Low-temperature controllable synthesis of monolayer-to-multilayer thick MoS2 with tuneable morphology is demonstrated by using plasma enhanced atomic layer deposition (PEALD). The characteristic self-limiting ALD growth with a growth-per-cycle of 0.1 nm per cycle and digital thickness control down to a monolayer are observed with excellent wafer scale uniformity. The as-deposited films are found to be polycrystalline in nature showing the signature Raman and photoluminescence signals for the mono-to-few layered regime. Furthermore, a transformation in film morphology from in-plane to out-of-plane orientation of the 2-dimensional layers as a function of growth temperature is observed. An extensive study based on high-resolution transmission electron microscopy is presented to unravel the nucleation mechanism of MoS2 on SiO2/Si substrates at 450 °C. In addition, a model elucidating the film morphology transformation (at 450 °C) is hypothesized. Finally, the out-of-plane oriented films are demonstrated to outperform the in-plane oriented films in the hydrogen evolution reaction for water splitting applications.

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Edge-enriched transition metal dichalcogenides, such as WS 2 , are promising electrocatalysts for sustainable production of H 2 through the electrochemical hydrogen evolution reaction (HER). The reliable and controlled growth of such edge-enriched electrocatalysts at low temperatures has, however, remained elusive. In this work, we demonstrate how plasma-enhanced atomic layer deposition (PEALD) can be used as a new approach to nanoengineer and enhance the HER performance of WS 2 by maximizing the density of reactive edge sites at a low temperature of 300 °C. By altering the plasma gas composition from H 2 S to H 2 + H 2 S during PEALD, we could precisely control the morphology and composition and, consequently, the edge-site density as well as chemistry in our WS 2 films. The precise control over edge-site density was verified by evaluating the number of exposed edge sites using electrochemical copper underpotential depositions. Subsequently, we demonstrate the HER performance of the edge-enriched WS 2 electrocatalyst, and a clear correlation among plasma conditions, edge-site density, and the HER performance is obtained. Additionally, using density functional theory calculations we provide insights and explain how the addition of H 2 to the H 2 S plasma impacts the PEALD growth behavior and, consequently, the material properties, when compared to only H 2 S plasma.

The surface reactions during atomic layer deposition (ALD) of Al2O3 from Al(CH3)3 and H2O have been studied with broadband sum-frequency generation to reveal what is limiting the growth at low temperatures. The –CH3 surface coverage was measured for temperatures between 100 and 300 °C and the absolute reaction cross sections, describing the reaction kinetics, were determined for both half-cycles. It was found that –CH3 groups persisted on the surface after saturation of the H2O half-cycle. From a direct correlation with the growth per cycle, it was established that the reduced reactivity of H2O towards –CH3 is the dominant factor limiting the ALD process at low temperatures.

We have elucidated the reaction mechanism and the role of the reactive intermediates in the atomic layer deposition (ALD) of aluminum oxide from trimethyl aluminum in conjunction with O(3) and an O(2) plasma. In situ attenuated total reflection Fourier transform infrared spectroscopy data show that both -OH groups and carbonates are formed on the surface during the oxidation cycle. These carbonates, once formed on the surface, are stable to prolonged O(3) exposure in the same cycle. However, in the case of plasma-assisted ALD, the carbonates decompose upon prolonged O(2) plasma exposure via a series reaction kinetics of the type, A (CH(3)) --> B (carbonates) --> C (Al(2)O(3)). The ratio of -OH groups to carbonates on the surface strongly depends on the oxidizing agent, and also the duration of the oxidation cycle in plasma-assisted ALD. However, in both O(3) and O(2) plasma cycles, carbonates are a small fraction of the total number of reactive sites compared to the hydroxyl groups.