Unveiling the Growth Mechanism of MoS2 with Chemical Vapor

Subscriber access provided by READING UNIV

Article

Unveiling the Growth Mechanism of MoS2 with Chemical Vapor Deposition: From 2D Planar Nucleation to Self-Seeding Nucleation Dong Zhou, Haibo Shu, Chenli Hu, Li Jiang, Pei Liang, and Xiaoshuang Chen Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01486 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 25, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Crystal Growth & Design is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Crystal Growth & Design

Unveiling the Growth Mechanism of MoS2 with Chemical Vapor Deposition: From 2D Planar Nucleation to Self-Seeding Nucleation Dong Zhou,† Haibo Shu,†,* Chenli Hu,† Li Jiang,† Pei Liang,† and Xiaoshuang Chen ‡ †

‡

College of Optical and Electronic Technology, China Jiliang University, 310018 Hangzhou, China National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Science, 200083 Shanghai, China ∗

Corresponding author. Haibo Shu, Phone: +86-0571-86875622, E-mail: [email protected]

ABSTRACT: The deep understanding of the nucleation and growth mechanisms is fundamental for the precise control of the size, layer number, and crystal quality of two-dimensional (2D) transition-metal dichalcogenides (TMDs) with the chemical vapor deposition (CVD) method. In this work, we present a systematic spectroscopic study of CVD-grown MoS2, and two types of MoS2 flakes have been identified: one type of flakes contains a central nanoparticle with the multilayer MoS2 structure and the other is dominated by triangular flakes with monolayer or bilayer structures. Our results demonstrate that two types of flakes can be tuned by changing the growth temperature and carrier-gas flux, which originates from their different nucleation mechanisms that essentially depends on the concentration of MoO3-x and S vapor precursors: a lower reactant concentration facilitates the 2D planar nucleation that leads to the monolayer/bilayer MoS2 and a higher reactant concentration induces the self-seeding nucleation which easily produces few-layer and multilayer MoS2. The reactant-concentration dependence of nucleation can be used to control the growth of MoS2 and understand the growth mechanism of other TMDs.

Keywords: Transition metal dichalcogenides; Molybdenum disulfide; Chemical vapor Deposition; Nucleation; Growth mechanism

ACS Paragon Plus Environment

Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 17

1. INTRODUCTION Two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted considerable attention due to their importance for both fundamental science and technology fields in developing nanoscale electronic and optoelectronic devices.1-4 Among these 2D materials, molybdenum disulfide (MoS2) are one of the most studied targets because of its numerous outstanding properties,5-7 including relatively high carrier mobility (~10 cm2V-1s-1),8 moderate band gap in the range of 1.29-1.9 eV,9,10 and strong spin-valley coupling,11 which makes it great potential for building new kinds of atomically thin devices, such as field-effect transistors (FETs),12-14 photodetectors,15 light emitting diodes (LEDs),16 sensors,17 photovoltaic and spin-valleytronic devices18,19. Motivated by these potential applications and the great demand for high-quality 2D MoS2, a variety of synthetic methods have been developed in the past few years. The most widely used methods include (i) Scotch Tape mechanical exfoliation,20 (ii) hydrothermal method,21,22 (iii) solution sonication,23 (iv) electrochemical exfoliation,24 (v) molecular beam epitaxy, and (vi) chemical vapor deposition (CVD)25-27. Among these methods, the CVD growth technique exhibits numerous advantages to produce MoS2 and other TMD materials, including of large area, high crystal quality, and monolayer and few-layer 2D single crystals.28 Hence, the CVD growth method is regarded as the most promising approach for fabricating large-scale and high-quality TMD materials.29,30 The CVD growth of MoS2 is generally based on the reaction of transition-metal oxide powders (i.e., MoO3) and sulfide powder in the vapor phase. The growth behaviors of 2D MoS2 have been revealed to be correlated with several key growth parameters, including carrier gas, growth temperature, precursors, substrates, and promoters,31-33 arising from that the growth parameters can affect the nucleation and growth modes of MoS2. To improve the crystal quality and scale of MoS2, the understanding of growth mechanisms in the CVD process is an important precondition. Through extensive efforts, two popular growth mechanisms have been proposed: (1) the initial reaction between MoO3 and S produces the intermediate volatile MoO3-xSy, then the further sulfuration and reaction of MoO3-xSy on the desired substrates lead to a complete conversion into 2D MoS2 grains;33,34 (2) MoO3 and S react only in the vapor phase, and MoS2 species deposit directly onto the substrate to form the MoS2 nuclei.35 The two mechanisms have been also widely used to understand the CVD growth of other TMD materials, such as


