The synthesis of atomically thin transition-metal disulfides (MS2) with layer controllability and large-area uniformity is an essential requirement for their application in electronic and optical devices. In this work, we describe a process for the synthesis of WS2 nanosheets through the sulfurization of an atomic layer deposition (ALD) WO3 film with systematic layer controllability and wafer-level uniformity. The X-ray photoemission spectroscopy, Raman, and photoluminescence measurements exhibit that the ALD-based WS2 nanosheets have good stoichiometry, clear Raman shift, and bandgap dependence as a function of the number of layers. The electron mobility of the monolayer WS2 measured using a field-effect transistor (FET) with a high-k dielectric gate insulator is shown to be better than that of CVD-grown WS2, and the subthreshold swing is comparable to that of an exfoliated MoS2 FET device. Moreover, by utilizing the high conformality of the ALD process, we have developed a process for the fabrication of WS2 nanotubes.

The effective synthesis of two-dimensional transition metal dichalcogenides alloy is essential for successful application in electronic and optical devices based on a tunable band gap. Here we show a synthesis process for Mo1−xWxS2 alloy using sulfurization of super-cycle atomic layer deposition Mo1−xWxOy. Various spectroscopic and microscopic results indicate that the synthesized Mo1−xWxS2 alloys have complete mixing of Mo and W atoms and tunable band gap by systematically controlled composition and layer number. Based on this, we synthesize a vertically composition-controlled (VCC) Mo1−xWxS2 multilayer using five continuous super-cycles with different cycle ratios for each super-cycle. Angle-resolved X-ray photoemission spectroscopy, Raman and ultraviolet–visible spectrophotometer results reveal that a VCC Mo1−xWxS2 multilayer has different vertical composition and broadband light absorption with strong interlayer coupling within a VCC Mo1−xWxS2 multilayer. Further, we demonstrate that a VCC Mo1−xWxS2 multilayer photodetector generates three to four times greater photocurrent than MoS2- and WS2-based devices, owing to the broadband light absorption.

This work reports the self-limiting synthesis of an atomically thin, two dimensional transition metal dichalcogenides (2D TMDCs) in the form of MoS2. The layer controllability and large area uniformity essential for electronic and optical device applications is achieved through atomic layer deposition in what is named self-limiting layer synthesis (SLS); a process in which the number of layers is determined by temperature rather than process cycles due to the chemically inactive nature of 2D MoS2. Through spectroscopic and microscopic investigation it is demonstrated that SLS is capable of producing MoS2 with a wafer-scale (~10 cm) layer-number uniformity of more than 90%, which when used as the active layer in a top-gated field-effect transistor, produces an on/off ratio as high as 108. This process is also shown to be applicable to WSe2, with a PN diode fabricated from a MoS2/WSe2 heterostructure exhibiting gate-tunable rectifying characteristics.

We investigated nucleation and growth characteristics of atomic layer deposition (ALD) HfO 2 on exfoliated and chemical vapor deposition (CVD) graphene by using two Hf precursors, tetrakis(dimethylamino)hafnium (TDMAH) and hafnium tetrachloride (HfCl 4 ). Experimental results and theoretical calculations indicate that HfO 2 nucleation is more favorable on CVD graphene than on exfoliated graphene due to the existence of defect sites. Also, the TDMAH precursor showed much more unfavorable nucleation and growth than HfCl 4 due to different initial adsorption mechanisms, affecting lower leakage currents and breakdown electric field. ALD growth characteristics of HfO 2 will be fundamentally and practically significant for realizing the fabrication of graphenebased electronic devices. ■ INTRODUCTIONGraphene has attracted a great deal of attention for potential applications in electronic devices due to its novel electrical properties, such as high electron and hole mobility above 100,000 cm 2 /(V s). 1 In order to realize graphene-based electronic devices, high quality high-k dielectric thin films are required. 2 However, since conventional techniques of thin film deposition employ energetic radicals in the plasma state for sputtering or chemical reactions of precursors for chemical vapor deposition (CVD), the physical properties of graphene which is composed of an ideal single atomic layer are easily affected by deposition environments. 3 Thus, the damage-free deposition method is needed to form high-k dielectrics on graphene. Compared to other deposition techniques, atomic layer deposition (ALD) produces dense and pinhole-free since ALD films are formed through a layer-by-layer growth manner based on the surface self-saturated reaction of precursors. In addition, damages on an original surface in ALD are less significant than those in CVD and physical vapor deposition (PVD). 4 Therefore, ALD has been one of the essential fabrication methods for graphene-based devices and has been widely used.In earlier studies, ALD dielectrics such as Al 2 O 3 and HfO 2 were attempted on graphite surfaces which have chemically identical surface properties to those of graphene. 5−10 Selective growth of ALD dielectrics along the step edge sites of highly ordered pyrolytic graphite (HOPG) was observed since the step edge sites are chemically more reactive than the basal planes. 5−7 In the following studies, dielectric deposition by ALD on graphene prepared from the exfoliation of HOPG shows similar results to previous reports about ALD on HOPG since there was no chemically available adsorption site on graphene surfaces. 8−10 In other research, interestingly, on graphene synthesized by CVD, there was no selectivity in the growth of ALD dielectrics rather island growth over all the surface. 11−13 Different growth behaviors of ALD dielectrics on graphene surfaces are probably affected by nonideal surface properties of graphene originating from different synthesis methods and preparation processes. Although exfoliated graphene from HOPG and ...

We report the fabrication of graphene-encapsulated nanoballs with copper nanoparticle (Cu NP) cores whose size range from 40 nm to 1 μm using a solid carbon source of poly(methyl methacrylate) (PMMA). The Cu NPs were prone to agglomerate during the annealing process at high temperatures of 800 to 900 °C when gas carbon source such as methane was used for the growth of graphene. On the contrary, the morphologies of the Cu NPs were unchanged during the growth of graphene at the same temperature range when PMMA coating was used. The solid source of PMMA was first converted to amorphous carbon layers through a pyrolysis process at the temperature regime of 400 °C, which prevented the Cu NPs from agglomeration, and they were converted to few-layered graphene (FLG) at the elevated temperatures. Raman and transmission electron microscope analyses confirmed the synthesis of FLG with thickness of approximately 3 nm directly on the surface of the Cu NPs. X-ray diffraction and X-ray photoelectron spectroscopy analyses, along with electrical resistance measurement according to temperature changes showed that the FLG-encapsulated Cu NPs were highly resistant to oxidation even after exposure to severe oxidation conditions.