Controlled growth of Mo₂C pyramids on liquid Cu surface

2D materials provide unusual mechanical, chemical, and electronic properties that can be influenced by their spatial structures^[1-3]. As a new member of 2D materials, Mo₂C has been gaining great strengthen in the past few years because of its potential applications in energy storage and catalysis and so on^[4-8]. Recent advances in chemical vapor deposition (CVD) methods have enabled the large-scale production of high-quality 2D Mo₂C crystals^[9-11]. Meanwhile, interesting Mo₂C/graphene vertical heterostructure has been achieved by a one-step CVD process^[12-14]. However, there still exist several issues to be resolved. Currently, the CVD method in shaping Mo₂C have fundamental limitations that are based on a seed-induced 2D growth mechanism^[8] and do not allow one to fabricate in situ complex spatial Mo₂C structures during the growth process. Moreover, the detailed mechanism for Mo₂C formation during CVD is less explored and the related kinetics process is still elusive. The lack of understanding on the growth mechanism of Mo₂C has hindered further studies of their intrinsic properties and wide applications.

Here we demonstrate spatially controlled growth of large-area Mo₂C pyramid structures on liquid Cu surface by ambient pressure CVD method. A large amount of carbon atoms has been introduced as an accelerator to bond with Mo atoms to achieve the structures. Clear observations of terraces and highly symmetrical regular shaped contours under atomic force microscope (AFM) characterization reveal the existence of pyramid features in the as-grown Mo₂C. An inside-out lateral diffusion mechanism was proposed to elucidate the formation process as supported by atomic-resolution transmission electron microscopy (ARTEM) imaging. The proposed inside-out growth model provides a general guide for engineering spatial growth of 2D materials. This approach stands as a complementary method to CVD growth while permitting growth of complex Mo₂C based spatial structures.

2.1. CVD growth of Mo₂C pyramids

2.2. Transfer of Mo₂C pyramids structures

2.3. Characterization

The whole growth process is schematically shown in Fig. 1. It is seen that during the CVD growth, the CH₄ is introduced into the chamber as carbon source, while several pieces of Cu foils sitting on a Mo foil as the substrate at a temperature of 1150 °C (see Methods). Note that, the high temperature (above melting point of metal Cu) allows the Cu into liquid phase^[15-17], thus facilitating Mo atoms to diffuse from Cu-Mo interface to the surface of Cu to form Mo₂C. Unlike with the previously reported growth^{[9, 10]} in our case, after segregation of Mo atoms onto the Cu surface, the high temperature and large flow rate of CH₄ enables excessive segregation of Mo atoms, leading to large-area growth of typical Mo₂C pyramids with obvious featured terraces.

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Fig. 2(a) shows the optimized crystal structure of Mo₂C. Mo₂C holds an orthorhombic structure with space group Pbcn. The optimized lattice contants are 4.77, 5.99, and 5.21 Å, respectively. From the top view, Mo₂C shows regular hexagonal profile like MoS₂, MoSe₂ and many other layered 2D materials. From the side view, each Mo (C) atomic layer is combinedly bonded by two C (Mo) layers. The first-principles calculations are performed to explore its electronic structure in Figs. 2(b) and 2(c). It is found that both the electronic bands near the Fermi level are mainly contributed by d orbitals of Mo. The rather delocalization of electrons near the Fermi level makes Mo₂C a metal. Fig. 2(d) shows the Mo₂C pyramid structures fabricated on liquid Cu surface at 1150 °C under 0.2 sccm methane, 50 sccm hydrogen and 300 sccm argon with a growth time of 60 min. Notably, the argon has been introduced as tunable gas, facilitating decomposition of the methane during the growth process^[18]. The large-area growth of Mo₂C pyramid-like structures (Fig. S1) suggested that a general growth rule might work in shaping Mo₂C and the detailed growth mechanism will be present in the following section. In our experiments, it is found that all the pyramid structures have highly symmetrical shapes, a typical characteristic of well-defined crystals. As for the central part within the pyramid structures, obvious hexagonal profile is observed and thus results in formation of terrace with respect to the outer one. All the Mo₂C pyramid structures in Fig. 2(d) have regular hexagonal profiles, indicating as-formed terraces with respect to Cu substrate. It is worth noting that the Mo₂C pyramid structures with terraces is first demonstrated, which is largely different from the as-reported Mo₂C with the same thickness among the whole flake^{[8, 11]}. Further, the large-area growth of those pyramid structures strongly confirms the high controllability of our method over spatial stacking of 2D materials.

