Ultrafast Nanoimaging of the Photoinduced Phase Transition Dynamics in VO₂

Many phase transitions in correlated matter exhibit spatial inhomogeneities with expected yet unexplored effects on the associated ultrafast dynamics. Here we demonstrate the combination of ultrafast nondegenerate pump–probe spectroscopy with far from equilibrium excitation, and scattering scanning near-field optical microscopy (<i>s</i>-SNOM) for ultrafast nanoimaging. In a femtosecond near-field near-IR (NIR) pump and mid-IR (MIR) probe study, we investigate the photoinduced insulator-to-metal (IMT) transition in nominally homogeneous VO₂ microcrystals. With pump fluences as high as 5 mJ/cm², we can reach three distinct excitation regimes. We observe a spatial heterogeneity on ∼50–100 nm length scales in the fluence-dependent IMT dynamics ranging from <100 fs to ∼1 ps. These results suggest a high sensitivity of the IMT with respect to small local variations in strain, doping, or defects that are difficult to discern microscopically. We provide a perspective with the distinct requirements and considerations of ultrafast spatiotemporal nanoimaging of phase transitions in quantum materials

Metal Contacts on Physical Vapor Deposited Monolayer MoS₂

The understanding of the metal and transition metal dichalcogenide (TMD) interface is critical for future electronic device technologies based on this new class of two-dimensional semiconductors. Here, we investigate the initial growth of nanometer-thick Pd, Au, and Ag films on monolayer MoS₂. Distinct growth morphologies are identified by atomic force microscopy: Pd forms a uniform contact, Au clusters into nanostructures, and Ag forms randomly distributed islands on MoS₂. The formation of these different interfaces is elucidated by large-scale spin-polarized density functional theory calculations. Using Raman spectroscopy, we find that the interface homogeneity shows characteristic Raman shifts in E_2g¹ and A_1g modes. Interestingly, we show that insertion of graphene between metal and MoS₂ can effectively decouple MoS₂ from the perturbations imparted by metal contacts (<i>e.g.</i>, strain), while maintaining an effective electronic coupling between metal contact and MoS₂, suggesting that graphene can act as a conductive buffer layer in TMD electronics