 30, 1800865. doi: 10.1002/adma.201800865

  6. [6]

     Voiry, D.; Mohite, A.; Chhowalla, M. Phase engineering of transition metal dichalcogenides, Chem. Soc. Rev. 2015, 44, 2702–2712. doi: 10.1039/C5CS00151J

  7. [7]

     Mirõ, P.; Ghorbani-Asl, M.; Heine, T. Two dimensional materials beyond MoS2: noble-transition-metal dichalcogenides, Angew. Chemie-Int. Ed. 2014, 53, 3015–3018. doi: 10.1002/anie.201309280

  8. [8]

     Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A. Single-layer MoS2 transistors, Nat. Nanotechnol. 2011, 6, 147–150. doi: 10.1038/nnano.2010.279

  9. [9]

     Jariwala, D.; Sangwan, V. K.; Lauhon, L. J.; Marks, T. J.; Hersam, M. C. Emerging device applications for semiconducting two-dimensional transition metal dichalcogenides, ACS Nano 2014, 8, 1102–1120. doi: 10.1021/nn500064s

  10. [10]

      Li, H.; Wu, J.; Yin, Z.; Zhang, H. Preparation and applications of mechanically exfoliated single-layer and multilayer MoS2 and We2 nanosheets, Acc. Chem. Res. 2014, 47, 1067–1075. doi: 10.1021/ar4002312

  11. [11]

      Tan, S. J. R.; Abdelwahab, I.; Chu, L.; Poh, S. M.; Liu, Y.; Lu, J.; Chen, W.; Loh, K. P. Quasi-monolayer black phosphorus with high mobility and air stability, Adv. Mater. 2018, 30, 1–8.

  12. [12]

      Xia, F.; Wang, H.; Hwang, J. C. M.; Neto, A. H. C.; Yang, L. Black phosphorus and its isoelectronic materials, Nat. Rev. Phys. 2019, 1, 306–317. doi: 10.1038/s42254-019-0043-5

  13. [13]

      Lin, Y. C.; Komsa, H. P.; Yeh, C. H.; Björkman, T.; Liang, Z. Y.; Ho, C. H.; Huang, Y. S.; Chiu, P. W.; Krasheninnikov, A. V.; Suenaga, K. Single-layer ReS2: two-dimensional semiconductor with tunable in-plane anisotropy, ACS Nano 2015, 9, 11249–11257. doi: 10.1021/acsnano.5b04851

  14. [14]

      Keyshar, K.; Gong, Y.; Ye, G.; Brunetto, G.; Zhou, W.; Cole, D. P.; Hackenberg, K.; He, Y.; Machado, L.; Kabbani, M.; Hart, A.; Li, B.; Galvao, D. S.; George A.; Vajtai, R.; Tiwary, C. S.; Ajayan, P. M. Chemical vapor deposition of monolayer rhenium disulfide (ReS2), Adv. Mater. 2015, 27, 4640–4648. doi: 10.1002/adma.201501795

  15. [15]

      Aivazian, G.; Gong, Z.; Jones, A. M.; Chu, R. L.; Yan, J.; Mandrus, D. G.; Zhang, C.; Cobden, D.; Yao, W.; Xu, X. Magnetic control of valley pseudospin in monolayer WSe2, Nat. Phys. 2015, 11, 148–152. doi: 10.1038/nphys3201

  16. [16]

      Zheng, L.; Zhou, J.; Shi, J.; Zeng, Q.; Chen, Y.; Niu, L.; Liu, F.; Yu, T.; Suenaga, K.; Liu, X.; Lin, J. InSe monolayer: synthesis, structure and ultra-high second-harmonic generation, 2D Mater. 2018, 5, 25019. doi: 10.1088/2053-1583/aab390

  17. [17]

      Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L. J.; Loh, K. P.; Zhang, H. The chemistry of two-dimensional layered transition metal dichalcogenide nanosheets, Nat. Chem. 2013, 5, 263–275. doi: 10.1038/nchem.1589

  18. [18]

      Geim, A. K. Graphene: Status and prospects, Science 2009, 324, 1530-1534. doi: 10.1126/science.1158877

  19. [19]

      Fang, A.; Kroenlein, K.; Riccardi, D.; Smolyanitsky, A. Highly mechanosensitive ion channels from graphene-embedded grown ethers, Nat. Mater. 2018, 18, 76-81.