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X-ray photoelectron spectroscopy (XPS) has been performed to probe the element information of the as-grown Mo₂C crystals. It is obviously found (Figs. 2(e) and 2(f)) that there exist two typical signals correlated with the corresponding elements C and Mo, respectively. It should be noted in Fig. 1(b) that the peak derived from the C–Mo bond is labeled, indicating the flake is composed of Mo and C with an atomic ratio of ~2 : 1.

After CVD growth, a mild Cu etchant, (NH₄)₂S₂O₈ (see Methods) is used for transferring Mo₂C flakes, similar to the transfer CVD-grown other 2D materials^[19]. AFM images of three transferred hexagonal layered Mo₂C structures are clearly shown in Figs. 2(g)–2(i). As for each structure, the size of inner hexagonal profile is much smaller than that of outer one. Terrace is formed at the edge of inner flake, serving as an indicator of thickness. In the three Mo₂C structures, hexagonal shapes are strictly kept, suggesting a certain growth mode.

Fig. 3(a) shows typical optical images of large-area as-transferred Mo₂C flakes on SiO₂/Si substrate, exhibiting a clear color contrast. Further characterizations show different colors on Cu surface or SiO₂/Si substrates, allowing rapid thickness identification. Raman mapping measurement has been conducted on the Mo₂C structure. It is evidenced that the Mo₂C pyramid structures owns a characterized peak at around 650 cm^–1 under Raman spectrum (Fig. 3(b)), highly related with the orthorhombic crystal structure of Mo₂C, corresponding to the stretching mode of Mo–C. From the mapping image, three hexagonal profiles are clearly found (Fig. S2), further confirming the pyramid structures of the Mo₂C flake. Uniform color contrast among the layered whole structure is observed, indicating the same feature for the three composed hexagonal parts. Based on Raman analysis, it is reasonably deduced that the pyramid structure is composed of three individual parts along lateral direction, rather than three hexagonal flakes via layer-by-layer mode along vertical direction.

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In our experiments, one interesting phenomenon is detected, posing implication on growth mechanism of hexagonal Mo₂C pyramid. The inner hexagonal part of the pyramid structures occasionally drops off during the growth or transferring process, with incomplete hexagonal profile left. Further analysis shows that the middle part falls off independently with a hexagonal loop profile, with the bare Cu surface left behind, while the inner part and outer part still keep intact (Figs. 3(c) and 3(d)). On the basis of the observations, it is suggested that the growth of Mo₂C pyramid structures does not follow a layer-by-layer behavior, whereas week interlayer force enables peeling off through layer-by-layer and thus results in the intact downside layer even dropping of the topside layer. Then an inner-outside growth mode is favorable, in which the center part is highly initialized by excessive segregation of Mo atoms, after that, driven by concentration gradient, the following growth takes place from inside to outside with a decreasing thickness. Under this circumstance, the above experimental observations are feasible.

Another clue involving left residues during transferring process further confirms the second growth mode. After transferring of the Mo₂C pyramid onto SiO₂/Si substrates, with the structures falling off, the whole hexagonal profile, especially the contour of inner part, are kept, showing clear terraces, similar with those structures on Cu surface (Fig. S3). The stamp phenomena only appear under the second growth mode, while in the first case, it will cause the whole uniform profile on the SiO₂/Si substrates. Based on the above discussion, it is deduced that the second growth mechanism works, resulting in the formation of novel hexagonal Mo₂C pyramid structures.

