Spontaneous Room-Temperature Solid-State Reaction at the MoS2/Ti Interface: Implications for Contact Engineering.

High-quality interfaces with metals are essential for exploiting two-dimensional materials in nanoscale solid-state devices. Conventional strategies rely on depositing metals with a suitable work function at low kinetic energies to minimize damage. Direct exfoliation of layered materials onto clean metal surfaces under ultrahigh vacuum offers a promising alternative, producing pristine, oxide-free interfaces while avoiding the disorder induced by high-energy metal deposition. TMDCs possess unique properties, including a direct band gap, high carrier mobility, and strong spin-orbit coupling, which makes them highly promising for high-performance electronic and optoelectronic devices such as transistors, photodetectors, and solar cells, as well as for emerging quantum technologies. While the metal-assisted exfoliation is well established for gold and other noble metals, its applicability to more reactive metals remains unclear. To address this question, we exfoliate large-area MoS2 layers on titanium under ultrahigh vacuum and characterize the resulting heterostructures using X-ray photoelectron and Raman spectroscopy. The monolayer yield is over 75%, but in contrast to noble metals, Ti reacts with exfoliated MoS2 at room temperature, producing metallic molybdenum and various sulfur species upon simple physical contact, and only ∼6% of MoS2 remains in its pristine form. The degradation also propagates to the second MoS2 layer in the bilayer case. In trilayer MoS2, the degradation and subsequent oxidation upon air exposure are significantly reduced, and the top layer remains intact and partially protects the interface. These findings likely apply to other material combinations and suggest opportunities for engineering low-resistance contacts, particularly with reactive and high-melting-point metals.