  20. [20]

      Gupta, A.; Rawal, T. B.; Neal, C. J.; Das, S.; Rahman, T. S.; Seal, S. Molybdenum disulfide for ultra-low detection of free radicals: electrochemical response and molecular modeling, 2D Mater. 2017, 4, 25077. doi: 10.1088/2053-1583/aa636b

  21. [21]

      Prasai, D.; Tuberquia, J. C.; Harl, R. R.; Jennings, G. K.; Bolotin, K. I. Graphene: corrosion-inhibiting coating, ACS Nano 2012, 6, 1102–1108. doi: 10.1021/nn203507y

  22. [22]

      Li, J.; Chen, M.; Cullen, D. A.; Hwang, S.; Wang, M.; Li, B.; Liu, K.; Karakalos, S.; Lucero, M.; Zhang, H.; Lei, C.; Xu, H.; Sterbinsky, G. E.; Feng, Z.; Su, D.; More, K. L.; Wang, G.; Wang, Z.; Wu, G. Atomically dispersed manganese catalysts for oxygen reduction in proton-exchange membrane fuel cells, Nat. Catal. 2018, 1, 935–945. doi: 10.1038/s41929-018-0164-8

  23. [23]

      Luo, Y.; Zhang, S.; Pan, H.; Xiao, S.; Guo, Z.; Tang, L.; Khan, U.; Ding, B. F.; Li, M.; Cai, Z.; Zhao, Y.; Lv, W.; Feng, Q.; Zou, X.; Lin, J.; Cheng, H. M.; Liu, B. Unsaturated single atoms on monolayer transition metal dichalcogenides for ultrafast hydrogen evolution, ACS Nano 2019, 14, 767-776.

  24. [24]

      Lu, J. M.; Zheliuk, O.; Leermakers, I.; Yuan, N. F. Q.; Zeitler, U.; Law, K. T.; Ye, J. T. Evidence for two-dimensional ising superconductivity in gated MoS2, Science 2015, 350, 1353–1357. doi: 10.1126/science.aab2277

  25. [25]

      Chatterjee, U.; Zhao, J.; Iavarone, M.; Di Capua, R.; Castellan, J. P.; Karapetrov, G.; Malliakas, C. D.; Kanatzidis, M. G.; Claus, H.; Ruff, J.; Weber, F.; van Wezel, J.; Campuzano, J. C.; Osborn, R.; Randeria, M.; Trivedi, N.; Norman, M. R.; Rosenkranz, S. Emergence of coherence in the charge-density wave state of 2H-NbSe2, Nat. Commun. 2015, 6, 6313. doi: 10.1038/ncomms7313

  26. [26]

      Xi, X.; Zhao, L.; Wang, Z.; Berger, H.; Forró, L.; Shan, J.; Mak, K. F. Strongly enhanced charge-density-wave rrder in monolayer NbSe2, Nat. Nanotechnol. 2015, 10, 765–769. doi: 10.1038/nnano.2015.143

  27. [27]

      Soluyanov, A. A.; Gresch, D.; Wang, Z.; Wu, Q.; Troyer, M.; Dai, X.; Bernevig, B. A. Type-Ⅱ Weyl semimetals, Nature 2015, 527, 495–498. doi: 10.1038/nature15768

  28. [28]

      Zhou, W.; Zou, X.; Najmaei, S.; Liu, Z.; Shi, Y.; Kong, J.; Lou, J.; Ajayan, P. M.; Yakobson, B. I.; Idrobo, J. C. Intrinsic structural defects in monolayer molybdenum disulfide, Nano Lett. 2013, 13, 2615–2622. doi: 10.1021/nl4007479

  29. [29]

      Rhodes, D.; Chae, S. H.; Ribeiro-Palau, R.; Hone, J. Disorder in van der Waals heterostructures of 2D materials, Nat. Mater. 2019, 18, 541–549. doi: 10.1038/s41563-019-0366-8

  30. [30]

      Pennycook, S. J.; Nellist, P. D. Scanning transmission electron microscopy: imaging and analysis; Springer New York 2011.

