Metal Induced Growth of Transition Metal Dichalcogenides at Controlled Locations

Metal induced nucleation is adopted to achieve the growth of transition metal dichalcogenides at controlled locations. Ordered arrays of MoS2 and WS2 have successfully been fabricated on SiO2 substrates by using the patterned Pt/Ti dots as the nucleation sites. Uniform MoS2 monolayers with the adjustable size up to 50 μm are grown surrounding these metal patterns and the mobility of such layer is about 0.86 cm2/V·s. The crystalline flakes of WS2 are also fabricated extending from the metal patterns and the electron mobility of these flakes is up to 11.36 cm2/V·s.

Scientific RepoRts | 6:38394 | DOI: 10.1038/srep38394 into the quartz tube and transferred to the substrate by the carrier gas, Ar. The Pt/Ti patterns are expected to act as the nucleation sites for the growth of TMD.
Characterization of MoS 2 . Figure 2a shows a typical SEM image for MoS 2 grown on the substrate by this metal induced nucleation method. It is obvious that the circular films exactly follow the patterned metal dots to form a well ordered array. The inset of Fig. 2a, the magnified image for a single unit, clearly demonstrates that the bright metal dot is in the middle of the dark circular film, indicating that this dark film actually grows from the metal dot. Figure 2c shows a series of Raman spectra taken at various points on a circular MoS 2 film (shown in Fig. 2b). There are two prominent peaks at ~387 cm −1 and 407 cm −1 at all the Raman spectra. It is well known that these two peaks are ascribed to the in-plane mode E 1 2g and out-of-plane mode A 1g of the MoS 2 , respectively. The presence of these two Raman peaks unambiguously gives the evidence that these dark films are indeed MoS 2 grown on the substrate. On the other hand, these two Raman modes of E 1 2g and A 1g , are very sensitive to the layer thickness 28,29 , which provide a convenient and reliable means to determine the thickness of MoS 2 film. The frequency difference value (Δ ) between these two peaks are about 19.5 cm −1 ~20.0 cm −1 at the points of 1, 2, 6 and 7 in Fig. 2b, respectively, confirming that the thickness of the MoS 2 film surrounding the Pt/Ti dots is one monolayer. The vibration modes of MoS 2 are also observed in the spectrum taken at the metal dot (point 4 in Fig. 2b), indicating that the MoS 2 is also grown on the top of the metal. Moreover, relative larger frequency differences (about 22∼ 27 cm −1 ) are obtained on the top and near the Pt/Ti dots, suggesting that the MoS 2 films at these places are multilayers ( Figure S2). The Raman mapping with the peak difference as the indicator (407 cm −1 ) given in Fig. 2d further reveal that the MoS 2 film grown out of the Pt/Ti dot is very uniform monolayer. In addition, the PL spectrum in Figure S3 exhibits the strongest emission at ~1.83 eV for the MoS 2 films grown around the metal dots, which is in agreement with the previous reports on MoS 2 monolayer 30,31 . The thickness of the MoS 2 film derived from AFM measurement in Fig. 2(e) is ~0.85 nm, which is also consistent with the values for MoS 2 monolayer 32 , and the results also revealed that there are some cracks in the MoS 2 units in Fig. 2(e), it is possible that the cracks are ascribe to the domain boundaries of the polycrystalline MoS 2 , where some domain boundaries of the polycrystalline MoS 2 are slightly oxidized in air. It is worth noting that the dimension of MoS 2 monolayer grown by this method can be easily controlled by adjusting the size of the metal dots. Figure 2f-h gives the SEM images of MoS 2 monolayer grown around the metal dots with the various sizes under the same growth condition. A SiO 2 /Si substrate containing an array of the metal dots with various sizes was used to grow the MoS 2 monolayer around various metal dots at the same time, as shown in Fig. 2f-h. It is obvious that the dimension of the MoS 2 monolayer increases from 18.3 μ m to 53.5 μ m as the size of the metal dot varies from 3 μ m to 10 μ m. Careful examinations reveal that the area of the grown MoS 2 film linearly depends on the circumference of the metal dot, indicating that the radial growth rate of MoS 2 around the metal dot might be kept the same during the growth process. (See Figure S4).
Transmission electron microscopy (TEM) was used to characterize the crystal structures of the obtained MoS 2 film. Figure 3c shows the low magnification image of MoS 2 film as well as the metal dot (the center black area) that transferred onto Cu grid. The boundary of the transferred the film was outlined by the red dashed line in Fig. 3a. Figure 3(b) shows a typical high-resolution TEM (HRTEM) image. The periodic atom arrangement is clearly observed and the specific inter-planar distances for the (100) plane is measured to be about 0.285 nm, which gives the direct evidence for the crystalline nature of such MoS 2 monolayer. The selected area electron diffraction (SAED) was taken on different location as marked by the numbers in Fig. 3a. In Figure 3c and d, there is only one set of the hexagon diffraction pattern at location 8 and 12 but with different orientation, indicating that the MoS 2 film in these areas are crystalline monolayer but in different crystalline domains. The cracks on the AFM image in Fig. 2e represent the domain boundaries between different domains, indicating the polycrystalline nature of the MoS 2 film. The presence of two set of the hexagon spots in the SAED patterns for the areas of the point 10 and point11 (Fig. 3(e)) shows that the few-layers MoS 2 is grown on the top and near the Pt/Ti dots. The element distribution in the MoS 2 film is measured by energy dispersive spectroscopy (EDS). Although the elements Pt and Ti can be found in the multilayer area on the top of the metal dot ( Fig. 3(f)), they are not observed in the regions far away from the metal dot ( Fig. 3(g)), and only the S and Mo elements are measured. These results suggested that the metal dot only acts as a nucleation site and the metals, Pt and Ti, do not diffuse into the MoS 2 film during the growth process, at least in our measurement accuracy.
Characterization of WS 2 . WS 2 films are also successfully grown on the substrate by using the same growth protocol. Figure 4(a) shows an optical image of the WS 2 films grown on the prepatterned substrates in which the films appear much brighter than the metal dots. Although the WS 2 films are not uniformly circular shape like the MoS 2 films, these WS 2 films certainly follow the periodicity of the metal pattern, indicating that the metal dots do act as the nucleation sites during the film growth process. The SEM image of the WS 2 unit in Fig. 4(b) clearly shows that the irregular WS 2 flakes extend from the center metal dot, in contrast to a symmetric circular shape of the MoS 2 monolayer in Fig. 2a. The chemical reaction process different to those of MoS 2 may account for the irregular shape of the WS 2 grown around the metal dot. The typical Raman spectrum for the films is shown in Fig. 4(c). Two peaks located at 360.1 cm −1 and 426.1 cm −1 are observed, which are the well known E 1 2g and A 1g modes for WS 2 33 . Figure 4(d) shows the Raman mapping image for the WS 2 unit in Fig. 4(b) by using the peak position (426.1 cm −1 ) as an indicator. The perfect match between the Raman mapping image and the SEM image directly reveals that the WS 2 films are not grown on the bare substrate but around the metal dots, further   Electrical measurements. In order to characterize the electronic properties of these grown TMD films, field effect transistors (FETs) were fabricated by e-beam lithography directly on the growth substrate without any transfer processes. Typical measurement results were shown in Fig. 5, where both the MoS 2 and WS 2 devices show good n-type field effects. The on/off ratio of the MoS 2 FET is about 10 5 , while it is only about 10 4 for the WS 2 FET. The extracted electron mobility are 0.86 cm 2 /V·s for MoS 2 and 11.36 cm 2 /V·s for WS 2 , respectively. The mobility of MoS 2 is relatively lower as compared with the previous work 24 , which is originated from the polycrystallinite nature of the grown MoS 2 monolayer. During the fabrication process of these devices, the FETs were purposively built far from the pre-defined Pt/Ti dots to avoid the influence of the metal dots. As a consequence, the channel of the MoS 2 FET is actually a MoS 2 monolayer and that of the WS 2 FET is a multilayered WS 2 . Therefore, the above measured results are understandable because that the multilayered channel is more difficult to be switched off than a monolayered channel but the higher mobility can be achieved in multilayered channel. The electrical properties of the FETs based on the grown MoS 2 and WS 2 films unambiguously confirm the high quality of the TMD films grown by this metal indunced nucleation method. Such observations also support our argument that large scale devices fabrication can be easily achieved via the defined location growth of TMD.
In summary, we reported a metal induced growth method for the transition metal dichalcogenides grown at controlled locations. Where the high quality monolayer MoS 2 arrays are grown orderly around the core of Pt/ Ti patterned on the substrates, and the WS 2 multilayers are also prepared at a controlled location by a simple CVD technology. The mobility of the MoS 2 films and the crystalline WS 2 flakes are about 0.86 cm 2 /V·s and up to 11.36 cm 2 /V·s, respectively. The results are hopeful for facilitating device fabrication for the integrated devices based on the transition metal dichalcogenides.

