Dynamic Structural Response and Deformations of Monolayer MoS₂ Visualized by Femtosecond Electron Diffraction

Abstract

Two-dimensional materials are subject to intrinsic and dynamic rippling that modulates their optoelectronic and electromechanical properties. Here, we directly visualize the dynamics of these processes within monolayer transition metal dichalcogenide MoS2 using femtosecond electron scattering techniques as a real-time probe with atomic-scale resolution. We show that optical excitation induces large-amplitude in-plane displacements and ultrafast wrinkling of the monolayer on nanometer length-scales, developing on picosecond time-scales. These deformations are associated with several percent peak strains that are fully reversible over tens of millions of cycles. Direct measurements of electron-phonon coupling times and the subsequent interfacial thermal heat flow between the monolayer and substrate are also obtained. These measurements, coupled with first-principles modeling, provide a new understanding of the dynamic structural processes that underlie the functionality of two-dimensional materials and open up new opportunities for ultrafast strain engineering using all-optical methods.

Light-induced picosecond rotational disordering of the inorganic sublattice in hybrid perovskites

“…2A, the relative intensity drop has a clear Q dependence: Higher-order peaks show larger decreases. Figure 2C shows that the logarithm of the intensity modulation for a particular Bragg peak depends nearly linearly on the corresponding Q 2 for this Bragg peak; that is, it follows a Debye-Waller–like dependence: $\mathrm{italicI}false(\mathrm{italicQ},\mathrm{italict}false)\propto \text{exp}true(-\frac{1}{3}{\mathit{Q}}^{2}true\langle {\mathit{u}}_{\text{RMS}}^{2}false(\mathrm{italict}false)true\rangle true)$, where $true\langle {\mathit{u}}_{\text{RMS}}^{2}false(\mathrm{italict}false)true\rangle$ is the time-dependent mean square displacement of the atoms [RMS (root mean square)] ( 30 , 31 ). The peak intensity decay is correlated with the rise of the lattice temperature (for small changes in diffraction intensity), with a measured time constant of 10 ± 1 ps under an excitation fluence of 66 μJ/cm 2 , corresponding to an estimated carrier density of 2.3 × 10 19 cm −3 (see the Supplementary Materials).…”

“…Photoexcitation at 400 nm generates electrons and holes that first equilibrate at an elevated carrier temperature T e within hundreds of femtoseconds through carrier-carrier scattering before relaxing to the band edge via carrier-phonon scattering and transferring their excess electronic energy to the lattice ( 32 , 33 ). Thus, the dynamics of the peak intensity decay track the hot-carrier cooling time scale ( 31 ). After ≈10 ps, both the carriers and the lattice reach an equilibrium, and no further thermalization is observed (figs.…”

“…S2D and S3). From the Q -dependent Bragg peak intensity drop, one may extract the photoinduced change in the RMS $true(\sqrt{true\langle {\mathit{u}}_{\text{RMS}}^{2}false(\mathrm{italict}false)true\rangle }true)$ displacements under the assumption of isotropic disorder ( 31 ). Maximum observed RMS displacements are ≈0.1 Å for temperature jumps of ≈8 K (see the Supplementary Materials), showing that weak photoexcitation induces large structural deformations.…”

Exploring Thermal Properties of M0S2 Using In Situ Quantitative STEM

“…6]. It is also possible that the measured DW factors can be affected by the dynamic structural response of the material upon illumination [7]. In the present work, we measure the DW factors of MoS2 layers as a function of temperature and number of layers in an in situ quantitative STEM condition.…”

Ultrafast terahertz-field-driven ionic response in ferroelectric BaTiO3

“…We note this effect is in contrast to the ordinary decrease in scattering intensity that would be associated with a temperature-induced Debye-Waller effect. 28,29 Inclusion of the oxygen atoms in the above analytical calculation (See Supplemental Materials) gives a slightly revised equation which can be written as: This changes the estimated magnitude of the induced RMS displacement by ≈ 10%. Based on the observed THz-field-induced out-of-plane RMS displacement and the MD simulation, one can derive an atomistic understanding of structural response of a ferroelectric upon ultrafast electric field excitation, directly relating the induced out-of-plane motions to a field-induced transient polarization rotation of local dipoles following the applied in-plane THz field.…”