  31. [31]

      Krivanek, O. L.; Chisholm, M. F.; Nicolosi, V.; Pennycook, T. J.; Corbin, G. J.; Dellby, N.; Murfitt, M. F.; Own, C. S.; Szilagyi, Z. S.; Oxley, M. P.; Pantelides, S. T.; Pennycook, S. J. Atom-by-atom structural and chemical analysis by annular dark-field electron microscopy, Nature 2010, 464, 571–574. doi: 10.1038/nature08879

  32. [32]

      Novoselov, K. S. Electric field effect in atomically thin carbon films, Science 2004, 306, 666–669. doi: 10.1126/science.1102896

  33. [33]

      Li, X.; Cai, W.; An, J.; Kim, S.; Nah, J.; Yang, D.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E.; Banerjee, S. K.; Colombo, L.; Ruoff, R. Large-area synthesis of high-quality and uniform graphene films on copper foils, Science 2009, 324, 1312–1314. doi: 10.1126/science.1171245

  34. [34]

      Zhou, J.; Lin, J.; Huang, X.; Zhou, Y.; Chen, Y.; Xia, J.; Wang, H.; Xie, Y.; Yu, H.; Lei, J.; Wu, D.; Liu, F.; Fu, Q.; Zeng, Q.; Hsu, C. H.; Yang, C.; Lu, L.; Yu, T.; Shen, Z.; Lin, H.; Yakobson, B. I.; Liu, Q.; Suenaga, K.; Liu, G.; Liu, Z. A library of atomically thin metal chalcogenides, Nature 2018, 556, 355–359. doi: 10.1038/s41586-018-0008-3

  35. [35]

      Lin, Z.; Carvalho, B. R.; Kahn, E.; Lv, R.; Rao, R.; Terrones, H.; Pimenta, M. A.; Terrones, M. Defect engineering of two-dimensional transition metal dichalcogenides, 2D Mater. 2016, 3, 022002. doi: 10.1088/2053-1583/3/2/022002

  36. [36]

      Van Der Zande, A. M.; Huang, P. Y.; Chenet, D. A.; Berkelbach, T. C.; You, Y.; Lee, G. H.; Heinz, T. F.; Reichman, D. R.; Muller, D. A.; Hone, J. C. Grains and grain boundaries in highly crystalline monolayer molybdenum disulphide, Nat. Mater. 2013, 12, 554–561. doi: 10.1038/nmat3633

  37. [37]

      Meyer, J. C.; Geim, A. K.; Katsnelson, M. I.; Novoselov, K. S.; Booth, T. J.; Roth, S. The structure of suspended graphene sheets, Nature 2007, 446, 60–63. doi: 10.1038/nature05545

  38. [38]

      Brivio, J.; Alexander, D. T. L.; Kis, A. Ripples and layers in ultrathin MoS2 membranes, Nano Lett. 2011, 11, 5148–5153. doi: 10.1021/nl2022288

  39. [39]

      Gong, Y.; Lin, J.; Wang, X.; Shi, G.; Lei, S.; Lin, Z.; Zou, X.; Ye, G.; Vajtai, R.; Yakobson, B. I.; Terrones, H.; Terrones, M.; Tay, B. K.; Lou, J.; Pantelides, S. T.; Liu, Z.; Zhou, W.; Ajayan, P. M. Vertical and in-plane heterostructures from WS2/MoS2 monolayers, Nat. Mater. 2014, 13, 1135–1142. doi: 10.1038/nmat4091

  40. [40]

      Dong, R.; Zhang, T.; Feng, X. Interface-assisted synthesis of 2D materials: trend and challenges, Chem. Rev. 2018, 118, 6189–6235. doi: 10.1021/acs.chemrev.8b00056

  41. [41]

      Hong, J.; Jin, C.; Yuan, J.; Zhang, Z. Atomic defects in two-dimensional materials: from single-atom spectroscopy to functionalities in opto-/electronics, nanomagnetism, and catalysis, Adv. Mater. 2017, 29, 1606434. doi: 10.1002/adma.201606434

  42. [42]

      Dan, J.; Zhao, X.; Pennycook, S. J. A machine perspective of atomic defects in scanning transmission electron microscopy, InfoMat 2019, 1, 359–375. doi: 10.1002/inf2.12026

  43. [43]

      Mendes, R. G.; Pang, J.; Bachmatiuk, A.; Ta, H. Q.; Zhao, L.; Gemming, T.; Fu, L.; Liu, Z.; Rümmeli, M. H. Electron-driven in situ transmission electron microscopy of 2D transition metal dichalcogenides and their 2D heterostructures, ACS Nano 2019, 13, 978-995.

