ISSN 1998-0124 CN 11-5974/O4

2021, 14(6): 1650–1658 https://doi.org/10.1007/s12274-020-3034-z

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Atomic-scale insights into the formation of 2D crystals from in situtransmission electron microscopy Yatong Zhu1, Dundong Yuan1, Hao Zhang1, Tao Xu1 (), and Litao Sun1,2 ()

1 SEU-FEI Nano-Pico Center, Key Laboratory of MEMS of Ministry of Education, Southeast University, Nanjing 210096, China 2 Center for Advanced Materials and Manufacture, Southeast University-Monash University Joint Research Institute, Suzhou 215123, China © Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020 Received: 29 May 2020 / Revised: 30 July 2020 / Accepted: 4 August 2020

ABSTRACT Two-dimensional (2D) crystals are attractive due to their intriguing structures and properties which are strongly dependent on the synthesis conditions. To achieve their superior properties, it is of critical importance to fully understand the growth processes and mechanisms for tailored design and controlled growth of 2D crystals. Due to the high spatiotemporal resolution and the capability to mimic the realistic growth conditions, in situ transmission electron microscopy (TEM) becomes an effective way to monitor the growth process in real-time at the atomic scale, which is expected to provide atomic-scale insights into the nucleation and growth of 2D crystals. Here we review the recent in situ TEM works on the formation of 2D crystals under electron irradiation, thermal excitation as well as voltage bias. The underlying mechanisms are also elucidated in detail, providing key insights into the nucleation and formation of 2D crystals.

KEYWORDS two-dimensional crystal, in situ transmission electron microscopy, formation mechanism, electron irradiation

1 Introduction Two-dimensional (2D) crystals with ultrathin thickness, which have been among the most studied materials since the discovery of mechanically exfoliated graphene in 2004 [1], are promising fundamental building blocks in the next-generation electronics and optoelectronics due to their unique geometry and extraor-dinary properties [2–5]. The properties of 2D crystals are expected to be determined by their morphology and crystal structure [6], so the capabilities for the synthesis of designed 2D crystals are of critical importance for the further study of their properties as well as exploration of potential applications. Up to now, various synthesis methods have been developed, such as top-down exfoliation, thermal decomposition, chemical vapor deposition (CVD), solution synthesis and so on [7–9]. Although the shape, size and uniformity can be tuned to a certain extent by modifying experimental parameters such as temperature, chemical reagents, concentrations, etc., the underlying growth mechanisms are still not fully understood because most of these methods are not suited to perform real-time observations with high spatial resolution. The lack of dynamical observation of nucleation and growth processes may make the synthesis mechanism inferred from ex situ characterizations unconvincing.

In recent years, some in situ characterization techniques have been developed, such as in situ X-ray absorption fine structures (XAFS), in situ X-ray diffraction (XRD), in situ transmission electron microscopy (TEM), and so on [10]. Among them, in situ TEM including in situ scanning TEM (STEM) is the only effective way to characterize dynamic changes in morphology, atomic structure, electronic state, and chemical

composition in materials at and below the nanoscale [11–17]. TEM uses electron beam to illuminate a specimen and produce enlarged images that are directly related to structure and morphology. Modern TEM has achieved lattice resolution and becomes indispensable in the study of sub-micron size objects. In situ TEM is carried out in a TEM instrument where external conditions and stimuli are applied to a specimen while the change in specimen remains under observation. In most cases, external stimuli (such as heat, stress, optical excitation, and magnetic or electric field) or exotic environments (such as gas and liquid) are applied to a specimen area through specially designed stages, which can be conveniently equipped in almost every TEM instrument. Then specimen and microscope requirements for in situ TEM are not much different from those of conventional TEM operation. With continuous im-provement and optimization, the influence of conditions and stimuli on resolution has been minimized, and stage-based in situ TEM can achieve spatial resolution close to conventional TEM imaging. Hence, it is not surprising that in situ TEM is responsible for the exploration of formation mechanism at the atomic scale, as shown in Fig. 1. It should be noticed that electron beam provides a stimulus and the interaction with the specimen may trigger local structural transformation, so in situ electron irradiation is also beneficial to explore the formation of nanophase [18–21].

