Metal-assisted vacuum transfer enabling in situ visualization of charge density waves in monolayer MoS₂

Recent advancements in quantum materials research have focused on monolayer transition metal dichalcogenides and their heterostructures, known for complex electronic phenomena. While macroscopic electrical and magnetic measurements provide valuable insights, understanding these electronic states requires direct experimental observations. Yet, the extreme two-dimensionality of these materials demands surface-sensitive measurements with exceptionally clean surfaces. Here, we present the metal-assisted vacuum transfer method combined with in situ measurements in ultrahigh vacuum (UHV), enabling pristine monolayer MoS₂ with ultraclean surfaces unexposed to ambient conditions. Consequently, in situ scanning tunneling microscopy revealed charge density waves (CDWs) in MoS₂/Cu(111), previously unobserved in monolayer MoS₂. Additionally, angle-resolved photoelectron spectroscopy identified notable Fermi surface nesting due to substrate interactions, elucidating the mechanisms behind CDW formation. This method is broadly applicable to other monolayer two-dimensional materials, enabling the high-fidelity in situ UHV characterization and advancing the understanding of correlated electronic behaviors in these material systems.