Page 3 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


WS2, MoSe2, and WSe2. However, no matter which growth mechanism has not been directly probed and confirmed due to the very short lifetime of reaction intermediates in the growth process. Recently, a self-seeding nucleation mechanism was proposed in the CVD growth of MoS2, WS2, MoSe2 and their heterostructures.36-38 These mechanisms provide a basic understanding for the CVD growth of MoS2, but several key issues have not been clarified: (1) whether the mechanisms mentioned above have coexisted in a CVD growth of MoS2? (2) What’s the inherent relationship between the nucleation mechanism and the growth parameters? (3) How to control the CVD synthesis of MoS2? To answer above questions, a detailed study to reveal the microscopic growth mechanism of MoS2 is strongly desired. In this work, we present systematic spectroscopic approaches for the understanding of nucleation and growth mechanisms of CVD-grown MoS2. Two types of typical MoS2 flakes have been identified: one is that the flakes with uniform planar thickness have few layer numbers (e.g., monolayer and bilayer) and the other is that the flakes contain a central nanoparticle with multilayer MoS2 structure. The formation of two types of MoS2 flakes can be controlled by tuning the growth temperature and carrier gas flux, which essentially derives from two different nucleation mechanisms (i.e., 2D planar nucleation and self-seeding nucleation) that depend on the reactant (MoO3-x and S) concentration of substrate surface. Moreover, the 2D planar nucleation mechanism facilitates the growth of monolayer and bilayer MoS2, while the self-seeding nucleation mechanism easily leads to few-layer and multilayer MoS2. Our study provides a clear picture on MoS2 growth and nucleation mechanisms and proposes a scheme for the controllable growth of 2D TMD materials.

EXPERIMENT SECTION

CVD Synthesis of 2D MoS2 Samples. The MoS2 samples were synthesized using the standard chemical vapor deposition (CVD) under atmospheric pressure. Figure 1a shows a schematic diagram of CVD experimental setup, and the growth reaction is performed in a quartz tube (3.0 cm in diameter) inserted into a large quartz tube (8.0 cm in diameter) in a three-zone tube furnace. In a typical growth, the high purity MoO3 powder (18 mg) was loaded onto a quartz boat in the central heating zone, the S powder (120 mg) was placed in the upstream zone, and an ultraclean Si substrate covered by a 300 nm thick SiO2 (i.e., SiO2/Si substrate) with the size of 0.5 cm×0.5 cm was inverted on a square quartz boat in the



Page 4 of 17

downstream zone (see Figure 1a). The temperature for the central heating zone was gradually heated up to a target value (i.e.,700 oC, 725 oC, 750 oC, 775 oC, 800 oC, and 850 oC) in 60 min and the temperature for the upstream S zone was controlled at a maximum temperature of ~180 oC, as shown in Figure 1b. After the central heating zone maintained at the target temperature for 20 min, the system cooled down naturally. During the growth, ultra-high purity Argon was used as the carrier gas and the pressure was maintained at the ambient pressure. For the study of the temperature effect on the CVD growth, the gas flow was fixed at 25 sccm. To investigate the role of gas flux in the growth, the flux of carrier gas was changed from 15 to 50 sccm with a separation of 5 sccm, and the growth temperature of the S zone and the central heating zone was set at 180 oC and 750 oC, respectively. A list of experimental results with different growth temperatures and gas fluxes are listed in S1 of Supporting Information (SI). Characterizations. Optical images were obtained by a NOVEL optical microscopy, and the scanning electron microscope (SEM) images were taken with Hitachi SU-8010. The Raman spectra and mapping images were recorded using HORIBA Raman microscope with 532 nm laser source, and the laser spot was ~ 1 µm. Atomic force microscope (AFM) images were performed using a Bruker Dimensional ICON system in tapping mode with the scan rate of 0.8 Hz.

Figure 1. (a) CVD experimental setup and the temperament curve of the reaction furnace. (a) Schematic diagram for the CVD setup for the fabrication of MoS2. The inset is a schematic view of SiO2/Si substrate illustrating the growth of MoS2 under different temperatures and gas fluxes. The distance between S and MoO3 is 23 cm and the separation between MoO3 and the substrate is 1 cm. (b) Temperature evolution of the reaction furnace. The black and red curves indicate the temperature evolution of central heating zone and S zone, respectively.


