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Vertically stacked van der Waals (vdW) heterostructures exhibit unique electronic, optical, and thermal properties that can be manipulated by twist-angle engineering. However, the weak phononic coupling at a bilayer interface imposes a fundamental thermal bottleneck for future two-dimensional devices. Using ultrafast electron diffraction, we directly investigated photoinduced nonequilibrium phonon dynamics in MoS₂/WS₂at 4° twist angle and WSe₂/MoSe₂heterobilayers with twist angles of 7°, 16°, and 25°. We identified an interlayer heat transfer channel with a characteristic timescale of ~20 picoseconds, about one order of magnitude faster than molecular dynamics simulations assuming initial intralayer thermalization. Atomistic calculations involving phonon-phonon scattering suggest that this process originates from the nonthermal phonon population following the initial interlayer charge transfer and scattering. Our findings present an avenue for thermal management in vdW heterostructures by tailoring nonequilibrium phonon populations. 

Interactions of quantum materials with strong laser fields can induce exotic non-equilibrium electronic states. Monolayer transition metal dichalcogenides, a new class of direct-gap semiconductors with prominent quantum confinement, offer exceptional opportunities for the Floquet engineering of excitons, which are quasiparticle electron–hole correlated states8. Strong-field driving has the potential to achieve enhanced control of the electronic band structure and thus the possibility of opening a new realm of exciton light–matter interactions. However, a full characterization of strong-field driven exciton dynamics has been difficult. Here we use mid-infrared laser pulses below the optical bandgap to excite monolayer tungsten disulfide and demonstrate strong-field light dressing of excitons in excess of a hundred millielectronvolts. Our high-sensitivity transient absorption spectroscopy further reveals the formation of a virtual absorption feature below the 1s-exciton resonance, which we assign to a light-dressed sideband from the dark 2p-exciton state. Quantum-mechanical simulations substantiate the experimental results and enable us to retrieve real-space movies of the exciton dynamics. This study advances our understanding of the exciton dynamics in the strong-field regime, showing the possibility of harnessing ultrafast, strong-field phenomena in device applications of two-dimensional materials. 

Abstract In situ tensile testing using transmission electron microscopy (TEM) is a powerful technique to probe structure‐property relationships of materials at the atomic scale. In this work, a facile tensile testing platform for in situ characterization of materials inside a transmission electron microscope is demonstrated. The platform consists of: 1) a commercially available, flexible, electron‐transparent substrate (e.g., TEM grid) integrated with a conventional tensile testing holder, and 2) a finite element simulation providing quantification of specimen‐applied strain. The flexible substrate (carbon support film of the TEM grid) mitigates strain concentrations usually found in free‐standing films and enables in situ straining experiments to be performed on materials that cannot undergo localized thinning or focused ion beam lift‐out. The finite element simulation enables direct correlation of holder displacement with sample strain, providing upper and lower bounds of expected strain across the substrate. The tensile testing platform is validated for three disparate material systems: sputtered gold‐palladium, few‐layer transferred tungsten disulfide, and electrodeposited lithium, by measuring lattice strain from experimentally recorded electron diffraction data. The results show good agreement between experiment and simulation, providing confidence in the ability to transfer strain from holder to sample and relate TEM crystal structural observations with material mechanical properties.