News

I gave an invited talk titled "Probing Spatiotemporal Modulation of Strain in van der Waals Materials" in the "Defects and Strain in Two-Dimensional Materials" symposium. 

I gave an invited talk titled "In Situ Imaging of Phase and Twist Boundaries in 2D Materials" in the "Advanced Manufacturing of 2D Materials at the Atomic Scale" symposium

I gave an invited talk titled "In situ Imaging of Atomic Interfaces of 2D Materials" in the "Moire Superlattices in 2D Materials" symposium. 

I presented a talk titled "In-situ Imaging of Thermally Activated Atomic Reconstruction of Twisted Bilayer Transition Metal Dichalcogenides" in the "Quantum Materials Under Electron Beam" symposium. Won the Postdoctoral Scholar Award.

I was selected to participate in an NSF workshop on grant application and review for young investigators.

I was invited to give a talk in the Spring Future Leaders Seminar series in the Department of Materials Science and Engineering at Northwestern. I talked about my PhD and postdoc work on "Visualizing Nano- to Atomic-scale Dynamics with Time-resolved Electron Microscopy". Thanks to the organizing committee for the invitation!

I presented a talk titled "Direct Imaging of Step-induced Phonon Softening with 4D Ultrafast Electron Microscopy " in "Electron Microscopy and Spectroscopy of 2D Materials". 

We used holey substrate to periodically strain pattern bilayer MoS2. This was an exciting collaboration effort at the UMN MRSEC. 

UMN News: Researchers find new way to manipulate properties of ultrathin semiconductors

Abstract: Key properties of two-dimensional (2D) layered materials are highly strain tunable, arising from bond modulation and associated reconfiguration of the energy bands around the Fermi level. Approaches to locally controlling and patterning strain have included both active and passive elastic deformation via sustained loading and templating with nanostructures. Here, by float-capturing ultrathin flakes of single-crystal 2H-MoS2 on amorphous holey silicon nitride substrates, we find that highly symmetric, high-fidelity strain patterns are formed. The hexagonally arranged holes and surface topography combine to generate highly conformal flake-substrate coverage creating patterns that match optimal centroidal Voronoi tessellation in 2D Euclidean space. Using TEM imaging and diffraction, as well as AFM topographic mapping, we determine that the substrate-driven 3D geometry of the flakes over the holes consists of symmetric, out-of-plane bowl-like deformation of up to 35 nm, with in-plane, isotropic tensile strains of up to 1.8% (measured with both selected-area diffraction and AFM). Atomistic and image simulations accurately predict spontaneous formation of the strain patterns, with van der Waals forces and substrate topography as the input parameters. These results show that predictable patterns and 3D topography can be spontaneously induced in 2D materials captured on bare, holey substrates. The method also enables electron scattering studies of precisely aligned, substrate-free strained regions in transmission mode. 

I started my postdoc position in the Department of Materials Science and Engineering at the University of Illinois, Urbana-Champaign. I am working with Prof. Pinshane Huang on atomic-resolution imaging of structural and phase transformations in 2D materials. Looking forward to the new journey!

We observed nanometer-variant phonon softening near nanometer-scale step edge using 4D ultrafast electron microscopy. 

Abstract: Step edges are an important and prevalent topological feature that influence catalytic, electronic, vibrational, and structural properties arising from modulation of atomic-scale force fields due to edge-atom relaxation. Direct probing of ultrafast atomic-to-nanoscale lattice dynamics at individual steps poses a particularly significant challenge owing to demanding spatiotemporal resolution requirements. Here, we achieve such resolutions with femtosecond 4D ultrafast electron microscopy and directly image nanometer-variant softening of photoexcited phonons at individual surface steps. We find large degrees of softening precisely at the step position, with a thickness-dependent, strain-induced frequency modulation extending tens of nanometers laterally from the atomic-scale discontinuity. The effect originates from anisotropic bond dilation and photoinduced incoherent atomic displacements delineated by abrupt molecular-layer cessation. The magnitude and spatiotemporal extent of softening is quantitatively described with a finite-element transient-deformation model. The high spatiotemporal resolutions demonstrated here enable uncovering of new insights into atomic-scale structure–function relationships of highly defect-sensitive, functional materials. 

We used 4D ultrafast electron microscopy to study correlation between coherent acoustic phonons and periodic lattice distortion in charge-density-wave material 1T-TaS2.

Abstract: Ultrafast manipulation of phase domains in quantum materials is a promising approach to unraveling and harnessing interwoven charge and lattice degrees of freedom. Here we find evidence for coupling of displacively excited coherent acoustic phonons (CAPs) and periodic lattice distortions (PLDs) in the intensely studied charge-density-wave material, 1T-TaS2, using 4D ultrafast electron microscopy (UEM). Initial photoinduced Bragg-peak dynamics reveal partial CAP coherence and localized c-axis dilations. Weak, partially coherent dynamics give way to higher-amplitude, increasingly coherent oscillations, the transition period of which matches that of photoinduced incommensurate domain growth and stabilization from the nearly-commensurate phase. With UEM imaging, it is found that phonon wave trains emerge from linear defects 100 ps after photoexcitation. The CAPs consist of coupled longitudinal and transverse character and propagate at anomalously high velocities along wave vectors independent from PLDs, instead being dictated by defect orientation. Such behaviors illustrate a means to control phases in quantum materials using defect-engineered coherent-phonon seeding. 

Our invited paper is selected as the cover article for a special issue "Time-Resolved Microscopy".

Abstract: The structural anisotropy of layered materials leads to disparate lattice responses along different crystallographic directions following femtosecond photoexcitation. Ultrafast scattering methods are well-suited to resolving such responses, though probe size and specimen structure and morphology must be considered when interpreting results. Here we use ultrafast electron microscopy (UEM) imaging and diffraction to study the influence of individual multilayer terraces and few-layer step-edges on acoustic-phonon dynamics in 1T-TaS2 and 2H-MoS2. In TaS2, we find that a multilayer terrace produces distinct, localized responses arising from thickness-dependent c-axis phonon dynamics. Convolution of the responses is demonstrated with ultrafast selected-area diffraction by limiting the probe size and training it on the region of interest. This results in a reciprocal-space frequency response that is a convolution of the spatially separated behaviors. Sensitivity of phonon dynamics to few-layer step-edges in MoS2 and the capability of UEM imaging to resolve the influence of such defects are also demonstrated. Spatial frequency maps from the UEM image series reveal regions separated by a four-layer step-edge having 60.0 GHz and 63.3 GHz oscillation frequencies, again linked to c-axis phonon propagation. As with ultrafast diffraction, signal convolution is demonstrated by continuous increase of the size of the selected region of interest used in the analysis. 

I presented a talk titled "Imaging Structure-Directed Phonon Dynamics in MoS2 with Ultrafast Electron Microscopy" and a poster titled "Feasibility of Layer-Number Determination of Few to Monolayer MoS2 via Combined Simulation and Electron Diffraction Experiments".

We observed anisotropic strain-wave dynamics and few-layer dephasing using 4D ultrafast electron microscopy. 

Abstract: 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. 

I presented a talk titled "Direct Imaging of Nanoscale Anisotropic Structural Dynamics with Ultrafast Electron Microscopy".

I presented a poster titled "Direct Imaging of Anisotropic Acoustic-Phonon Dynamics in MoS2". Won the Student Scholar Award.

I presented a talk titled "Visualization of Coherent Acoustic-Phonon Dynamics in MoS2 with Ultrafast Electron Microscopy ".