Page 5 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. Optical microscope (OM) and scanning electron microscope (SEM) images for the CVD-grown MoS2 flakes with varying growth temperature. The OM image of MoS2 with the growth temperature at (a) 725 oC, (b) 750 oC, (c) 775 oC, and (d) 800 oC. The SEM image of MoS2 with the temperature at (a) 750 oC and (f) 800 oC. The blue arrows point to the central nanoparticles.

RESULTS AND DISCUSSION

The structure and morphology of CVD-grown MoS2 have been firstly investigated over the temperature range of 700-850 oC (see Figure 2 and Table S1). Figure 2a-d show optical microscope (OM) images of the MoS2 samples with the temperature change from 725 oC to 800 oC. Most of MoS2 flakes maintain a triangular shape under different growth temperatures, which originates from that the flake shape depends on the growth rate of MoS2 edges. In the CVD growth process, the attachment of reactant atoms (Mo and S) at different MoS2 edges (i.e., the edge growth rate) is different. The edges with fast growth rate disappear and the shape of growing MoS2 flakes is thus determined by the slow-growing edges (see Figure S1), which is a simple consequence of kinetic Wulff construction (KWC) theory.39 From previous studies, the S-rich condition facilitates thermodynamically stable zigzag edges that have a slower growth rate,40-42 which contributes to the formation of triangular MoS2 flakes with the zigzag edges. As shown in Figure 2, two types of MoS2 flakes can be identified with different growth temperatures. The higher



Page 6 of 17

temperature (775-800 oC) facilitates the formation of thicker MoS2 flakes with island-like nanoparticles located at the center of flakes (see Figures 2c,d), while the lower temperature (725-750 oC) leads to thinner triangular MoS2 flakes without the central nanoparticles (see Figures 2a,b). This phenomenon can be further confirmed by the SEM images (see Figures 2e, f). Moreover, the structural characteristic of MoS2 flakes is not sensitive to the flake size. The white spots can be also identified at the center of arbitrary-sized flakes under a higher temperature (i.e., 800 oC, Figure 2f), even for the ultra-small flakes.

Figure 3. Atomic force microscope (AFM) images and the height profile of a MoS2 domain grown at (a) 725 oC and (b) 775 oC. The height profile is plotted along the dash line of the AFM image.

The layer thickness of these MoS2 flakes grown at different temperatures was explored by the AFM topographic images. Figure 3 shows the AFM image and height profile of a triangular MoS2 flake grown with the temperature of 725 oC and 775 oC, respectively. At 725 oC, the homogeneous color contrast in the AFM image (see Figure 3a) indicates that the basal plane of the MoS2 flake is flat and uniform, and its thickness (~7.75 Å) confirms that the flake is a monolayer. Meanwhile, a few bilayer MoS2 flakes have been identified under the growth condition (725 oC). Similar phenomenon has also appeared in the case of the growth temperature of 750 oC. In contrast, the AFM image of a MoS2 flake with the growth temperature of 775 oC includes a highly bright spot (i.e., a nanoparticle mentioned above) at the center of a triangular flake, and the height profile indicates that the central bright spot is as high as 12.29 nm


Page 7 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(see Figure 3b), corresponding to 15-16 layers of MoS2. We have also measured the height of bright spot in other several MoS2 flakes, and it is found that the height of bright spots is generally beyond 5.0 nm. On the other hand, we find that most of MoS2 flakes grown at higher temperature (775-800 oC) are not a monolayer. The basal-plane thickness of the flake shown in Figure 3b is 2.37 nm, indicating that it is a third-layer structure. In addition, some white spots can be identified in the AFM image of 775 oC sample (Figure 3b). The formation of white spots originates from the incomplete sulfurization of larger MoO3 clusters (i.e., MoO3-xSy clusters), and similar phenomenon has been also found in previous studies34.

Figure 4. Raman spectra of MoS2 flakes with four different temperatures. (a) OM images of triangle MoS2 flakes with four different temperatures (725 oC, 750 oC, 775 oC, and 800 oC). (b) Raman spectra taken from the circle regions of triangular MoS2 flakes shown in (a). (c) Frequencies of E12g (red) and A1g (blue) modes as a function of temperature.