  44. [44]

      Zhou, Y.; Song, E.; Zhou, J.; Lin, J.; Ma, R.; Wang, Y.; Qiu, W.; Shen, R.; Suenaga, K.; Liu, Q.; Wang, J.; Liu, Z.; Liu, J. Auto-optimizing hydrogen evolution catalytic activity of ReS2 through intrinsic charge engineering, ACS Nano 2018, 12, 4486–4493. doi: 10.1021/acsnano.8b00693

  45. [45]

      Wang, S.; Robertson, A.; Warner, J. H. Atomic structure of defects and dopants in 2D layered transition metal dichalcogenides, Chem. Soc. Rev. 2018, 47, 6764–6794. doi: 10.1039/C8CS00236C

  46. [46]

      Susarla, S.; Hachtel, J. A.; Yang, X.; Kutana, A.; Apte, A.; Jin, Z.; Vajtai, R.; Idrobo, J. C.; Lou, J.; Yakobson, B. I.; Tiwary, C. S.; Ajayan, P. M. Thermally induced 2D alloy-heterostructure transformation in quaternary alloys, Adv. Mater. 2018, 30, 1–6.

  47. [47]

      Yu, P.; Lin, J.; Sun, L.; Le, Q. L.; Yu, X.; Gao, G.; Hsu, C. H.; Wu, D.; Chang, T. R.; Zeng, Q.; Liu, F.; Wang, Q. J.; Jeng, H. T.; Lin, H.; Trampert, A.; Shen, Z.; Suenaga, K.; Liu, Z. Metal-semiconductor phase-transition in WSe_2(1-x)Te_2x monolayer, Adv. Mater. 2017, 29, 1–8.

  48. [48]

      Li, P.; Cui, J.; Zhou, J.; Guo, D.; Zhao, Z.; Yi, J.; Fan, J.; Ji, Z.; Jing, X.; Qu, F.; Yang, C.; Lu, L.; Lin, J.; Liu, Z.; Liu, G. Phase transition and superconductivity enhancement in Se-substituted MoTe₂ Thin Films, Adv. Mater. 2019, 31, 1–9.

  49. [49]

      Lin, J.; Zhou, J.; Zuluaga, S.; Yu, P.; Gu, M.; Liu, Z.; Pantelides, S. T.; Suenaga, K. Anisotropic ordering in 1T′ molybdenum and tungsten ditelluride layers alloyed with sulfur and selenium, ACS Nano 2018, 12, 894–901. doi: 10.1021/acsnano.7b08782

  50. [50]

      Wen, W.; Lin, J.; Suenaga, K.; Guo, Y.; Zhu, Y.; Hsu, H. P.; Xie, L. Preferential S/Se occupation in an anisotropic ReS_2(1-x)Se_2x monolayer alloy, Nanoscale 2017, 9, 18275–18280. doi: 10.1039/C7NR05289H

  51. [51]

      Shi, J.; Ma, D.; Han, G. F.; Zhang, Y.; Ji, Q.; Gao, T.; Sun, J.; Song, X.; Li, C.; Zhang, Y.; Lang, X. Y.; Zhang, Y.; Liu, Z. Controllable growth and transfer of monolayer MoS2 on Au foils and its potential application in hydrogen evolution reaction, ACS Nano 2014, 8, 10196–10204. doi: 10.1021/nn503211t

  52. [52]

      Jaramillo, T. F.; Jorgensen, K. P.; Bonde, J.; Nielsen, J. H.; Horch, S.; Chorkendorff, I. Identification of active edge sites for electrochemical H₂ evolution from MoS₂ nanocatalysts, Science 2007, 317, 100–102. doi: 10.1126/science.1141483

  53. [53]

      Ye, G.; Gong, Y.; Lin, J.; Li, B.; He, Y.; Pantelides, S. T.; Zhou, W.; Vajtai, R.; Ajayan, P. M. Defects engineered monolayer MoS₂ for improved hydrogen evolution reaction, Nano Lett. 2016, 16, 1097–1103. doi: 10.1021/acs.nanolett.5b04331

  54. [54]

      Lin, J.; Fang, W.; Zhou, W.; Lupini, A. R.; Idrobo, J. C.; Kong, J.; Pennycook, S. J.; Pantelides, S. T. AC/AB stacking boundaries in bilayer graphene, Nano Lett. 2013, 13, 3262–3268. doi: 10.1021/nl4013979

