Capture the growth kinetics of CVD growth of two-dimensional MoS2

Understanding the microscopic mechanism of chemical vapor deposition (CVD) growth of two-dimensional molybdenum disulfide (2D MoS2) is a fundamental issue towards the function-oriented controlled growth. In this work, we report results on revealing the growth kinetics of 2D MoS2 via capturing the nucleation seed, evolution morphology, edge structure and terminations at the atomic scale during CVD growth using the transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) studies. The direct growth of few- and mono-layer MoS2 onto graphene based TEM grids allow us to perform the subsequent TEM characterization without any solution-based transfer. Two forms of seeding centers are observed during characterizations: (i) Mo-oxysulfide (MoOxS2-y) nanoparticles either in multi-shelled fullerene-like structures or in compact nanocrystals for the growth of fewer-layer MoS2; (ii) Mo-S atomic clusters in case of monolayer MoS2. In particular, for the monolayer case, at the early stage growth, the morphology appears in irregular polygon shape comprised with two primary edge terminations: S-Mo Klein edge and Mo zigzag edge, approximately in equal numbers, while as the growth proceeds, the morphology further evolves into near-triangle shape in which Mo zigzag edge predominates. Results from density-functional theory calculations are also consistent with the inferred growth kinetics, and thus supportive to the growth mechanism we proposed. In general, the growth mechanisms found here should also be applicable in other 2D materials, such as MoSe2, WS2 and WSe2 etc.


Introduction
Two-dimensional molybdenum disulfide (2D MoS2), a representative member of the rediscovered transition metal dichalcogenide (TMDC) family, is holding promising interests owing to their excellent performance in electronic, optoelectronic and catalytic applications. [1][2][3][4][5][6][7] However, there are few challenging tasks still remain in reality towards the potential applications, for instance, to modulate the carrier type and concentration i.e., to achieve equivalent good performance for both n-and p-type in electronic devices, to precisely control the structure/morphology, to grow wafer-size sample and in high quality. Out of these, the controlled growth seems to be a key issue. So far, chemical vapor deposition (CVD), as one of the most successful routes, has been widely adopted to grow 2D TMD material from graphene, 8,9 hexagonal boron nitride 10 to 2D TMDC 3 materials, [11][12][13] while the growth mechanisms, particularly in the CVD growth of 2D TMDC materials, is less understood.
Previous studies have successfully revealed the roles of a number of important parameters determining the CVD growth behaviors of 2D MoS2 via CVD, i.e., hydrogen carrier gas, precursors/promotors and screw dislocations etc. 14- 22 Recently Rajan 18 et al. have proposed a generalized mechanistic model to quantitatively explain the shape evolution of MoS2 monolayers observed experimentally. In spite of these successes, our knowledge on the microscopic process during the growth including nucleation and growth kinetics seems to be still limited, partly due to the few practical difficulties, i.e., the challenge to transfer ultra-fine clusters formed at early stage growth, possible loss of intermediate-/by-products (some of them are aqueous soluble). As such, comprehensive studies are highly demanded to reveal the microscopic growth mechanism of 2D MoS2.
In this work, we carried out a CVD experiment and subsequent TEM study to capture the nucleation and subsequent growth kinetics of 2D MoS2. Here few-and mono-layer MoS2 were grown directly on graphene supported TEM grids, [23][24][25][26] and then loaded into the microscope chamber for further microscopic characterizations, during which no solution was involved in the transfer process, and thus allowing us to visualize the seeding centers, shape morphology, edge structures and the associated morphology evolved during the growth. We firstly confirmed there exists two form of seeding centers: (1) Mo-oxysulfide (MoOxS2-y, y≥x) nanoparticles either in nested multi-shelled fullerene-like structures or in compact forms of nanocrystals, for few-layer MoS2, and 4 (2) atomic MoS2 monolayer cluster. For the growth of mono-layer MoS2, it develops from an irregular polygon-shape morphology comprised with two primary forms of edge configurations: S-Mo Klein edge and Mo zigzag edge. As the growth proceeds, it turns to appear in a near-triangular shape predominantly terminated with Mo zigzag edge. Microscopic nucleation and the growth kinetics and the mechanism can be deduced based on our experimental finding, which were further supported by densityfunction theory (DFT) calculations.

