Theoretical study of the adsorption of Lewis acids on MoS2 in relation to atomic layer deposition of Al2O3

There is presently a critical need for a viable approach to form ultrathin, high-quality layers of oxides such as Al2O3 on MoS2 and related two-dimensional transition-metal chalcogenides. Atomic layer deposition (ALD), which is, in principle, the most suitable technique, has been problematic in this case as a result of the low reactivity of these substrates when free of strain, impurities, and defects. Lewis acid-base chemistry provides a possible solution, and the present work employs ab initio theory to examine the feasibility of adsorbing strongly Lewis-acidic Al reagents on MoS2. The investigation focuses on small Al precursor molecules that are able to adsorb at high coverages and, therefore, to produce a dense and uniform layer of Al(OH)(3) when reacted with H2O, which is highly desirable for initiating ALD growth. Of those species considered, it is found that Al(CH3)(3), the most widely used reagent for ALD of Al2O3, is actually the least favorable in terms of its interaction with the MoS2 basal plane. Other, more Lewis-acidic reagents are found to adsorb somewhat more strongly in the critical first step of the growth process and, therefore, to be potentially more useful for initiating ALD growth. The coverage dependence of the adsorption energy and geometry and the effects of AlX3 polymerization are also considered, and practical suggestions are given for how such species can be incorporated into an ALD process.