TION www. MaterialsViews. com www. advopticalmat. de perfect triangular shape and sharp edges are effectively formed on the 300 nm SiO 2/Si substrate through the CVD process. It is noted that, though triangular WS 2 monolayers can be readily formed on both bottom pieces which contain WO 3 powders and on the covering pieces which are blank, the WS 2 monolayers grown on the covering substrates are much cleaner and appear as isolated perfect triangles (Figure 1 b, c). This could be due to the existence of WO 3 powders and highly concentrated reaction sources on the bottom substrates (see Supporting Figure S1). Due to the sensitivity to the local environment of the CVD growth, the distribution of dimensions of the triangular crystals is quite wide, from a few to hundreds of micrometers, while the majority of the flakes are more than 100 micrometers large. The wide distribution of sizes is also observed in the previously reported CVD growth of MoS 2 and WS 2.[35, 36, 40] The zoomed-in optical, SEM, and AFM images show the clean surface of the WS 2 flakes, which is very different to the MoS 2 and WS 2 thin layers grown by CVD previously,[36, 40] where impurities or small second or even multiple layers existed on top of the monolayers. The thickness of these triangular crystals is determined by the height profile of AFM and Raman spectroscopy (discussed later). Figure 1 e, f present the transmission electron microscopy (TEM) study of the WS 2 monolayers. As with the optical, SEM, and AFM images, a typical low-magnification TEM image (Figure 1 e) visualizes the perfect triangular shape and sharp edges of the as-grown WS 2 monolayers. The high-resolution TEM (HRTEM) image (Figure 1 f) reveals the hexagonal ring lattice consisting of alternating tungsten atoms (dark dots) and sulfur (gray dots) atoms as schematically illustrated by the blue and yellow spheres. The TEM data also reveal that the sharp edges are at the microscale perpendicular to the [100] crystalline direction. Though the microscale edges are along the zigzag direction, it should be noticed that our HRTEM image near the edge shows that the edge is not atomically sharp, which ends up with the triangle-like features of a few nanometers (see Supporting Figure S2). A detailed study at the atomic level is needed. All data unambiguously show that the as-grown triangular WS 2 flakes are monolayer. The large dimension, perfect triangular shape, and clean surface indicate the WS 2 monolayers formed in this work could be a perfect candidate for studying fundamental knowledge and developing practical applications of 2D semiconductors. Raman spectroscopy has been widely used to study 2D materials, such as the determination of numbers [41–46] and stacking sequence [47–49] of layers, the external field and molecular doping effects,[50, 51] and the internal and external strain.[23, 24, 52–56] Figure 2 a shows Raman spectra of the asgrown WS 2 monolayer over a frequency range of 80–650 cm− 1 at room temperature with laser excitation at 532 nm. Fifteen Raman modes of WS 2 are present, as labeled, including the first order modes of LA (M), LA (K), E1