There is no doubt that in situ approach is valid for 2D materials [22–24]. The particular appeal of in situ TEM studies on 2D crystals is that the motion of individual atoms in/on 2D sheets may be monitored in real time without projection artifacts, which is expected to provide atomic insights into the nucleation and formation of 2D crystals. In

Address correspondence to Litao Sun, slt@seu.edu.cn; Tao Xu, xt@seu.edu.cn

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 Figure 1 Schematic diagram showing the application of in situ TEM in studies on the formation of nanomaterials.

this paper, we summarize many in situ TEM works on the formation of 2D crystals in three main sections depending on the stimulus: electron irradiation, thermal excitation and voltage bias. The works presented in each category are grouped according to the similarities among the physical processes.

2 Electron irradiation driven formation of 2D crystals Inside an electron microscope, 2D crystals are exposed to energetic electron bombardment when the instrument is on [25–27]. As a low-mass particle, the incident electron can be easily deflected by the Coulomb interactions with atomic nucleus (elastic scattering) or/and electrons surrounding the nucleus in the specimen (inelastic scattering). Both elastic and inelastic scattering can produce some adverse effects, such as ionization damage (radiolysis), displacement damage, and sputtering, which may remove materials (atom loss) [28].

2.1 Distortion-induced transformation

Electron beam irradiation can introduce distortions even if

there is no atom loss, resulting in local strain in the 2D lattice. The distorted structure may lead atom rearrangements in order to release the strain, and eventually bring in the formation of new phase. Lin et al. used in situ STEM to follow the structural transformation between 2H and 1T phases in monolayer Re-doped MoS2 in the temperature range 400–700 °C [29]. Thermal excitation provides activation energy for atom displacement, which is expected to promote the phase transition. As shown in Fig. 2(a), the transformation starts with the formation of α-phase precursors consisting of three to four constricted zigzag chains, which has a strong tendency to nucleate at the Re dopants because the initial out-of-plane protuberance of the Re–S bond could help the S out-of-plane displacement. The atoms at the corner preferentially glide to release the local strain when two non-parallel α-phases are in contact. Consequently, a triangular nucleus of 1T phase forms and further expands via migration of β and γ boundaries under electron irradiation. Besides, considering that high doping concentration can trigger transformation from 2H to 1T but the doping rate is relatively small in this work, the author proposed that continuous electron beam irradiation may play an electronic role in accumulating negative charge to trigger the phase transition [29]. Since the electron beam scanning area and the irradiation time can be controlled easily in STEM, the phase transition can be intentionally introduced in a chosen area with a predetermined size.

The loss of atoms induced by electron irradiation usually appears as the formation of vacancies [30–32]. The local strain caused by the as-created vacancies most likely serves as driving force which can stimulate the formation of new phase around the vacancies. Take MoTe2 as an example, it is not hard to understand why 1H-MoTe2 partially converted into 1T’-MoTe2 under electron irradiation considering that the phase transition barrier is low and that 1T’ phase becomes favored under strain [33].

Generally, the vacancies are easy to migrate and agglomerate into extended defects under electron beam excitation [34, 35], which is expected to play a significant role in the formation of nanodomains. In order to confirm the role of Se vacancies in the formation of grain boundary (GB) and inversion domains, Lin et al. used the electron beam to generate and excite Se vacancies within a monolayer MoSe2 and simultaneously monitored the dynamical structural evolution [36]. In situ STEM observation discovered that the nucleation of the inversion domains and migration of 60° GB are driven by the collective evolution of Se vacancies. As shown in Fig. 2(b),

 Figure 2 Distortion-induced formation of new phase or domain. (a) STEM image series showing the step-by-step progress of phase transformation from 2H to 1T in monolayer MoS2 at 600 °C (reproduced with permission from Ref. [29], © Macmillan Publishers Limited 2014). (b) STEM image series showing the growth of the inversion domain (reproduced with permission from Ref. [36], © American Chemical Society 2015).

