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.

Semiconducting two-dimensional (2D) transition metal dichalcogenides (TMDCs) are promising gas-sensing materials due to their large surface-to-volume ratio. However, their poor gas-sensing performance resulting from the low response, incomplete recovery, and insufficient selectivity hinders the realization of high-performance 2D TMDC gas sensors. Here, we demonstrate the improvement of gas-sensing performance of large-area tungsten disulfide (WS) nanosheets through surface functionalization using Ag nanowires (NWs). Large-area WS nanosheets were synthesized through atomic layer deposition of WO followed by sulfurization. The pristine WS gas sensors exhibited a significant response to acetone and NO but an incomplete recovery in the case of NO sensing. After AgNW functionalization, the WS gas sensor showed dramatically improved response (667%) and recovery upon NO exposure. Our results establish that the proposed method is a promising strategy to improve 2D TMDC gas sensors.

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.

Rare earth oxide (REO) atomic layer deposition (ALD) processes are investigated for hydrophobic coatings. Thermal and plasmaenhanced ALD (PE-ALD) Er 2 O 3 and Dy 2 O 3 are developed using the newly synthesized Er and Dy precursors bis-methylcyclopentadienyl-diisopropylacetamidinate-erbium and bis-isopropylcyclopentadienyl-diisopropyl-acetamidinate-dysprosium, with H 2 O and O 2 plasma counter oxidants. The Er and Dy precursors show typical ALD growth characteristics with no nucleation incubation, indicating that they are suitable ALD precursors. The hydrophobicities of ALD-grown Er 2 O 3 and Dy 2 O 3 are investigated, together with those of ALD-grown Y 2 O 3 , La 2 O 3 , and CeO 2 that were previously developed for high-k applications. All the ALD-grown REOs show high hydrophobicity, with water contact angles as high as 90°. After annealing at 500 °C in air for 2 h, hydrophobicity is degraded depending on the kind of material; this degradation is related to the hygroscopy of REOs. In addition, we demonstrate the fabrication of a superhydrophobic surface by depositing highly conformal ALD REO films on 3D Si nanowire nanostructures. The Si NWs are conformally coated with ALD Y 2 O 3 , yielding a surface with a water contact angle of about 158°. The ALD REOs reported herein should find widespread applicability in the fabrication of robust hydrophobic coatings.

A process for the self-limited layer synthesis (SLS) of WSe 2 on SiO 2 substrates has been developed that provides systematic layer number controllability with micrometer-scale (>90%) and wafer-scale (∼8 cm) uniformity suitable electronic and optoelectronic device applications. This was confirmed by the fabrication and testing of a WSe 2 back-gated field effect transistor (FET) using Pd (30 nm) as the contact metal, which exhibited p-type behavior with an on/off ratio of ∼10 6 and a field-effect hole mobility of 2.2 cm 2 V −1 s −1 value, which was higher than has been reported for WSe 2 -based FETs produced by conventional chemical vapor deposition. On the basis of these results, it is proposed that the SLS method is universally applicable to a range of device applications.