Article pubs.acs.org/cm
Shape Evolution of Monolayer MoS2 Crystals Grown by Chemical Vapor Deposition Shanshan Wang, Youmin Rong, Ye Fan, Mercè Pacios, Harish Bhaskaran, Kuang He, and Jamie H. Warner* Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, United Kingdom S Supporting Information *
ABSTRACT: Atmospheric-pressure chemical vapor deposition (CVD) is used to grow monolayer MoS2 two-dimensional crystals at elevated temperatures on silicon substrates with a 300 nm oxide layer. Our CVD reaction is hydrogen free, with the sulfur precursor placed in a furnace separate from the MoO3 precursor to individually control their heating profiles and provide greater flexibility in the growth recipe. We intentionally establish a sharp gradient of MoO3 precursor concentration on the growth substrate to explore its sensitivity to the resultant MoS2 domain growth within a relatively uniform temperature range. We find that the shape of MoS2 domains is highly dependent upon the spatial location on the silicon substrate, with variation from triangular to hexagonal geometries. The shape change of domains is attributed to local changes in the Mo:S ratio of precursors (1:>2, 1:2, and 1:1:2, meaning that the S-zz terminations grow faster than the Mo-zz terminations, so that S-zz terminations should be shorter than the Mo-zz one. The greater roughness of shorter sides supports its S-zz edge structure and corresponds to the analysis well. The third reason is that, this regular shape changing phenomenon not only exists along the gas flow direction but also is observed in the direction vertical to the gas flow direction on the substrate (Figure S2 of the Supporting Information), indicating the change in the Mo:S ratio is the most important reason for the shape change. Despite the shape change, the crystal size also experiences a regular change. This may relate to the concentration gradient of the gas phase MoO3 along the gas flow direction, which impacts the average growing rate of the MoS2 crystals. The effect of MoO3 precursor temperature on the MoS2 shape change has also been investigated, and we found that this morphology transformation phenomenon widely exists. However, as the temperature influences the evaporation amount of MoO3, leading to a different MoO3 concentration gradient in 6377
dx.doi.org/10.1021/cm5025662 | Chem. Mater. 2014, 26, 6371−6379
Chemistry of Materials
Article
increase in Ar gas flow rate from 10 to 100 sccm. In general, the loss of stability during crystal growth can be clearly observed, because the smoothness of the domain sides is severely reduced, and the crystal shape becomes dendritic-like in some areas. The high flow rate may promote the mass transfer process, which contributes to the increase in the crystal growth rate. In this case, instability may occur as atoms do not have enough time to move into the right lattice locations, where crystal domains could have the lowest surface free energy, and the probability of defect formation increases. Therefore, under the high-flow rate condition, the MoS2 crystals are more likely to grow under “kinetic” conditions rather than thermodynamic ones, which is typical for colloidal nanocrystals with high precursor feedstock. However, the regular shape domains with smooth sides and sharp edges can still be seen in some locations, as shown in section 3 in Figure 8. This is because the direction from section 1 to 3 is perpendicular to the Ar gas flow direction and there is a decrease in the concentration of MoO3 on the substrate surface, as the distance from the MoO3 powder precursor to each spot increases. The decrease in precursor concentration has an effect of slowing the crystal growth rate, which can balance the positive influence from the high flow rate, thus leading to stable crystal growth under thermodynamic control. Furthermore, in the direction parallel to the gas flow, the MoO3 concentration on the first substrate decreases as the sections of investigation move farther from the MoO3 powder. This is why in section 8 in Figure 8, crystal domains transform back into more regular shapes compared to those before it. However, as the MoO3 concentration in the central area of the substrate is too high, we are not able to achieve isolated monolayer domains there, making the direct comparison of crystal shape along the gas flow direction not feasible. From these studies, we found that the increase in Ar flow rate could turn the control of crystal growth from the thermodynamic to kinetic regime, resulting in the formation of dendritic morphologies, which are not favorable for the production of high-quality 2D crystal. However, with a decrease in the precursor concentration, a new balance can be achieved to stabilize the crystal growth and shift the growing conditions to the thermodynamic control. In conclusion, we observed the regular morphology evolution of CVD-grown MoS2 domains along the gas flow direction on one substrate. The microstructure and properties of the MoS2 crystals were measured by SEM, Raman spectra, AFM, and PL spectra, confirming that the MoS2 film was a uniform, single layer with high crystallinity. A possible explanation for shape evolution was based on the principles of crystal growth. In addition, the effects of MoO3 precursor temperature and Ar gas flow rate on MoS2 crystal shape were also investigated. It is anticipated that these studies will allow the shape-controllable synthesis of MoS2 and improve the ability to discover of more shape-dependent properties.
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oxide (MoO3) powder (≥99.5%, Sigma-Aldrich). The exact location of the MoO3 powder is directly below the tiny gap between the first and second substrate. The ceramic boat covered with four substrates was loaded into a 1 in. quartz tube together with another boat containing 80 mg of sulfur powder (≥99.5%, Sigma-Aldrich). The boat containing S is 18 cm from the boat of MoO3 and was at the upstream of the tube. They were then put into two different furnaces (furnace 1 having S and furnace 2 having MoO3). The CVD growth occurred at atmospheric pressure while ultra-high-purity argon was flowing. The CVD system was first flushed with 500 sccm of Ar gas for 1.5 h when the temperatures of the first and second furnaces were set to 30 and 150 °C, respectively. Then the second furnace was heated at a rate of 15 °C/min to 760 °C under a flow rate of 10 sccm of Ar, held at the setting temperature for 30 min, and then slowly cooled at a cooling rate of −8 °C/min followed by a fast cooling under 500 sccm of Ar. The temperature programming for the first furnace having S was as follows: temperature held at 30 °C until the second furnace was heated for 15 min, reaching a temperature of 375 °C, and then increased with a ramping rate of 3 °C/min to 150 °C and held for 30 min followed by rapid cooling. Characterization. The crystal structures of the resulting products were analyzed with a scanning electron microscope (Hitachi-4300) under an accelerating voltage of 3.0 kV. The thickness and surface topology were measured by an atomic force microscope (Asylum Research MFP-3D). Typical scans were conducted in AC mode with a silicon AC160TS cantilever (Olympus, spring constant of ∼42 N/m and resonant frequency of ∼300 kHz). Raman spectroscopy and photoluminescence were conducted using a JY Horiba LabRAM ARAMIS imaging confocal Raman microscope under an excitation wavelength of 532 nm.
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
S Supporting Information *
Shape evolution along the direction vertical to the Ar gas flow, demonstrating the S concentration is uniform, and PL and Raman spectra of MoS2 in the same plot. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS J.H.W. thanks the Royal Society for support. S.W. acknowledges financial support from the China Scholarship Council and thanks S. Zhou for discussions.
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EXPERIMENTAL METHODS
The results presented in this paper were reproduced more than five times, and the phenomenon of the domain shape change in the same place on the chip along the flow direction always existed. We used CVD to grow MoS2 with two furnaces to control the temperature of MoO3 and S separately, and a ceramic boat to create a wider change in MoO3 concentration on the substrates. Growth substrates were Si with a 285 nm layer of SiO2. Substrates were cleaned in acetone for 30 min and then 2-propanol for 15 min, followed by O2 plasma for 5 min. After being cleaned, four substrates were tightly aligned and placed face down above a ceramic boat containing 15 mg of molybdenum(VI) 6378
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dx.doi.org/10.1021/cm5025662 | Chem. Mater. 2014, 26, 6371−6379