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the migration of 60° GB is initiated by the formation of Se vacancies (highlighted by the circle). The Se deficiency leads to a slight displacement of the nearby Se column and Mo atom; the displacement of the Mo atom then triggers the neighboring Mo atoms in the GB region to undergo displacements along the same direction one by one (highlighted by the arrows). Eventually, the inversion domain expands via the 60° GB migration after these displacements and corresponding recon-structions [36]. Similar process is expected to occur during thermal treatment when sufficient thermal energy is supplied to overcome the barrier for migration of chalcogen vacancies in defective transition metal dichalcogenide (TMDC) [37]. It is easy to understand that much larger inversion domains can be achieved when both electron irradiation and thermal excitation are applied to the specimen since the additional thermal energy induces the higher mobility of chalcogen vacancies [38]. Naturally, a growth strategy of 2D crystals with large-area inversion domains is concluded that electron beam is used to control the nucleation while thermal treatment is used to promote the growth.

2.2 Atom-loss-induced transformation

Under continuous electron irradiation, transformation between the phases with different stoichiometry is feasible if the loss ratio of different elements does not match with the initial stoichiometry [39]. Take layered tin dichalcogenides as an example, the progressive removal of chalcogen atoms can be achieved by electron irradiation in a controllable manner, which initially results in the formation of mixed mono- and dichalcogenide followed by complete conversion to highly anisotropic orthorhombic monochalcogenides [40]. Figure 3(a), as an example, presents the transformation of few layers SnS2 into SnS via a Sn2S3 intermediate under electron irradiation at room temperature. The electron beam induced transformation at higher temperature is similar, but the as-formed SnS shows larger size, uniform thickness as well as long-range crystalline order due to longer-range mass transport [40]. The transformation pathway for SnSe2 is quite different. SnSe2 always directly transforms into basal plane oriented SnSe because there is no Sn2Se3 intermediate [40].

Another novel example is the transformation of bilayer PdSe2 into monolayer Pd2Se3 by interlayer melding accompanied with the emission of Se atoms under electron irradiation [41], as shown in Fig. 3(b). After the introduction of Se vacancies by electron irradiation in the layered PdSe2 matrix, the undercoordinated Pd atoms try to bond with the nearest Se atom in the adjacent layer, which is expected to create quantum force that pulls two layers towards each other. The interlayer distance decreases as the concentration of Se vacancies increases, causing the melding of two layers into one when the distance reaches close to the length range of typical Pd–Se bond [41].

In the limit, some elements are totally lost under the electron irradiation, which may lead to the formation of out-of-equilibrium 2D crystals. A novel example is the formation of monolayer Mo by selectively ionizing Se atoms in monolayer MoSe2 using STEM [42], as shown in Fig. 3(c). MoSe2 is the most appropriate template for growing a Mo membrane compared to other Mo compounds because the relatively weak Mo–Se bonds are easy to dissociate. Either vacancy complexes or line defects are triggered by the formation and subsequent aggregation of Se vacancies, and contribute to the growth of Mo membranes. It should be noticed that the as-formed Mo membrane is not the considered body centered cubic structure in bulk Mo crystal but retains the close-packed structure of the sandwiched Mo layer in MoSe2 sheet. Such a formation strategy

 Figure 3 Atom-loss-induced formation of new phase or domain. (a) Transformation of few layers SnS2 to SnS via electron beam induced loss of S (reproduced with permission from Ref. [40], © American Chemical Society 2016). (b) The formation of monolayer Pd2Se3 extending to the few-layers PdSe2 matrix by emission of Se atoms under electron excitation (reproduced with permission from Ref. [41], © American Physical Society 2017). (c) Formation of monolayer Mo by selectively ionizing Se atoms in monolayer MoSe2 (reproduced with permission from Ref. [42], © WILEY- VCH Verlag GmbH & Co. KGaA, Weinheim 2018).

may extend to other pure metal-based membranes, enlarging the diversity of 2D crystals.

