Current Research Topics being pursued by the group include:

• Dynamics of Layered Materials
• Imaging Charge Carriers and Phonons
• Plasmonic Nanocrystals
• Ultrafast Electron Microscopy
• Pulsed-Beam TEM
• Ultrafast Phase Transitions
• Strongly-Correlated Materials

(Funding)

Materials are rarely pristine.  The relatively large number of atoms and molecules in even nanoscale systems compared to individual constituents increases the likelihood that imperfections will be present.  We are interested in determining how such features influence energy generation, transport, conversion, and decay in nanoscale and nanostructured materials.  To accomplish this, we develop methods based on femtosecond electron imaging and diffraction to directly resolve - in space and time - dynamics occurring at discrete defect sites.  For example, we have directly imaged phonon-dephasing behavior at crystal step edges in MoS₂ that are a single unit cell in height [Nano Lett. 2019].  In WSe₂, we have imaged the nucleation and launch of a single acoustic-phonon wavefront at a step edge within the first picosecond following in situ photoexcitation [Nat. Commun. 2016].  With these approaches, we are uncovering new physical insights into fundamental energy-defect behaviors.

Strong photoexcitation of semiconducting materials results in a variety of non-linear effects that evolve across a wide range of space and time.  Coupling of charge-carrier dynamics to atomic and molecular constituents comprising the material lattice is dependent upon a variety of both intrinsic and extrinsic properties and perturbations.  One result of photoexcitation is the generation of coherent lattice oscillations, which are major energy carriers in semiconductors and impact numerous electronic and structural properties.  We have developed novel femtosecond electron imaging techniques derived from pump-probe spectroscopy and bright-field TEM imaging to directly observe the generation and dispersion behaviors of hypersonic phonons in a number of semiconducting materials [Phys. Rev. Mater. 2017].  We find that, among other things, mode generation is dependent upon the excitation conditions, and phonon wavetrains can be highly coherent and long lived relative to electron-phonon coupling times.  The results of this work are shedding new light on fundamental phonon behaviors in archetypal materials of major technological importance.

Metallic nanocrystals can be made to display strong field-enhancement effects upon photoexcitation simply by controlling both the particle shape and the properties of the incident light.  Such effects have been explored for a variety of applications, such as photocatalysis and sensing.  Ultrafast spectroscopy has been used to study electron behaviors - and by extension, structural responses - in single particles spanning a large temporal range.  In addition to few-femtosecond plasmon dynamics, highly-coherent oscillations of the entire nanocrystal occur following femtosecond photoexcitation.  Using femtosecond electron imaging, we are able to directly visualize - on a single-particle basis - both incoherent and coherent structural responses, spanning from initial electron-phonon coupling to relaxation of coherent acoustic modes [Nano Lett. 2016].  Further, we have discovered complex behaviors of coupling and excitation within few-particle clusters.  In this way, the energetics within complex clusters having varied boundary conditions can be spatiotemporally mapped.

While spatial and energy resolutions of TEMs have reached the meV and sub-angstrom scales, respectively, temporal resolutions of conventional approaches remain mired in the millisecond domain.  This constitutes a significant discrepancy in scales - fundamental atomic-scale phenomena operate on femtosecond to picosecond durations.  We overcome this gap in scales by taking a decidedly unconventional approach via modification of an otherwise conventional TEM to allow optical access to the electron source and the specimen [Chem. Mater. 2015].  In this way, we are able to use concepts of pump-probe ultrafast spectroscopy to conduct ultrafast electron microscopy (UEM) measurements, thus improving TEM temporal resolution by a factor of 10 billion over conventional time-resolved approaches.  In addition to applications, paradigm tests, and new discoveries with UEM, we actively pursue advances in technology that lead to improvements in combined resolutions and increased capabilities [Chem. Phys. Lett. 2017].

A core challenge in the field of electron microscopy is to obtain a fundamental, quantitative understanding of the temporal aspects of electron-beam effects.  Indeed, the inherent need to irradiate the specimen with many energetic electrons in order to obtain a reasonable amount of information in a relatively short period of time is difficult to reconcile with the associated excitations, bond breaking and reformation, and species migration that is seemingly unavoidable.  While many approaches to slowing the deleterious effects of beam damage have been developed, few focus on understanding and leveraging mechanistic timescales.  We thus have applied our femtosecond laser-driven pulsed-beam TEM approach to this problem.  Through a series of rigorous systematic studies, we demonstrated, for the first time, a clear distinction between the conventional and pulsed beam approaches - all else being the same - on a model n-alkane system (hexatriacontane, C₃₆H₇₄) [Nano Lett. 2019].  We are now extending our approach to other weakly-stable systems (e.g., MAPbI₃) to both reduce damage and to explore molecular-scale mechanisms [ACS Omega 2020].

Transformations of molecules and materials from one (meta)stable state to another following perturbation involves reconfiguration of charges and atoms.  Such transformations can be ultrafast, occurring on the order of femtoseconds to picoseconds, and can be characterized by strongly-correlated behaviors between, for example, electrons and atoms.  Interestingly, the existence of complex potential energy surfaces enables control of trajectories for preferential product or structure formation.  Despite intense investigation, there remains much to learn about precisely how matter rearranges itself on the molecular level during such transformations.  We thus are applying correlative methods of femtosecond electron imaging and diffraction to elucidate such behaviors [Philos. Trans. R. Soc. A 2020].  Being sensitive to picometer changes, we aim to develop rich pictures of how, for example, charge excitation produces atomic-to-nanoscale reconfigurations, passing through a transition state toward new-structure formation.

Order through charge pooling can produce associated coherent lattice deformations having periodicities on the order of nanometers - significantly larger than unit-cell and molecular dimensions.  Certain structural and compositional configurations lead to the display of mulitple charge-ordering states occupying discrete potential energy sites across a complex phase space.  Thus, perturbations to these systems - for example, via femtosecond photoexcitation - can launch dynamics that are interwoven between charge order/disorder and nanoscale lattice reconfigurations.  As with phase transformations, however, there is still much to learn about the trajectories associated with order-disorder-reordering and the connections to atomic-to-nanoscale responses.  Accordingly, we are probing charge order/disorder/reorder dynamics and the oscillatory coherent periodicity in charge-density-wave systems with ultrafast electron microscopy.  One major aim is to uncover new physical insights into local behaviors - especially in relation to discontinuities associated with defects.