Two-dimensional (2D) monolayer molybdenum disulfide (MoS 2 ) semiconductors are an emerging material with interesting device applications. MoS 2 crystals grown on a substrate often have random orientations due to weak van der Waals (vdW) interaction with the substrate. This leads to multiple grain boundaries when random orientated crystals coalesce. Understanding the conditions and mechanism to grow 2D crystals with an aligned orientation is crucial for high-quality single-crystal growth. Here, we study the introduction of oxidation etching in chemical vapor deposition to grow aligned MoS 2 crystals and elucidate the mechanism of the guided growth by a sapphire lattice through vdW interaction. Under proper oxygen flow conditions, single crystals are found to grow in two preferential orientations with triangle crystal edges aligned to the [112̅ 0] or [11̅ 00] direction of the sapphire substrate. These two orientations correspond to a superlattice of (3×3) MoS 2 on (2×2) sapphire and (5×5) MoS 2 on (3×3) sapphire and occur in Mo oxide-and sulfur-rich growth environments, respectively. This aligned orientation growth is realized by a carefully balanced etching and growth competition, which acts as a selection mechanism to grow energetically stable structures while etching less stable structures away. The commeasure of MoS 2 crystals with the sapphire lattice in the superlattice increases the bonding of MoS 2 to the sapphire lattice, thereby becoming the preferred stable structure for nucleation orientations. This study demonstrates the important role of etching−growth competition in the substrate lattice-guided 2D material growth and paves the way for the future development of vdW single-crystal epitaxy.

Non-radiative energy transfer (NRET) from quantum dots (QDs) to monolayer MoS2 has been shown to greatly enhance the photoresponsivity of the MoS2 photodetector, lifting the limitations imposed by monolayer absorption thickness. Studies were often performed on a photodetector with a channel length of only a few μm and an active area of a few μm2. Here, we demonstrate a QD sensitized monolayer MoS2 photodetector with a large channel length of 40 μm and an active area of 0.13 mm2. The QD sensitizing coating greatly enhances photoresponsivity by 14-fold at 1.3 μW illumination power, as compared with a plain monolayer MoS2 photodetector without QD coating. The photoresponsivity enhancement increases as QD coating density increases. However, QD coating also causes dark current to increase due to charge doping from QD on MoS2. At low QD density, the increase of photocurrent is much larger than the increase of dark current, resulting in a significant enhancement of the signal on/off ratio. As QD density increases, the increase of photocurrent becomes slower than the increase of dark current. As a result, photoresponsivity increases, but the on/off ratio decreases. This inverse dependence on QD density is an important factor to consider in the QD sensitized photodetector design.

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