Ultrafast transmission electron microscopy of a structural phase transition
by Thomas Christian Danz
Date of Examination:2021-07-12
Date of issue:2021-08-09
Advisor:Prof. Dr. Claus Ropers
Referee:Prof. Dr. Claus Ropers
Referee:Prof. Dr. Christian Jooß
Referee:Prof. Dr. Nuh Gedik
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Abstract
English
High hopes are placed on materials with tunable properties and excitations for future applications in energy conversion devices. Functionality of devices, however, not only arises from the properties of individual materials but also from their interplay and nanoscale structuring. While a number of established experimental techniques are capable of tracking electronic excitations on femtosecond time and nanometer length scales, no time-resolved nanoimaging of a structural order parameter had previously been reported. Addressing this challenge, the present cumulative thesis reports on the development and application of a time-resolved dark-field electron microscopy scheme implemented at the Göttingen Ultrafast Transmission Electron Microscope (UTEM). This nanoimaging approach combines femtosecond temporal and 5 nm spatial resolution with sensitivity to the structural component of a charge-density wave phase transition in 1T-polytype tantalum disulfide. Ultrashort laser pulses locally induce the phase transition, while the subsequent spatiotemporal relaxation dynamics of the structural order parameter is tracked using ultrashort electron pulses. Order parameter sensitivity is obtained by means of a dark-field aperture array, tailored to filter the periodicities of the charge-density wave in the diffraction plane of the microscope. In the first publication contributing to this thesis, the preparation technique for the thin films of tantalum disulfide is introduced. Specimens obtained by ultramicrotomy are ideal for electron and x-ray experiments in a transmission geometry, as exemplified by the investigation of manganese- and iron-intercalated tantalum disulfide. Static optical microscopy, electron diffraction and x-ray magnetic circular dichroism measurements serve to characterize these ferromagnetic thin films and to verify that the properties reflect those of the bulk crystals. The second article describes the implementation of the ultrafast nanoimaging approach. A central aspect of the experiment is the design of a specimen that spatially structures the optical excitation pattern and allows for stroboscopic probing of the phase transition in tantalum disulfide at hundreds of kilohertz repetition rates. Based on parameters extracted from a steady-state heating experiment, the optically induced evolution of nanoscale charge-density wave domains in the free-standing thin film is reproduced in time-dependent Ginzburg-Landau simulations. Finally, perspectives for future nanoimaging experiments are discussed. Allowing for sensitivity to further structural degrees of freedom in complex materials, ultrafast dark-field imaging will contribute to a better understanding of actively controlled processes in energy conversion devices.
Keywords: ultrafast transmission electron microscopy; UTEM; ultrafast nanoimaging; dark-field imaging; tailored dark-field aperture; ultrafast dynamics; structural dynamics; order parameter; phase transition; charge-density wave; CDW; transition metal dichalcogenide; tantalum disulfide; 1T-TaS2; ultramicrotomy; solid-state physics