Surface Engineering of Monolayer MoS₂ for Atomic Layer Deposition of TiO₂

We investigate methods to improve the growth of high-quality TiO₂ films on MoS₂ via atomic layer deposition (ALD). Untreated MoS₂ surfaces are hydrophobic, and as a result, ALD results in discontinuous films that nucleate only on defect sites. Since pristine MoS₂ monolayers have low defect density, argon ion bombardment is used to create sulfur vacancies. The vacancy concentration is monitored in-situ with x-ray photoelectron spectroscopy. We use thiols to passivate the vacancies and act as a seed layer for ALD. However, spontaneous oxidation of the defective MoS₂ layer can occur, and ALD on argon sputtered MoS₂ results in uniform TiO₂ films with and without the thiol treatment. Without thiol treatment, dissociative adsorption of oxygen molecules on the sulfur vacancies may seed the ALD by leaving behind adsorbed atomic oxygen. With density functional theory, we investigate the electronic properties of defective, thiol- and oxygen-passivated sulfur vacancies in MoS₂. Thiol- or oxygen-passivated MoS₂ retains desirable electronic properties. Careful control of the sulfur vacancy concentration followed by functionalization can provide a means of tuning the electronic properties of MoS₂ and providing seed sites for high quality ALD film growth.