2.3 Diffusion and rearrangement of adatoms

It is a remarkable fact that most of the excluded atoms at the surface do not leave the specimen. These atoms are weakly bound to the surface by physisorption or/and chemisorption, which can frequently diffuse along the surface because the activation energy for surface diffusion is typically lower than adsorption energy by a factor of 3 or more [43]. These adatoms are expected to aggregate and rearrange themselves on the surface or at the defect sites, presenting as epitaxial growth or healing [44]. Figure 4(a) presents the healing of Bi2Te3 nanopore under 300 kV electron irradiation [45]. Both Bi and Te adatoms ejected from the lattice provide the source for healing. In situ TEM observations reveal that adatoms pre-ferentially occupy sites with more surrounding atom columns, which have larger binding energy due to more created bonds. Similar phenomena were observed in MoS2 and can extend to other 2D TMDC sheets. It’s worth noting that electron irradiation can induce ejection and reconstruction simultaneously, corresponding to etching and growth respectively. These two processes may have different rates depending on the electron beam intensity [46], so it is important to control the electron beam intensity to an optimal level, allowing the growth to be the dominant process. In addition, such growth process can be accelerated with the combination of thermal treatment because extra thermal energy can promote the diffusion of adatoms [47].

Novel 2D or quasi 2D structures can also be transformed from clusters on specimen surface if the incoming energy is

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 Figure 4 The formation of 2D or quasi 2D crystals resulting from diffusion of adatoms at the surface. (a) The healing of Bi2Te3 nanopores (reproduced with permission from Ref. [45], © WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2018). (b) Formation of a monolayer CuO on graphene substrate (reproduced with permission from Ref. [50], © IOP Publishing Ltd 2016). (c) Catalytic growth of graphene by a single Cr atom at the edge (reproduced with permission from Ref. [52], © Tsinghua University Press and Springer-Verlag GmbH Germany 2017).

sufficient to trigger the reconstruction. Zhao et al. found that Fe residuals were able to entirely seal small perforations in graphene and form free-standing crystalline single-atom-thick layer [48]. The lattice constant for such Fe membranes was larger than bulk Fe and expanded towards the center of the membranes due to the strain resulting from the mismatch of lattice between graphene and suspended Fe sheet. Unfortunately, the largest stable Fe monolayer is only 10–12 atoms wide. Similarly, metal oxide monolayer can form either on graphene substrates or inside graphene nanopores [49]. It has been observed for crystalline single-atom-thick CuO membrane on graphene substrate [50] and is shown in Fig. 4(b). Small CuO clusters on the graphene surface are mobile around the graphene edges and finally transform into a crystalline monolayer structure with a square Cu sub-lattice under prolonged electron irradiation. The presence of a van der Waals (vdW) interaction between the as-formed CuO layer with the graphene substrate was identified, which suggests that a large-scale free-standing monolayer CuO may exist. These studies confirm the potential of graphene as a support for the growth of 2D membranes and pave the way for new 2D structures from a variety of elements to emerge.

When the adatoms diffuse to the edge with foreign atoms, single atom catalytic growth can be resolved [51]. Ta et al. directly observed single Cr atom catalytic growth of graphene using in situ TEM [52], as shown in Fig. 4(c). The Cr atoms in graphene vacancies migrated in no particular direction along the edge under electron irradiation, and additional carbon atoms inserted between the initial and final positions of the Cr atoms, forming new hexagonal structures at the edge. Cr atoms were only observed to induce graphene growth, while Fe atoms diffused along a graphene edge either etching and growing graphene by removing or adding C atoms [53]. The results suggest that Cr atom is more stable and efficient catalytic atom for graphene growth than Fe atom. This work also provides key insights into the fundamental processes and mechanisms for the growth of graphene by metal catalysts.