In order to further understand the temperature effect on the structure and layer number of MoS2 flakes. Raman spectra of MoS2 samples with four different temperatures (725 oC, 750 oC, 775 oC, and 800 oC) were collected for comparisons. Raman spectra of these MoS2 samples are taken from the central region of triangular flakes, and the central region of high-temperature (775 oC and 800 oC) samples corresponds an island-like particle (see Figure 4a). As shown in Figure 4b, two typical Raman vibration modes E12g (i.e., in-plane vibration of Mo and S atoms) and A1g (i.e., out-of-plane vibration of S atoms) can be identified in all MoS2 samples, suggesting that these samples with different growth



Page 8 of 17

temperatures exhibit a good crystalline quality, including of the planar and particle region of triangular flakes. With the rising temperature from 725 oC to 800 oC, the frequency of E12g peak decreases (red shift) and that of A1g peak increases (blue shift), resulting in an increasing frequency difference between the two vibration modes (∆ω) from 19.9 cm-1 to 23.5 cm-1. The result means that the increase of growth temperature leads to an increase in the layer number of MoS2 flakes.43 Especially for the particle region of high-temperature samples (e.g., 800 oC), its ∆ω value (23.5 cm-1) are obviously larger than that of the basal plane (21.27 cm-1). Thus, the particle region of the MoS2 flakes maintains the MoS2 crystal phase but has a multilayer structure.

Figure 5. Raman intensity mapping of MoS2 flakes with two different growth temperatures. Raman mapping of a MoS2 flake grown at 725 oC for (a) its E12g mode at 382 cm-1 and (b) its A1g mode at 402 cm-1, and that of a MoS2 flake grown at 775 oC for its E12g mode at (c) 382 cm-1 and (d) 380 cm-1 and its A1g mode at (e) 402 cm-1 and (f) 405 cm-1, respectively.

We have also made a comparison between the Raman intensity mapping of the samples with the growth temperature of 725 oC and 775 oC in Figure 5. For a MoS2 flake with the growth temperature of 725 oC, there is uniform Raman intensity distribution for both the E12g mode at 382 cm-1 (Figure 5a) and the A1g mode at 402 cm-1 (Figure 5b). This suggests that the flake is uniform in thickness. For a MoS2 flake grown at 775 oC, Raman intensity mapping signals shows an obvious difference in both E12g and A1g modes (see Figures 5c-f). Raman intensity signal in the planar region becomes dark as compared to


Page 9 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


that of the particle region when the E12g frequency moves from 382 cm-1 to 380 cm-1 (see Figures 5c, e) or the A1g frequency moves from 403 cm-1 to 405 cm-1 (see Figures 5d, f). Such a result confirms that the thickness of MoS2 flakes in the particle region is far larger than that of the planar region.

Figure 6. Schematic nucleation and growth kinetics of MoS2 flakes with (a) a lower concentration of precursors and (b) a higher concentration of precursors. Here the concentration of MoO3 and S precursors on SiO2/Si substrate is sensitive to the temperature and gas flux.

The above results indicate two types of MoS2 flakes grown at different temperatures, implying that there are two potential nucleation mechanisms. The evaporation, transport and diffusion rates of MoO3 and S precursors strongly depend on the temperature, resulting in the difference of concentration for the precursors on substrate surface with different growth temperatures. Under the lower temperature (700 oC < T ≤ 750 oC), the deposited MoO3-x molecular clusters are relatively small and limited. Therefore, the MoO3-x clusters are easily reduced into MoS2-x clusters in the initial growth stage, consequently leading to 2D nucleus on SiO2/Si substrate (see Figure 6a). Then, the continuous edge growth leads to the formation of monolayer/bilayer MoS2 flakes, which is supported by the result of AFM characterization where different sized MoS2 flakes remain a monolayer structure (see Figure S2a,b). Under the condition of higher growth temperature (775 oC ≤ T ≤ 850 oC), a large number of MoO3 molecular clusters can be transported to the SiO2/Si substrate surface, then the sulfurized MoO3 clusters with a high concentration rapidly merge into island-like (or cap-like) MoO3-xSy nanoparticles. Similar finding has been also reported in recent experiments on the growth of WS2 and MoSe2.37,38 The MoO3-xSy nanoparticles sever



