Atomic Layer Deposition of TiO2 on MoTe2: Chemical Changes, Band Offsets, and Photophysics

Transition metal dichalcogenides (TMDCs) are well suited to optoelectronic applications due to their strong absorption of a broad spectrum of wavelengths and their layered structure, which permits the assembly of van der Waals heterostructures. The behavior of TMDC-TMO (transition metal oxide) interfaces is an important topic for the potential development of TMDC-based optoelectronics, as they may be effective either as active components of optoelectronic devices or as interlayers between TMDCs and metals, improving contact efficiencies. Here, TiO₂–MoTe₂ junctions were studied to determine the effectiveness of TiO₂ synthesized with atomic layer deposition (ALD) as a potential charge-separating layer for MoTe₂. Band alignments measured with X-ray photoelectron spectroscopy (XPS) suggested a driving force for carrier separation. However, transient absorption spectroscopy (TAS) showed no evidence of charge injection, with relaxation mechanisms and lifetimes changing minimally after the deposition of TiO₂. High exciton binding energies are proposed as a likely cause of this behavior. XPS was also used to analyze the interface chemistry of the heterojunctions, revealing the formation of a thin MoO_x layer on bare MoTe₂ samples, which was partially reduced during the process of TiO₂ deposition. This work provides important information about chemical changes and the resulting electronic behavior of interfaces between two-dimensional (2D) and three-dimensional (3D) solids.