2.4 Electron beam induced crystallization

Electron beam induced crystallization of 2D or quasi 2D crystals is also resolved. Borrnert et al. investigated electron beam- induced heating for graphitization of amorphous carbon [54], as shown in Fig. 5(a). When amorphous carbon was supported on graphene or hexagonal BN membranes which served as the template, the planar graphene growth occurred layer by layer through vdW interactions from the substrate and the number of layers could be controlled by the thickness of the amorphous carbon. Such a transformation is somewhat similar to high temperature induced crystallization. Generally, the temperature rise induced by electron beam is not considered significant for materials with good thermal conductivity. However, if the specimen volume reduces and heat dissipation is inhibited then the local temperature may exceed the melting temperature and even sublimation temperature [54]. For amorphous carbon cluster on graphene, the energy transferred from electron beam is difficult to dissipate through graphene substrate or by radiative processes. Thus, a cluster could potentially heat up significantly, resulting in the graphitization of amorphous carbon. This heating process was confirmed by the molecular dynamics simulations which showed that the interaction of the electron beam and the amorphous carbon cluster was similar to a thermal explosion [54]. Another example is the crystallization of ultrathin amorphous MoS2 films [55], as shown in Fig. 5(b). Density functional theory (DFT) calculations confirmed a thermodynamic driving force behind the experimental observation. The energy input from the electron beam, which can emulate ex situ processing at elevated tem-perature, is helping to overcome kinetic barriers to crystallization. Similarly, the energy input from laser beam can increase local temperature, and then trigger the morphology modification or the transition between the amorphous and crystalline [56, 57].

These findings demonstrate that the electron beam can be an effective stimulus to trigger the nucleation and growth of

 Figure 5 Crystallization of 2D crystals by electron beam-induced heating. (a) Graphitization of amorphous carbons supported on graphene (reproduced with permission from Ref. [54], © WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2012). (b) Crystallization from amorphous-MoS2 to nanocrystalline-MoS2 domains (reproduced with permission from Ref. [55], © American Chemical Society 2018).

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2D materials although the size of as-formed crystals seems small. More importantly, the energy input from the electron beam can emulate materials processing under other stimulus, such as elevated temperature. Thus, the atomic-scale insights into the formation of 2D crystals under electron irradiation are readily extendable to other synthesis and processing conditions.

3 Thermal driven formation of 2D crystals Many crystal nucleation and growth, recrystallization as well as phase transitions in solids take part at elevated temperatures [58, 59]. In situ heating stages for TEM are normally realized by placing a heating element at the tip of a specimen holder and use precise Joule heating and temperature dissipation to obtain tunable and stable specimen temperatures. Nowadays, modern heating stages can reach up to 1,200 °C while showing high temperature stability and low specimen drift, which allow imaging with lattice resolution at elevated temperature. On this basis, thermal driven crystallization of 2D sheets can be studied by varying the specimen temperature. Julio et al. reported the transformation from amorphous carbon to graphene at high temperatures in the presence of catalytically active metals [60]. At temperatures over 600 °C, the amorphous carbon dissolved in metals, and then saturation of the metal with carbon and bulk diffusion of carbon in the metal preceded the nucleation and growth of graphene. Such a transformation was driven by a lowering system energy, while the transformation barrier was lowered by catalytic metals.