  55. [55]

      Toh, C. T.; Zhang, H.; Lin, J.; Mayorov, A. S.; Wang, Y. P.; Orofeo, C. M.; Ferry, D. B.; Andersen, H.; Kakenov, N.; Guo, Z.; Abidi, I. H.; Sims, H.; Suenaga, K.; Pantelides, S. T.; Özyilmaz, B. Synthesis and properties of free-standing monolayer amorphous carbon, Nature 2020, 577, 199–203. doi: 10.1038/s41586-019-1871-2

  56. [56]

      Li, M. Y.; Shi, Y.; Cheng, C. C.; Lu, L. S.; Lin, Y. C.; Tang, H. L.; Tsai, M. L.; Chu, C. W.; Wei, K. H.; He, J. H.; Chang, W. H.; Suenaga, K.; Li, L. J. Epitaxial growth of a monolayer WSe2-MoS2 lateral p-n junction with an atomically sharp interface, Science 2015, 349, 524–528. doi: 10.1126/science.aab4097

  57. [57]

      Huang, C.; Wu, S.; Sanchez, A. M.; Peters, J. J. P.; Beanland, R.; Ross, J. S.; Rivera, P.; Yao, W.; Cobden, D. H.; Xu, X. Lateral heterojunctions within monolayer MoSe2-WSe2 semiconductors, Nat. Mater. 2014, 13, 1096–1101. doi: 10.1038/nmat4064

  58. [58]

      Lin, J.; Cretu, O.; Zhou, W.; Suenaga, K.; Prasai, D.; Bolotin, K. I.; Cuong, N. T.; Otani, M.; Okada, S.; Lupini, A. R.; Idrobo, J. C.; Caudel, D.; Burger, A.; Ghimire, N.; Yan, J.; Mandrus, D. G.; Pennycook, S. J.; Pantelides, S. T. Flexible metallic nanowires with self-adaptive contacts to semiconducting transition-metal dichalcogenide monolayers, Nat. Nanotechnol. 2014, 9, 436–442. doi: 10.1038/nnano.2014.81

  59. [59]

      Liu, X.; Xu, T.; Wu, X.; Zhang, Z.; Yu, J.; Qiu, H.; Hong, J. H.; Jin, C. H.; Li, J. X.; Wang, X. R.; Sun, L. T.; Guo, W. Top-down fabrication of sub-nanometre semiconducting nanoribbons derived from molybdenum disulfide sheets, Nat. Commun. 2013, 4, 1–6.

  60. [60]

      Brehm, J. A.; Lin, J.; Zhou, J.; Sims, H.; Liu, Z.; Pantelides, S. T.; Suenaga, K. Electron-beam-induced synthesis of hexagonal 1H-MoSe₂ from square β-FeSe decorated with Mo adatoms, Nano Lett. 2018, 18, 2016–2020. doi: 10.1021/acs.nanolett.7b05457

  61. [61]

      Lin, J.; Zuluaga, S.; Yu, P.; Liu, Z.; Pantelides, S. T.; Suenaga, K. Novel Pd₂Se₃ two-dimensional phase driven by interlayer fusion in layered PdSe₂, Phys. Rev. Lett. 2017, 119, 1–6.

  62. [62]

      Lin, J.; Pantelides, S. T.; Zhou, W. Vacancy-induced formation and growth of inversion domains in transition-metal dichalcogenide monolayer, ACS Nano 2015, 9, 5189–5197. doi: 10.1021/acsnano.5b00554

  63. [63]

      Lin, Y. C.; Dumcenco, D. O.; Huang, Y. S.; Suenaga, K. Atomic mechanism of the semiconducting-to-metallic phase transition in single-layered MoS2. Nat. Nano 2014, 9, 391–396. doi: 10.1038/nnano.2014.64

  64. [64]

      Lin, J.; Zhang, Y.; Zhou, W.; Pantelides, S. T. Structural flexibility and alloying in ultrathin transition-metal chalcogenide nanowires, ACS Nano 2016, 10, 2782–2790. doi: 10.1021/acsnano.5b07888

  65. [65]

      Mahjouri-Samani, M.; Lin, M. W.; Wang, K.; Lupini, A. R.; Lee, J.; Basile, L.; Boulesbaa, A.; Rouleau, C. M.; Puretzky, A. A.; Ivanov, I. N.; Xiao, K.; Yoon, M.; Geohegan, D. B. Patterned arrays of lateral heterojunctions within monolayer two-dimensional semiconductors, Nat. Commun. 2015, 6, 1–6.