Methods
Growth process of the MoS 2 . Pure MoO 3 powder was placed in a quartz boat at the centre of furnace, cleaned substrates with patterned Pt/Ti cores were placed on the downstream, and a separate quartz boat with sulfur powder was placed on the upstream, which was heated up to 190 °C using a separate heating system. The furnace was heated from room temperature to 850 °C at a ramp rate of 15 °C/min under an argon (99.999%) flow of 100 sccm. The temperature was held constant for 30 min during the MoS 2 growth, and the furnace chamber was then rapidly cooled to room temperature by opening the furnace door.
Growth process of the WS 2 . Pure WO 3 powder and NaCl powder with the molar ratio of 1.4:1 were mixed and ball-milled in a grinding container for 2 h, using alcohol as a solvent, then dried at 95 °C 34 . According to the ref. 34, WO 3 is the tungsten precursor but NaCl acts as a growth promoter. Afterwards the ball-milled powders were placed in a quartz boat at the centre of furnace, a separate quartz boat with sulfur powder was placed on the upstream, which was heated up to 190 °C using a separate heating system, and cleaned substrates with patterned Pt/Ti cores were placed on the downstream. The furnace was heated from room temperature to 900 °C at a ramp rate of 15 °C/min. The temperature was held constant for 30 min during the WS 2 growth, and the furnace chamber was then rapidly cooled to room temperature by opening the furnace door.
Device fabrication and testing. The MoS 2 and WS 2 field effect transistors were fabricated by e-beam lithography using Raith 150. After exposure, the source and drain electrodes (Ti/Au film of 10/30 nm thick) was deposited by using e-beam evaporator (K. J. Lesker with base vacuum of 7× 10 −8 torr) followed by lift-off process. The bottom Si was used as gate electrode. The filed effect properties of the fabricated devices were measured by using probe station and Keithley 4200 Semiconductor parameter analyzer at room temperature in air.
Characterizations. The morphologies and microstructures of transition metal dichalcogenides were characterized by optical microscopy, scanning electron microscopy (FEI-Quanta 200 F), atomic force microscope (Veeco Dimension 3100) and transmission electron microscopy (JEOL JEM-2100 at 80 keV). Raman and PL spectra were taken by Horiba Jobin Yvon LabRAM H8000 system with laser excitation wavelength of 488 nm.