3.1 Thermal decomposition

In situ TEM studies on the growth of 2D TMDC crystals during thermal decomposition of amorphous solid precursors have been made by several groups. Fei et al. observed distinct growth stages and crystallization behaviors of MoS2 flakes on an amorphous Si3N4 substrate from ammonium thiomolybdates ((NH4)2MoS4) at elevated temperatures [61]. In the initial stage (400 °C), the solid precursor (NH4)2MoS4 decompose into MoS2

and vertically aligned MoS2 structures grow in a layer-by-layer mode, as shown in Fig. 6(a). The growth of new layers is always initialized from the step edges of the old one because the step edges are more energetically active. As the temperature rises to 780 °C, the vertically aligned structures transform to horizontal structures as a result of the minimization of the system energy. By further increasing temperatures or providing more precursors, MoS2 flakes can enlarge through oriented attachment and Ostwald ripening [61]. Interestingly, the initial growth pathway is partially consistent with the sulfidation of molybdenum oxide in an H2S/H2 atmosphere at 690 °C performed in an environmental TEM (ETEM) [62]. In situ ETEM observation demonstrated that single-layer MoS2 preferentially nucleated and grew heterogeneously, and then new layers homogeneously formed on old ones from either the middle or the edge, leading to the formation of multi-layer MoS2 in a layer-by-layer mode [62]. Besides, Sang et al. also observed the formation of MoS2 through thermal decomposition of (NH4)2MoS4 using both in situ TEM and in situ STEM [63]. They found that the growth started with the formation of MoS2 nanograins, followed by grain coalescence through oriented attachment and GB movement. Then the influence of reaction temperature, growth substrate, the initial precursor morphology as well as electron irradiation on the as-formed 2D MoS2 flakes were explored, which may provide new understanding for controllable synthesis of large-area 2D TMDC crystals [63]. About the same time, Kondekar et al. investigated the effects of added Ni on the dynamic crystallization and growth process of MoS2 during the thermal decomposition of (NH4)2MoS4 precursor [64]. Combining in situ TEM and other ex situ characterizations, they found that low concentrations of Ni could cause larger crystals. In contrast, high concentrations of Ni may suppress MoS2 formation, instead resulting in the formation of Ni and Ni-sulfides [64]. Zhang et al. explored the influence of precursor on the as-formed flakes [65], and found that the as-formed WS2 and MoS2 are vertically aligned rather than horizontally oriented during thermal decomposition of K2WS4/ K2MoS4 at 900 °C. Combining with density functional theory

 Figure 6 Thermolysis driven growth of 2D crystals. (a) Layer-by-layer growth of vertical MoS2 structure from solid precursor at 400 °C (reproduced with permission from Ref. [61], © Fei, L. F. et al. 2016). (b) Epitaxial growth of graphene on SiC by thermal decomposition (reproduced with permission from Ref. [66], © Tsinghua University Press and Springer-Verlag GmbH Germany 2017).

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(DFT) simulations, the author revealed that K element plays a critical role in the growth and evolution of vertically aligned structure. These results provide fundamental understanding on controllable growth of emerging 2D crystals by thermal decomposition.

Another intuitive example is the epitaxial growth of graphene on 6H-SiC (11

_00) surface at elevated temperatures [66], as

shown in Fig. 6(b). Since the carbon density in one SiC (11_00)

layer is only a third of that in graphene, at least three (11_00)

layers are required to be decomposed to form one complete graphene layer. Si atoms are sublimated layer by layer while C atoms remain on the surface and rearrange to form different transition structures. After the complete decomposition of the first layer, carbon clusters which serve as nuclei for graphene growth are formed on the SiC surface (II). The carbon atoms generated by the second layer of sublimation connect to the clusters to form a random carbon network with a disordered structure (III). The decomposition of three SiC layers produces just enough carbon atoms to form a complete graphene layer (IV). Single layer graphene cannot prevent the sublimation of silicon atoms from the surface and the subsequent layers can continue to decompose and repeat the processes mentioned above.