  66. [66]

      Zuluaga, S.; Lin, J.; Suenaga, K.; Pantelides, S. T. Two-dimensional PdSe₂-Pd₂Se₃ junctions can serve as nanowires, 2D Mater. 2018, 5, 035025. doi: 10.1088/2053-1583/aac34c

  67. [67]

      Cretu, O.; Komsa, H. P.; Lehtinen, O.; Algara-Siller, G.; Kaiser, U.; Suenaga, K.; Krasheninnikov, A. V. Experimental observation of boron nitride chains, ACS Nano 2014, 8, 11950–11957. doi: 10.1021/nn5046147

  68. [68]

      Kibsgaard, J.; Tuxen, A.; Levisen, M.; Lægsgaard, E.; Gemming, S.; Seifert, G.; Lauritsen, J. V; Besenbacher, F. Atomic-scale structure of Mo6S6 nanowires, Nano Lett. 2008, 8, 3928–3931. doi: 10.1021/nl802384n

  69. [69]

      Murugan, P.; Kumar, V.; Kawazoe, Y.; Ota, N. Assembling nanowires from Mo-S clusters and effects of iodine doping on electronic structure, Nano Lett. 2007, 7, 2214–2219. doi: 10.1021/nl0706547

  70. [70]

      Meden, A.; Kodre, A. Atomic and electronic structure of Mo6S9-xIx nanowires, Nanotechnology 2005, 16, 1578–1583. doi: 10.1088/0957-4484/16/9/029

  71. [71]

      Uplaznik, M.; Bercic, B.; Strle, J.; Ploscaru, M. I.; Dvorsek, D.; Kuar, P.; Devetak, M.; Vengust, D.; Podobnik, B.; Mihailovic, D. D. Conductivity of single Mo6S9-XIx molecular nanowire bundles, Nanotechnology 2006, 17, 5142–5146. doi: 10.1088/0957-4484/17/20/017

  72. [72]

      Zhou, J.; Liu, F.; Lin, J.; Huang, X.; Xia, J.; Zhang, B.; Zeng, Q.; Wang, H.; Zhu, C.; Niu, L.; Wang, X.; Fu, W.; Yu, P.; Chang, T. R.; Hsu, C. H.; Wu, D.; Jeng, H. T.; Huang, Y. Z.; Lin, H.; Shen, Z.; Yang, C.; Lu, L.; Suenaga, K.; Zhou, W.; Pantelides, S. T.; Liu, G.; Liu, Z. Large-area and high-quality 2D transition metal telluride, Adv. Mater. 2017, 29, 1603471. doi: 10.1002/adma.201603471

  73. [73]

      Burch, K. S.; Mandrus, D.; Park, J. G. Magnetism in two-dimensional van der Waals materials, Nature 2018, 563, 47–52. doi: 10.1038/s41586-018-0631-z

  74. [74]

      Fei, Z.; Huang, B.; Malinowski, P.; Wang, W.; Song, T.; Sanchez, J.; Yao, W.; Xiao, D.; Zhu, X.; May, A. F.; et al. Two-dimensional itinerant ferromagnetism in atomically thin Fe3GeTe2, Nat. Mater. 2018, 17, 778–782. doi: 10.1038/s41563-018-0149-7

  75. [75]

      Bonilla, M.; Kolekar, S.; Ma, Y.; Diaz, H. C.; Kalappattil, V.; Das, R.; Eggers, T.; Gutierrez, H. R.; Phan, M. H.; Batzill, M. Strong room-temperature ferromagnetism in VSe2 monolayers on van der Waals substrates, Nat. Nanotechnol. 2018, 13, 289–293. doi: 10.1038/s41565-018-0063-9