Page 10 of 17

as the nucleation sites (or seeding) for growing few-layer MoS2, rather than the monolayer form. As the growth proceeds, the further sulfurization of MoO3-xSy nanoparticles leads to the formation of cap-like multilayer MoS2 nuclei (see Figure 6b). Then, the edge growth of a nucleus induces few-layer MoS2. The result can be also confirmed by the AFM images (see Figure S2c,d). The cap-like nucleus located at the center of MoS2 flakes exhibits multilayer structure regardless of flake size, which is responsible for growing few-layer MoS2. If the nucleation and growth mechanisms of MoS2 depend on the concentration of MoO3 and S precursors on the substrate surface, we should also observe two types of MoS2 flakes by tuning other growth parameters to change the reactant concentration. The flux of carrier gas (Ar) is an important parameter to tune the concentration of MoO3-x and S vapor precursors. Different from the temperature, the carrier-gas flux is too large or too small, both resulting in a low concentration of precursors. This is because a small gas flux is difficult to transport gaseous MoO3-x and S molecules onto the substrate and the large gas flux can blow off MoO3-x and S molecules from the substrate. Hence, a medium gas flux will induce the highest reactant concentration on the substrate surface. In order to verify the correlation between the nucleation mechanism and reactant concentration, the CVD-grown MoS2 with different gas fluxes is explored. Figure 7 shows OM image of MoS2 samples with varying gas flux from 15 to 50 sccm. At the gas flux (qv) ≤ 15 sccm, we cannot find MoS2 flakes formed on the substrate due to the ultralow concentration of S and MoO3-x vapor precursors (Figure 7a). When qv is beyond 25 sccm, a large number of triangular MoS2 flakes are found on the substrate (see Figures 7b-f). Two types of flakes mentioned above can be identified by varying gas fluxes. MoS2 flakes with a central nanoparticle widely appear at 35 sccm ≤ qv ≤ 40 sccm, but their number will decrease with the further increase of gas flux qv from 35 to 50 sccm due to the gas-flux dependence of the reactant concentration. This phenomenon has been also confirmed by the SEM images (see Figure S3). It needs to be noted that the effect of gas flux is different from the temperature effect on the concentration of precursors, thus they show different roles in the nucleation and growth of MoS2.44,45 Here the temperature effect mainly causes the change of MoO3-x concentration and diffusion ability of reactant atoms/molecules on substrate surface. With the rise of temperature, increasing MoO3-x concentration contributes to the change of nucleation


Page 11 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


mechanisms from the 2D planar nucleation to the self-seeding nucleation and the enhanced diffusion of reactant atoms is responsible for the formation of few-layer MoS2 with the larger size (see Figure 2). In contrast, the increase of carrier-gas flux leads to the change in the concentration of both MoO3-x and S vapor sources, which is responsible for the nucleation of MoS2. But, the gas flux has the relatively small influence on the diffusion of reactant atoms. Thus, we can observe the change of nucleation mechanisms but there is no obvious change for the size of MoS2 flakes with increasing carrier-gas flux (see Figure 7).

Figure 7. Optical microscope (OM) images for CVD-grown MoS2 grains with varying gas flux. The OM image of MoS2 with the gas flux at (a) 15 sccm, (b) 25 sccm, (c) 30 sccm, (d) 35 sccm, (e) 40 sccm, and (f) 50 sccm. The red arrows point to the central nanoparticles.

The above results suggest that the nucleation mechanism of MoS2 is strongly related to the reactant concentration: a relatively lower reactant concentration facilitates the 2D planar nucleation, which is responsible for the formation of monolayer MoS2. In contrast, a higher reactant concentration induces a self-seeding nucleation mechanism that easily produces few-layer MoS2. It is a well-known fact that electrical and optical properties of MoS2 are sensitive to the layer thickness, which arises from the layer dependence of electronic structure. With the reduction of layer number from the multilayer to monolayer, there is an indirect-to-direct band-gap transition in MoS2.46 Moreover, the band-gap value of MoS2 correspondingly increases with the reduction of layer number. Therefore, the control of the concentration



Page 12 of 17

of S and MoO3-x vapor precursors on the substrate surface is of particular importance for fabricating the high-quality 2D structure and tuning electronic and optical properties of MoS2.

CONCLUSIONS

In summary, we have identified two types of CVD-grown MoS2 flakes obtained by tuning the growth temperature and flux of carrier gas, and the structure, layer number, and morphology of the MoS2 flakes were investigated by diverse analytical techniques. One type of MoS2 flakes with the uniform planar thickness exhibit a lower layer number (i.e., monolayer or bilayer) and the other type of MoS2 flakes include a central nanoparticle with a multilayer MoS2 structure. The formation of the two types of MoS2 flakes are ascribed to two different nucleation mechanisms, which can be controlled by tuning the concentration of MoO3-x and S vapor precursors. Our results indicate that a lower reactant concentration on the substrate surface facilitates the 2D planar nucleation mechanism that is responsible for monolayer or bilayer MoS2 and a higher reactant concentration induces the self-seeding nucleation mechanism that easily produces few-layer and multilayer MoS2. The mechanism can be applied to guide the precise control of the growth of MoS2 and understand the growth mechanism of other TMD materials, such as MoSe2, WS2, and WSe2. ASSOCIATED CONTENT Supporting Information The details on CVD-grown MoS2 samples, shape evolution of MoS2 flake based on KWC theory, AFM image and height profile of MoS2 domains grown at 750 oC and 775oC, and the SEM images of MoS2 with different gas fluxes. This material is available free of charge via the internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] Notes