3.2 Thermal dominated diffusion

If the thermal energy is sufficient, the atoms in the substrate will diffuse to the surface and then rearrange themselves to form adlayer on the substrate. Sang et al. dynamically observed homoepitaxial growth of hexagonal TiC (h-TiC) adlayers on monolayer Ti3C2 MXene substrates, generating new 2D materials Ti4C3 and Ti5C4 [67]. The homoepitaxial growth of 2D h-TiC is explained as a dedicate interplay between energy barriers, including low diffusion barrier of Ti and C adatom on h-Ti surface, high surface energy of h-Ti surface, high step-edge energy and high binding energy of h-TiC adlayer. Ti and C atoms in monolayer Ti3C2 flakes migrate onto the h-Ti surface under thermal excitation, providing Ti and C adatoms as the source material for growth and leaving pores in the substrate. The self-assembly of C and Ti adatoms on the surface forms TiC dimers, which serve as nucleation sites for bonding of additional TiC dimers, leading to the island growth of h-TiC adlayers on the h-Ti surface. Due to the non-equilibrium nature of fast diffusion and growth at high temperatures, the h-TiC adlayers exhibit irregular shapes. The bottom-up synthesis methods can be used for controllable growth of larger-scale and higher quality single-layer transition metal carbides with a careful selection of the alternative substrates.

To summarize, TEM equipped with heating stage can accurately control the specimen temperature and dynamically monitor the nucleation and growth process, which provides atomic insights into thermal driven growth mechanism of 2D materials and is expected to guide the controllable growth and tailoring of new 2D materials with desired properties.

4 Electrical driven formation of 2D crystals

4.1 Current induced crystallization

TEM can record structural evolution of specimen under bias voltage while simultaneously measuring electrical transport properties using electrical biasing stages where the specimen is connected into a circuit. When a bias voltage is applied to the specimen, the temperature rise caused by Joule heating is expected to trigger the crystallization of amorphous precursors. In essence, such a crystallization is the same to those carried

out by heating stage. The difference is that heating stage uses Joule heating of heating element while electrical biasing stage uses Joule heating of the specimen, and local temperature under electrical field, which depends on the specimen, may reach far beyond the range of heating stage [68, 69]. Current- induced transformation of carbon precursor to graphene has been investigated by several groups. Westenfelder et al. investigated the transformations of carbon adsorbates to graphene by controlling the current via two steps, including the transformation of physisorbed hydrocarbon adsorbates into amorphous carbon monolayers around 1,000 K and transformation into a completely polycrystalline graphene at temperatures exceeding 2,000 K [70]. They found that most of the annealed edges of individual grains present in armchair configuration because the energy cost of the armchair edge is the lowest. Barreiro et al. also investigated the formation of graphene from hydrocarbons [71]. Hydrocarbons in the TEM are decomposed by electron beam and amorphous carbon deposits on underlying graphene substrate. The small amorphous carbon clusters rearrange themselves and crystalize into high quality graphene before reaching the sublimation temperature of the amorphous carbon under electrical field.

4.2 Electrically driven intercalation

Under a voltage bias, small atoms or ions can be intercalated into 2D sheets by electromigration, resulting in the formation of new crystal or new phase [72, 73]. Figure 7(a) presents a novel example that Li atoms are assembled as ultrathin close-packed order inside a bilayer graphene flake [74]. The intercalated Li exhibits rapid lateral diffusion, establishing a uniform distribution of Li in the bilayer graphene. First prin-ciple calculations confirm that the multi-layered close-packed Li is conceivable. Moreover, close-packing Li atoms between bilayer graphene consume similar energy as the forming of C6LiC6 which is an energetically favored configuration for a single layer Li inside bilayer graphene, and this may be the reason for the accommodation of a large amount of Li in this system within such a short time.

