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 MoOx 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.