Page 13 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grant no. 61775201, 11404309, and 51402275) and the Fund of Shanghai Science and Technology Foundation (Grant no. 13JC1408800). REFERENCES AND NOTES (1) Jariwala, D.; Sanwan, 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. (2) 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. (3) Wang, Q. H.; Zadeh, K. K.; Kis, A.; Coleman, J. N.; Strano, M. S. Electronics and Optoelectronics of Two-Dimensional Transition Metal Dichalcogenides. Nat. Nanotechnol. 2012, 7, 699-712. (4) Shu, H.; Luo, P.; Liang, P.; Cao, D.; Chen, X. Layer-Dependent Dopant Stability and Magnetic Exchange Coupling of Iron-Doped MoS2 Nanosheets. ACS Appl. Mater. Interfaces 2015, 7, 7534-7541. (5) Shu, H.; Li, F.; Hu, C.; Liang, P.; Cao, D.; Chen, X. Capacity Fading Mechanism and Improvement of Cycling Stability in MoS2-Based Anode Materials for Lithium-Ion Batteries. Nanoscale 2016, 8, 2918-2926. (6) Mcdonnell, S.; Addou, R.; Buie, C.; Wallace, R. M.; Hinkle, C. L. Defect-Dominated Doping and Contact Resistance in MoS2. ACS Nano 2014, 8, 2880-2888. (7) Xie, S.; Xu, M.; Liang, T.; Huang, G.; Wang, S.; Xue, G.; Meng, N.; Xu, Y.; Chen, H.; Ma, X.; Yang, D. High-Quality Round-Shaped Monolayer MoS2 Domain and its Transformation. Nanoscale 2016, 8, 219-225. (8) Schmidt, H.; Wang, S.; Chu, L.; Toh, M.; Kumar, R.; Zhao, W.; Neto, A. H. C.; Martin, J.; Adam, S.; Ozyilmaz, B.; Eda, G. Transport Properties of Monolayer MoS2 Grown by Chemical Vapor Deposition. Nano Lett. 2014, 14, 1909-1913. (9) Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F. Atomically Thin MoS2: A New Direct-Gap Semiconductor. Phys. Rev. Lett. 2010, 105, 136805. (10) Splendiani, A.; Sun, L.; Zhang, Y.; Li, T.; Kim, J.; Chim, C. Y.; Li, G. G.; Wang, F. Emerging Photoluminescence in Monolayer MoS2. Nano Lett. 2010, 10, 1271-1275.



Page 14 of 17

(11) Xiao, D.; Liu, G.-B.; Feng, W.; Xu, X.; Yao, W. Coupled Spin and Valley Physics in Monolayers of MoS2 and Other Group-VI Dichalcogenides. Phys. Rev. Lett. 2012, 108, 196802. (12) Amani, M.; Chin, M. L.; Birdwell, A. G.; O’Regan, T. P.; Najmaei, S.; Liu, Z.; Ajayan, P. M.; Lou, J.; Dubey, M. Electrical Performance of Monolayer MoS2 Field-Effect Transistors Prepared by Chemical Vapor Deposition. Appl. Phys. Lett. 2013, 102, 193107. (13) Kang, J.; Liu, W.; Banerjee, K. High-Performance MoS2 Transistors with Low-Resistance Molybdenum Contacts. Appl. Phys. Lett. 2014, 104, 093106. (14) Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A. Single-Layer MoS2 Transistors. Nat. Nanotechnol. 2011, 6, 147-150. (15) Kufer, D.; Konstantatos, G. Highly Sensitive, Encapsulated MoS2 Photodetector with Gate Controllable Gain and Speed. Nano Lett. 2015, 15, 7307-7313. (16) Cheng, R.; Li, D.; Zhou, H.; Wang, C.; Yin, A.; Jiang, S.; Liu, Y.; Chen, Y.; Huang, Y.; Duan, X. Electroluminescence and Photocurrent Generation from Atomically Sharp WSe2/MoS2 Heterojunction p-n Diodes. Nano Lett. 2014, 14, 5590-5597. (17) Wang, T.; Zhu, R.; Zhuo, J.; Zhu, Z.; Shao, Y.; Li, M. Direct Dectection of DNA below ppb Level Based on Thionin-Functionalized Layered MoS2 Electrochemical Sensors. Anal. Chem. 2014, 86, 12064-12069. (18) Mak, K. F.; McGill, K. L.; Park, J.; McEuen, P. L. The Valley Hall Effect in MoS2 Transistors. Science 2014, 344, 1489-1492. (19) Sallen, G.; Bouet, L.; Marie, X.; Wang, G.; Zhu, C. R.; Han, W. P.; Lu, Y.; Tan, P. H.; Amand, T.; Liu, B. L.; Urbaszek, B. Robust Optical Emission Polarization in MoS2 Monolayer through Selective Valley Excitation. Phys. Rev. B 2012, 86, 081301(R). (20) Li, H.; Wu, J.; Yin, Z.; Zhang, H. Preparation and Applications of Mechanically Exfoliated Single-Layer and Multilayer MoS2 and WSe2 Nanosheets. Acc. Chem. Res. 2014, 47, 1067-1075. (21) Xie, J.; Zhang, J.; Li, S.; Grote, F.; Zhang, X.; Zhang, H.; Wang, R.; Lei, Y.; Pan, B.; Xie, Y. Controllable Disorder Engineering in Oxygen-Incorporated MoS2 Ultrathin Nanosheets for Efficient Hydrogen Evolution. J. Am. Chem. Soc. 2013, 135, 17881-17888. (22) Liu, Q.; Liu, Q.; Kong, X. Anion Engineering on Free-Standing Two-Dimensional MoS2 Nanosheets toward