Another intuitive example is the phase transformation by intercalation of alkali metals into TMDC sheets [75–77]. Figure 7(b) presents the transformation from 2H to 1T in MoS2 sheets after the intercalation of Li ions [78]. The phase transition of MoS2 crystal is found along with the wavy surface (pointed out by arrows) resulting from a sliding motion of MoS2 slabs and changing of the Mo sites. Both high resolution TEM and electron diffraction images reveal that the pristine 2H structure transforms to 1T phase after the Li ion intercalation. At the reaction front, the area near the lithiated side (pointed by a red arrow) presents a wavy periodic structure, and the area next to the unreacted part (pointed by a black arrow) is full of slip dislocations. The dislocation zone indicates the presence of strain introduced by the shear transformation of MoS2 slabs. It is believed that the Li ion intercalation provides the driving force to induce the slabs sliding and promotes the phase transformation. Besides, the transformation by intercalation of Na ions has also been investigated. Gao et al. reported a two-phase transition from trigonal prismatic 2H-MoS2 to octahedral 1T-NaMoS2 during Na intercalation in MoS2 nano-sheets using in situ electrical TEM platform [79]. The structural distortion of the phase caused by the Na ion intercalation induces the super-structural of the 1T-NaMoS2 phase. They also revealed the fast nucleation and formation of the expanded and defective LiSnS2 phase in the SnS2 matrix with a clear phase boundary through a two-phase reaction during Li intercalation [80].

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5 Conclusions and outlook The controlled growth and tailored design of 2D materials are keys for achieving their superior properties and potential applications. It is of critical importance to directly observe the growth processes and understand the growth mechanism in order to guide synthesis as well as tailoring functional properties of these materials. In recent years, in situ TEM has been proved to be a powerful tool to monitor the growth at the atomic scale due to the high spatiotemporal resolution and the capability to mimic the actual growth environments. In this paper, we summarize the recent in situ TEM works on the atomic scale nucleation and formation of 2D crystals under electron irradiation, thermal excitation and voltage bias. The underlying nucleation and growth mechanisms are also elucidated in detail, providing key insights into the growth of 2D materials.

Despite the recent success in this exciting research area, there are still many technology challenges and opportunities. Firstly, how to identify the side effects of electron beam irradiation and reduce their influence needs specific criteria. Electron beam enables visualization of the structural evolution under external stimuli while providing a stimulus. No matter what beam energy, and electron imaging conditions are chosen, electron beam irradiation effects to specimens can never go to zero especially after long time observation. The structural changes actually stem from a combined effect of the applied fields and the electron beam irradiation. Secondly, atomically resolved observations of the formation of 2D crystals during realistic processing remain a difficult challenge by TEM. It is clear that until now only a small fraction of the developed

growth techniques of 2D crystals have found their route to detailed mechanistic examination by the in situ TEM. Most of the real-time observation is restrained to the nucleation and growth from a solid precursor in the microscope where gas and liquid precursors are hardly preserved. Although the dynamical behavior in small amount of gas or liquid can be monitored with the use of dedicated specimen stages, the spatial resolution is not enough to achieve imaging at the atomic scale. It can be expected that new approach will be developed to reveal the growth of 2D crystals in gas or liquid at the atomic scale. On the other hands, multimodal operation capable of detecting multiple signals in situ may gain importance. If the structural evolution and the change of chemical information can be obtained simultaneously, a deeper understanding of the growth process can be inspired. Besides, the emerging of 2D heterostructures with a variety of properties and a range of potential applications arouses the development of scalable manufacturing, in situ TEM study on the formation of 2D heterostructures may bring new opportunities.

Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 11525415, 61974021, 61601116, and 51420105003) and the Natural Science Foundation of Jiangsu Province (No. BK20181284).

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 Figure 7 The formation of 2D crystals induced by the electromigration of alkali metals. (a) Formation of ultrathin Li crystal inside bilayer grapheneunder voltage bias (reproduced with permission from Ref. [74], © Springer Nature Limited 2018). (b) Morphological evolution of MoS2 nanosheet during the lithiation (reproduced with permission from Ref. [78], © American Chemical Society 2014).

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