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  Figure 1  Atomic-resolution STEM images of representative monolayer crystalline materials in different phases and representative 0D defects on the surface of 2D materials. Atomic resolved STEM images of MoS₂ in the 1 H phase (a), PtSe₂ in the 1 T phase (b), WTe₂ in the 1 T′ phase (c) and ReSe₂ in the 1 T″ phase (d), with their corresponding FFT patterns and atomic structural models. Reprint from Ref. [34]. (e) STEM image of monolayer V_Re-ReS₂ and corresponding FFT patterns (inset). V_Re was emphasized by the yellow circles. (f) Optimized structural model of the V_Re-ReS₂ defect. Reprint from Ref. [44]. (g) Mo SAs on MoS₂ monolayer and (f) W SAs on WS₂ monolayer, showing high densities of Mo and W SAs as adatoms on the surface. The scale bar is 1 nm for e and f. Reprint from Ref. [23]

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  Figure 2  Atomic resolution STEM images of WSe_2(1–x)Te_2x (x = 0~1) and ReS_1.4Se_0.6 alloyed monolayers. (a) Pristine WSe₂ monolayer in 2H phase, (b) Alloyed WSe_1.0Te_1.0 monolayer in 2H phase, (c) Alloyed monolayer in 1Td phase, and (d) Monolayer WTe₂ in 1Td phase. Corresponding FFT patterns and atomic structure models are shown in the inset. Scale bar: 0.5 nm. Reprint from Ref. [47]. STEM images of the representative WSe_2xTe_2(1–x) alloyed monolayer in the 2H phase (e) and 1T' phase (g), and corresponding atom-by-atom mapping in the 2H phase (f) and 1T' phase (h). Te concentration in this region is ∼24%. Reprint from Ref. [49]. (i) STEM image of ReS_1.4Se_0.6 monolayer. (j) Intensity profiles along the lines marked in (i). (k) Filtered STEM image in (i), illustrating the spatial distribution of Re, S and Se atoms. (l) Scheme of atomic structure corresponding to the STEM image in (k). Reprint from Ref. [50]

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  Figure 3  SEM image (a) and STEM image (b) of CVD grown MoS₂ with 30s oxygen plasma treatment. (c, d) STEM images of CVD grown MoS₂ with hydrogen annealing. Reprint from Ref. [53]. Z-contrast STEM images of the atomically sharp lateral interfaces along the zigzag (e) and armchair (f) directions. The atomic models on the right correspond to the structure in the highlighted regions. Scale bars: 0.5 nm. (g) Z-contrast STEM image of the step edge of the WS₂/MoS₂ bilayer, with a FFT pattern in the inset. (h) Schematic of the 2H stacking in the stacked WS₂/MoS₂ heterostructure. Reprint from Ref. [39]. (i) Optimized structural model of strained ripples in bilayer graphene. (j) Top view of the optimized structural model. (k) Simulated STEM-ADF images based on the structural model in panel (i). (l) Experimental STEM image (filtered), Scale bars: 1 nm. Reprint from Ref. [54]. (m) HRTEM image of MAC, 10 × 10 nm², and the corresponding FFT pattern of the selected region (inset). (n) Large-scale atom-by-atom mapping of the selected region, 5 × 5 nm², in (m). (o) Theoretical model of MAC, replicating the MAC features in (n). Reprint from Ref. [55]

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  Figure 4  Dynamical defect reconstruction in 2D materials. (a) Nucleation process of the inversion domain from 4|4E GB-like structure and the typical triangular inversion domain embedded within the MoSe₂ monolayer. Reprinted from Ref. [62]. (b) Sequential STEM images of the nucleation and growth of a hexagonal 1H-MoSe₂ flake out of the square β-FeSe lattice decorated with dispersed Mo atoms. Reprinted from Ref. [60] (c) High resolution ADF STEM image of layered PdSe₂ sample and in situ observation of the reconstruction process process from PdSe₂ bilayer to Pd₂Se₃ monolayer. Reprinted from Ref.[61]

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  Figure 5  Creating MoSe nanowires by precise STEM pattering and the corresponding electronic properties. (a) Serial STEM Z-contrast snapshots of the sculpting process of MoSe nanowires and the corresponding atomic structure. Dashed red triangles indicate the orientation of each layer in the nanowire. Reprintd from Ref. [58]. (b) Experimental Z-contrast STEM images of MoSe nanowires with structural flexibility: rotational twisting, axial kinking, branching of an individual nanowire and X-junctions (from left to right). (c) Z-contrast STEM image of an alloyed MoS_0.79Se_0.21 and its atomic structure. (d) Total density of states (DOS) near the Fermi energy in the MoS_xSe_1–x with different alloying concentrations. Reprintd from Ref. [64]

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