Page 15 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Hydrogen Evolution. Inorg. Chem. 2017, 56, 11462-11465. (23) Nguyen, E. P.; Carey, B. J.; Daeneke, T.; Ou, J. Z.; Latham, K.; Zhuiykov, S.; Kalantar-zadeh, K. Investigation of Two-Solvent Grinding-Assisted Liquid Phase Exfoliation of Layered MoS2. Chem. Mater. 2015, 27, 53-59. (24) Su, C. Y.; Lu, A. Y.; Xu, Y.; Chen, F. R.; Khlobystov, A. N.; Li, L. J. High-Quality Thin Graphene Films from Fast Electrochemical Exfoliation. ACS Nano 2011, 5, 2332-2339. (25) Lunceford, C.; Borcean, E.; Drucker, J. Uniform and Repeatable Cold-Wall CVD Synthesis of Single-Layer MoS2. Cryst. Growth Des. 2016, 16, 988-995. (26) Wang, S.; Pacios, M.; Bhaskaran, H.; Warner, J. H. Substrate Control for Large Area Continuous Films of Monolayer MoS2 by Atmospheric Pressure Chemical Vapor Deposition. Nanotechnology 2016, 27, 085604. (27) Yang, S. Y.; Shim, G. W.; Seo, S. B.; Choi, S. Y. Effective Shape-Controlled Growth of Monolayer MoS2 Flakes by Powder-Based Chemical Vapor Deposition. Nano Res. 2017, 10, 255-262. (28) 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. (29) Xie, L. M. Two-Dimensional Transition Metal Dichalcogenide Alloys: Preparation, Characterization and Applications. Nanoscale 2015, 7, 18392-18401. (30) Chen, J.; Zhao, X.; Tan, S. J. R.; Xu, H.; Wu, B.; Liu, B.; Fu, D.; Fu, W.; Geng, D.; Liu, Y.; Liu, W.; Tang, W.; Li, L.; Zhou, W.; Sum, T. C.; Loh. K. P. Chemical Vapor Deposition of Large-Size Monolayer MoSe2 Crystals on Molten Glass. J. Am. Chem. Soc. 2017, 139, 1073-1076. (31) Kumar, P.; Viswanath, B. Effect of Sulfur Evaporation Rate on Screw Dislocation Driven Growth of MoS2 with High Atomic Step Density. Cryst. Growth Des. 2016, 16, 7145-7154. (32) Ubaldini, A.; Jacimovic, J.; Ubrig, N.; Giannini, E. Chloride-Driven Chemical Vapor Transport Method for Crystal Growth of Transition Metal Dichalcogenides. Cryst. Growth Des. 2013, 13, 4453-4459. (33) Han, G. H.; Kybert, N. J.; Naylor, C. H.; Lee, B. S.; Ping, J.; Park, J. H.; Kang, J.; Lee, S. Y.; Lee, Y. H.; Agarwal, R.; Johnson, A. T. C. Seeded Growth of Highly Crystalline Molybdenum Disulphide Monolayers at Controlled Locations. Nat. Commun. 2015, 6, 6128. (34) Zhu, D.; Shu, H.; Jiang, F.; Lv, D.; Asokan, V.; Omar, O.; Yuan, J.; Zhang, Z.; Jin, C. Capture the Growth Kinetics of CVD Growth of Two-Dimensional MoS2. npj 2D Mater. Appl. 2017, 1, 8-15.