Results and Discussions
Figures 1a shows the schematic of our CVD system in which the graphene supported TEM grids (home-made) were placed facing-down to the MoO2-containing boat. few-layer regions bound with thicker/heavier nanoparticles or nanorods (marked with dotted circles in green) and monolayer MoS2 with a near-triangle shape (see dotted triangle in red). Contrastingly, in the latter case (high Ar rate), as-grown samples contain predominating monolayer MoS2, as shown in Figure 1c.
Further ADF-STEM and energy dispersive X-ray spectra (XEDS) were carried out to probe the atomic structure and the chemical compositions of those nanoparticles-or nanorods-like cores on few-layer MoS2 as shown in Figure 2 and Figure S1a During the preparation of this manuscript, we became aware of another work reporting similar fullerene as the seeding materials monolayer MoSxSe(2-x). 27 Importantly, another form of nanoparticles rather than the multi-shelled fullerene/tubular structure was experimentally observed which was further proved to Given the results shown above, we could assign these nanoparticles either fullerene/tubular like structure or compact MoOxS2-y nanocrystals as the centers for the nucleation and feeding source for the growth. Under this condition, the sublimated molecular clusters of MoO2 are mostly appearing in large sizes, and thus may not be completely sulfurized due to the limited reaction time before their deposition onto graphene substrate. As such, MoOxS2-y nanoparticles are formed, and then serving as the heterogeneous nucleation sites for the growth of MoS2 in few-layer forms. Such nuclei in large sizes should also facilitate for the nucleation and growth of few-layer MoS2, rather than the monolayer form, either from a few-layer nuclei or through a layeron-layer growth. As reaction proceeds, the nanoparticles have two different routes to serve as the nucleation sites: (1) The chemical conversion occurs much faster than the 7 diffusion of the sulfur gas into the MoOxS2-y nanoparticles which will be further sulfurized to form the nested multiple-fullerene nanostructures as shown earlier. 28 As the sulfurization, these as-deposited MoOxS2-y nanoparticles may also serve as the feeding source for growing MoS2. The fully or partially empty cores observed on those multiple fullerenes may be formed as a result of self-sacrifice as the seeding source.
Other mechanisms such as Kirkendall effect 29 during the sulfurization may also lead to the observed empty-core structures. In some cases, the outer shells in high qualities may block the mass transport and thus the sulfurization, leading the central core having a higher concentration of oxygen, as shown in Figure 1b. (2) When the c-axis of these asdeposited MoOxS2-y nanoparticles is perpendicular to the graphene plane, the particles will serve as the center of epitaxial growth, so the multiple-fullerene nanostructures will not form. We then start to address the evolution of shape morphology and edge structure of MoS2 monolayers, another key issue for the growth mechanism. Figure 4  MoS2 monolayers, serving as chemically active sites for the epitaxy addition of Mo-S molecules or atomic clusters from the supply, either by direct deposition in gas phase or in solids phase after a surface diffusion process. From the results shown above, we can infer that the morphology of MoS2 monolayers changes from irregular polygonal shape to triangular one with an increasing size (Figures 4a to 4j).
Accompanying the evolution of structure and mophology during the growth, the edge structure and termination also changed as shown in Figure 5 In the next section, we discuss the microscopic growth process of MoS2 monolayers, which seems to be different with that in few-layer case. Since no residual MoOxS2-y nanostructures were observed on as-prepared MoS2 monolayers as shown in Figure 1c and Figure 4, we can infer that the sublimated MoO2 precursor should mostly form tiny molecular clusters (MoO2, (MoO3) n etc.), 32 and then sulfurized completely, followed by its deposition on graphene substrate as the nucleation center, or its addition onto asformed nucleation center either from vapor ambient or via surface diffusion on graphene substrate. Relative ratios of these clusters can change as the carrier gas flow changes, the presence of MoOxS2-y is reduced as the sulfur concentration in the reaction zone. As such, it can also well explain the difference in the as-formed products where few-layer MoS2 or monolayer MoS2 dominates, as shown in Figures 1b and 1c.
According to the DFT calculations, suggesting that S-Mo and Mo-zz edges are dominant and with comparable ratio at the initial stage during the growth, as shown in Figure 4. Comparing to the Mo-zz edge, the S-Mo edge should possess relatively higher chemical reactivity due to the bare Mo atoms that will facilitate the incorporation of sourcing clusters, thus leading to a faster growth rate. According to the classic crystal growth theory, 33 fast growing edges/facets gradually disappear, while slow-growing edges/facets remain. In this case, it will eventually lead to the formation of triangle- 11 shape MoS2 monolayer decorated with Mo-zz edges, as most frequently observed. The slightly truncated shape observed on MoS2 mono-layers shown in Figures 4j and 4k may be formed due to insufficient growth time, which again provides clear evidence for the proposed kinetics in edge structures, as discussed previously.

Method
Graphene films used here were grown on polycrystalline copper foils, and then transferred onto molybdenum (Mo) based TEM grids via a PMMA-assisted wetchemistry process. 8 Graphene supported TEM grids were mounted onto a home-built ceramic carrier and loaded into a CVD system, facing down above an aluminum boat containing 1 mg of MoO2 precursors (Sigma-Aldrich, 99%). Here we choose MoO2, rather than MoO3 as the precursor for two considerations because of the single-step chemical reaction MoO2 +3S  MoS2 + SO2: 1) to reduce the reaction complexity, as a multi-step reduction reaction occurs for the sulfurization of MoO3, 2) to grow high 13 quality MoS2 monolayer. 13 The whole CVD setup is shown in Figure 1a. Within a typical CVD process, the furnace was firstly heated to 300°C in 10 mins and hold for additional 10 mins, and then heated to 750°C in 40 mins and kept for next 25 mins. At about 15 mins after the furnace temperature reached 750°C, heating of 300mg of sulfur source (Aladdin, 99.999%) started with its temperature reaching 180°C in 2 mins, and then hold for next 10 mins. During the whole process, argon (99.999%) was used as the carrier gas, with an optimized flow rate of 200 standard-state cubic centimeter per minute (sccm) for growing few-layer samples and of 500 sccm for monolayer samples. Energy dispersive X-ray spectroscopy (EDS) was carried out on a Bruker super-X detection system.