Photo-induced lattice contraction in layered materials

Observation of Anisotropic Strain-Wave Dynamics and Few-Layer Dephasing in MoS2 with Ultrafast Electron Microscopy

The large elastic strains that can be sustained by transition metal dichalcogenides (TMDs), and the sensitivity of electronic properties to that strain, make these materials attractive targets for tunable optoelectronic devices. Defects have also been shown to influence the optical and electronic properties, characteristics that are especially important to understand for applications requiring high precision and sensitivity. Importantly, photoexcitation of TMDs is known to generate transient strain effects but the associated intralayer and interlayer low-frequency (tens of GHz) acoustic-phonon modes are largely unexplored, especially in relation to defects common to such materials. Here, with femtosecond electron imaging in an ultrafast electron microscope (UEM), we directly observe distinct photoexcited strain-wave dynamics specific to both the ab basal planes and the principal c-axis crystallographic stacking direction in multilayer 2H-MoS₂, and we elucidate the microscopic interconnectedness of these modes to one another and to discrete defects, such as few-layer crystal step edges. By probing 3D structural information within a nanometer-picosecond 2D projected UEM image series, we were able to observe the excitation and evolution of both modes simultaneously. In this way, we found evidence of a delay between mode excitations; initiation of the interlayer (c-axis) strain-wave mode precedes the intralayer (ab plane) mode by 2.4 ps. Further, the intralayer mode is preferentially excited at free basal-plane edges, thus suggesting the initial impulsive structural changes along the c-axis direction and the increased freedom of motion of the MoS₂ layer edges at terraces and step edges combine to launch in-plane strain waves at the longitudinal speed of sound (here observed to be 7.8 nm/ps). Sensitivity of the c-axis mode to layer number is observed through direct imaging of a picosecond spatiotemporal dephasing of the lattice oscillation in discrete crystal regions separated by a step edge consisting of four MoS₂ layers. These results uncover new insights into the fundamental nanoscale structural responses of layered materials to ultrafast photoexcitation and illustrate the influence defects common to these materials have on behaviors that may impact the emergent optoelectronic properties.

Laser Annealing Improves the Photoelectrochemical Activity of Ultrathin MoSe2 Photoelectrodes

Understanding light-matter interactions in transition-metal dichalcogenides (TMDs) is critical for optoelectronic device applications. Several studies have shown that high intensity light irradiation can tune the optical and physical properties of pristine TMDs. The enhancement in optoelectronic properties has been attributed to a so-called laser annealing effect that heals chalcogen vacancies. However, it is unknown whether laser annealing improves functional properties such as photocatalytic activity. Here, we show that high intensity supra band gap illumination improves the photoelectrochemical activity of MoSe₂ nanosheets for iodide oxidation in indium doped tin oxide/MoSe₂/I^-, I₃^-/Pt liquid junction solar cells. Ensemble-level photoelectrochemical measurements show that, on average, illuminating MoSe₂ thin films with 1 W/cm² 532 nm excitation increases the photoelectrochemical current by 142% and shifts the photocurrent response to more favorable (negative) potentials. Scanning photoelectrochemical microscopy measurements reveal that pristine bilayer (2L)-MoSe₂, trilayer (3L)-MoSe₂, and multilayer-thick nanosheets are initially inactive for iodide oxidation. The light treatment activates 2L-MoSe₂ and 3L-MoSe₂ materials, and the activation process initiates at the edge sites. The photocurrent enhancement is more significant for 2L-MoSe₂ than for 1L-MoSe₂. Multilayer-thick MoSe₂ remains inactive for iodide oxidation even after the laser treatment. Our microscopy measurements reveal that the laser-induced enhancement effect depends critically on MoSe₂ layer thickness. X-ray photoelectron spectroscopy measurements further show that the laser treatment oxidizes Mo(IV) species that are initially associated with Se vacancies. Ambient oxygen fills the Se vacancies and removes trap states, thereby increasing the overall photogenerated carrier collection efficiency. To the best of our knowledge, this work represents the first report on using laser to enhance the photoelectrocatalytic properties of few-layer-thick TMDs. The simple and rapid laser annealing procedure is a promising strategy to tune the reactivity of TMD-based photoelectrochemical cells for electricity and chemical fuel production.