(310h) Nonequilibrium Light-Matter Interactions Investigated with Ultrafast Electron Microscopy

While static characterization of electronic materials is indispensable in applied materials science and engineering, advances in this area also rely on complementary in situ understanding of the processing, performance, and transformation of materials under dynamic, nonequilibrium conditions. Photoexcited semiconductors, for example, undergo a series of strongly-correlated many-body interactions which overlap in space and time during subsequent relaxation to the ground state [1]. The thermalization of excited carriers creates a nonequilibrium phonon population, resulting in an evolution of structural dynamics whose elucidation allows for both fundamental insights into the quantum nature of matter and control of device and materials behaviors in electronic and optoelectronic applications. Additionally, as the dimensions of electronic device components approach molecular-scale limits, the associated increased importance of surfaces, interfaces, and defects adds an additional layer of complexity to the structural behavior. For example, it is known that interfaces change the electronic structure in their surrounding regions, strongly influencing carrier dynamics, and it has been shown in our previous work that phonons are structure-directed, propagating specifically from interfaces and repeatably traveling perpendicularly to the launching interface [2]. As such, experimental investigation with high combined spatio-temporal resolution and access to real-space behavior is well-suited for increasing understanding of the complex processes that guide light-matter interactions.

Ultrafast electron microscopy (UEM), in which a pump-probe methodology is used to extend transmission electron microscopy (TEM) into the ultrafast (sub-picosecond) regime, is just such a technique. This is accomplished by modifying a conventional TEM to have two optical ports with which a femtosecond pulsed laser is used to both generate discrete photoelectron (probe) packets in the electron source region and to photoexcite (pump) the specimen in situ. By varying the arrival time of the pump laser pulse and the probe photoelectron packet at the specimen, an electron-microscopy imaging video of materials dynamics – with sub-picosecond temporal resolution –can be generated [3]. Importantly, direct real-space imaging eliminates the need for inferring structural responses from spatially-averaged spectroscopic data in order to connect observations with underlying materials behaviors. Indeed, owing to the lens arrays present in conventional TEMs, correlative studies using imaging in conjunction with diffraction or electron energy-loss spectroscopy – all extended to ultrafast timescales – can enhance the information available even further. Further, because analysis of defects is readily possible with TEM, the impact of such features on electronic and structural behavior can be elucidated as well, illuminating processes critical to understanding device operation.

Here, to illustrate the capabilities described above, we present two examples of how UEM can be used to resolve nonequilibrium structural behaviors of photoexcited optoelectronic materials. Firstly, coherent acoustic phonon properties such as generation and propagation, phase velocity, and dispersion in the archetypal semiconductor gallium arsenide (GaAs) are used to gain quantitative insight into nanoscale and nanostructured architectures stemming from structural ordering and lamella thickness. The use of this technique to measure transient optoelectronic properties during phonon propagation is discussed. Secondly, the impact of discrete, individual defect structures in molybdenum disulfide (MoS₂) is directly imaged, and spatially-disparate phonon behaviors are correlated to molecular-layer step edges having heights of as little as one crystallographic unit cell, demonstrating the extreme sensitivity of UEM to structural inhomogeneities [4,5]. In summary, here we will show how UEM can be utilized to access photoinduced nonequilibrium atomic, molecular, and nanoscale structural behaviors of optoelectronic materials with high combined spatiotemporal resolutions and correlative reciprocal and momentum-space techniques.

References

[1] A. Othonos, J. Appl. Phys. 83, (1998), 1789.

[2] D. R. Cremons, et al., Phys. Rev. Mater. 1, (2017), 073801.

[3] D. J. Flannigan and A. H. Zewail, Acc. Chem. Res. 45, (2012), 1828.

[4] Y. Zhang and D. J. Flannigan, Nano Lett. 19, (2019), 8216.

[5] S. A. Reisbick, et al., J. Phys. Chem. A 124, (2020), 1877.