Page 16 of 17

(35) Ji, Q.; Zhang, Y.; Zhang, Y.; Liu, Z. Chemical Vapor Deposition of Group-VIB Metal Dichalcogenide Monolayers: Engineered Substrates from Amorphous to Single Crystalline. Chem. Soc. Rev. 2015, 44, 2587-2602. (36) Liang, T.; Xie, S.; Huang, Z.; Fu, W.; Cai, Y.; Yang, X.; Chen, H.; Ma, X.; lwai, H.; Fujita, D.; Hanagata, N.; Xu, M. Elucidation of Zero-Dimensional to Two-Dimensional Growth Transition in MoS2 chemical Vapor Deposition Synthesis. Adv. Mater. Interfaces 2016, 4, 1600687. (37) Wiesel, I.; Arbel, H.; Yaron, A. A.; Biro, R. P.; Gordon, J. M.; Feuermanm, D.; Tenne, R. Synthesis of WS2 and MoS2 Fullerene-Like Nanoparticles from Solid Precursors. Nano Res. 2009, 2, 416-424. (38) Cain, J. D.; Shi, F.; Wu, J.; Dravid, V. P. Growth Mechanism of Transition Metal Dichalcogenide Monolayers: The Role of Self-Seeding Fullerene Nuclei. ACS Nano 2016, 10, 5440-5445. (39) Shu, H.; Chen, X.; Ding, F. The Edge Termination Controlled Kinetics in Graphene Chemical Vapor Deposition Growth. Chem. Sci. 2014, 5, 4639-4645. (40) Wang, S.; Rong, Y.; Fan, Y.; Pacios, M.; Bhaskaran, H.; He, K.; Warner, J. H. Shape Evolution of Monolayer MoS2 Crystals Grown by Chemical Vapor Deposition. Chem. Mater. 2014, 26, 6371-6379. (41) Cao, D.; Shen, T.; Liang, P.; Chen, X.; Shu, H. Role of Chemical Potential in Flake Shape and Edge Properties of Monolayer MoS2. J. Phys. Chem. C 2015, 119, 4294-4301. (42) Zhang, P.; Kim, Y.-H. Understanding Size-Dependence Morphology Transition of Triangular MoS2 Nanoclusters: The Role of Metal Substrate and Sulfur Chemical Potential. J. Phys. Chem. C 2017, 121, 1809-1816. (43) Lee, C. G.; Yan, H. G.; Brus, L. E.; Heinz, T. F.; Hone, J.; Ryu, S. M. Anomalous Lattice Vibrations of Single- and Few-Layer MoS2. ACS Nano, 2010, 4, 2695-2700. (44) Karthika, S.; Radhakrishnan, T. K.; Kalaichelvi, P. A Review of Classical and Nonclassical Nucleation Theories. Cryst. Growth Des. 2016, 16, 6663-6681. (45) Nie, Y.; Liang, C.; Zhang, K.; Zhao, R.; Eichfeld, S. M.; Cha, P.-R.; Colombo, L.; Robinson, J. A.; Wallace, R. M.; Cho, K. First Principles Kinetics Monte Carlo Study on the Growth Patterns of WSe2 Monolayer. 2D Mater. 2016, 3, 025029. (46) Yun, W. S.; Han, S. W.; Hong, S. C.; Kim, I. G.; Lee, J. D. Thickness and Strain Effects on Electronic Structures of Transition Metal Dichalcogenides: 2H-MX2 Semiconductors (M = Mo, W; X = S, Se, Te). Phys. Rev. B 20162, 85, 033305.


Page 17 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


For Table of Content Use Only

Unveiling the Growth Mechanism of MoS2 with Chemical Vapor Deposition: From 2D Planar Nucleation to Self-Seeding Nucleation Dong Zhou,† Haibo Shu,†,* Chenli Hu,† Li Jiang,† Pei Liang,† and Xiaoshuang Chen ‡

Two types of CVD-grown MoS2 flakes were obtained by tuning the growth temperature and carrier-gas flux, which originates from the precursors-concentration dependence of nucleation mechanisms during the growth of MoS2.


Unveiling the Growth Mechanism of MoS2 with Chemical Vapor

Recommend Documents