Moreover, direct observation of the novel growth process of Mo₂C pyramids is realized. In our CVD growth, another kind Mo₂C pyramid structure is also controllably prepared through lowering flow rate of hydrogen to 10 sccm, whereas the profile of center part exhibits six-folded star shape, while the outer part still owns hexagonal profile. The large-area structures have been obtained over the whole substrate as shown in Fig. 4(a) and Fig. S4(a). It is clearly seen that most of those Mo₂C pyramids are similar with each other, indicating a uniform growth process. Similar with the former hexagonal case, the novel star-shaped structures also exhibit a relatively high contrast with the substrate, indicating a thickness range. The thickness determination has been performed on the as-transferred samples as shown in Fig. 4(b), whereas a distinctive terrace was detected along the central star-shaped area.

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During growth process of the novel structures, middle stages are detected (Figs. 4(c)–4(f)). At the initial stage, the nuclei of Mo₂C gradually expand, following a diffusion-limited rule under the poor concentration of carbon atoms, and thus lead to star-shaped flakes (Fig. 4(c)). With a continuous feed of carbon source, the carbon atoms are preferably attached onto the font of star-shaped flake, whereas the much higher energy of these sites allow favorable deposition of carbon atoms (Figs. 4(d) and 4(e)). After that, the second nucleation happens, obeying concentration-limited rule, and then results in anisotropic growth with hexagonal shape. It is worth noting that the second growth at the six fonts of the star-shaped flakes is not simultaneous, whereas the structures with one, two, three, four, five and six fonts are all occasionally observed (Fig. S4(b)). Further, the as-grown hexagonal parts of the original flake start to approach with each other and finally emerge into compact flake, while still keeping the hexagonal profile (Fig. 4(f)). The whole process is correspondingly schemed in Fig. 4(g). The direct observation of the intermediates of star-shape centered structures strongly supports the aforementioned inside-out growth mechanism. Further, the intermediates of hexagonal-center Mo₂C pyramid structures have also been found (Fig. S5), suggesting a generality of the proposed growth mode.

To further characterize the hexagonal pyramid-like Mo₂C structure, aberration corrected scanning transmission electron microscopy annular dark field (STEM-ADF) imaging have been performed, as shown in Fig. 5. A typical STEM-ADF image of the inter-edge region between the two layers of pyramid Mo₂C structures is clearly depicted in Fig. 5(a), where a distinct step structure is observed. It is seen that the two layers show uniform thickness, respectively, while displaying an obvious step structure at the middle region, reflected by the different Z contrast. Geometric phase analysis (GPA) is further applied to analyze the level of strain associated with the grain boundaries^[20]. A symmetric strain matrix series (Ɛ_xx, Ɛ_xy, Ɛ_yy and rotation_xy) is used to map the strain fields and the results are given in the sub-panel in Fig. 5(b). Apparently, the contrast between the two layers is evident. No strain is detected in the two single parts and both of them preserve extremely high crystallinity. Step edges have been imaged and these are atomically sharp. Therefore, it can be concluded that in the growth of the pyramid Mo₂C structures, the crystallinity, homogeneity and uniformity of each layer are strictly kept. On the basis, the step-like feature of the pyramid structure is confirmed.

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To sum up, novel Mo₂C pyramid structure has been prepared, whereas the vertical spatial control is also realized. The formation of the terraces within the structures arises from the supersaturation-induced center nucleation and concentration-gradient driven diffused growth of a new Mo₂C layer on the edge and along with the previous one. Direct observation of the whole evolution process has been demonstrated, further suggesting the in-out growth mechanism for the structures. The controlled synthesis of highly symmetrical pyramid Mo₂C structures offer impetus for the design of new 3D spatial structures of other 2D materials.

This work was supported by Peiyang Talents Project of Tianjin University (201920701321016), Independent Innovation Fund of Tianjin University (20190903067036)