Illustration of the ultrafast electron microscopy technique with electrons passing through a tailored DF aperture, consisting of an array of 72 individual apertures. Credit: Dr. Florian Sterl, Sterltech Optics.
Dark-field imaging in the ultrafast transmission electron microscope. (a) Sectional drawing of the experimental setup. Electron (green) and optical (red) pulses are incident close to perpendicular on the specimen. (b) Scanning electron image of the DF aperture array placed in the diffraction plane of the microscope. (c) Ultrafast DF images of the 1T-TaS2 sample obtained in the laser pump/electron probe scheme at two different temporal delays ?t.The layered inorganic compound, tantalum disulfide, possesses an unusual property that could make it ideal for nanoelectronic applications. The material has so-called charge-density wave (CDW) phases where the crystal distorts like a wave in response to temperature and light intensity. These structural changes have a profound effect on the material’s electronic properties including resistivity, which enables the material to demonstrate a metal-to-insulator transition. Now researchers have used an ultrafast transmission electron microscope (UTEM) to capture a movie of the material’s unusual dynamic behavior [Danz et al., Science 371(2021) 371–374, https://doi.org/10.1126/science.abd2774].
“The UTEM in Göttingen combines the nanometer spatial imaging capabilities of conventional transmission electron microscopies with femtosecond temporal resolution,” explains first author of the study, Thomas Danz of the University of Göttingen.
Together with his colleague Till Domröse and senior author Claus Ropers, group leader at the University of Göttingen and director at the Max Planck Institute for Biophysical Chemistry, Danz used an optical laser pulse to modify the properties of a sample of tantalum disulfide and a pulsed electron beam to image the resulting changes.
“By varying the optical pump-to-electron probe delay Δt, we can assemble a ‘movie’ of the dynamics in the sample … [and] we use a tailored dark-field (DF) imaging technique to enhance the subtle signal of the structural phase transition,” he says.
The idea of using laser light to alter the properties of materials with high precision is well known and widely used in technologies such as rewritable DVDs, but the process is so fast and takes place at such a small scale that it has never been directly observed. To get around this problem, the researchers introduced a tailored DF aperture, consisting of an array of 72 individual apertures, into the diffraction plane to cut out electrons without any useful information. The resulting tailored DF imaging enables processes on femtosecond timescales and nanometer length scales to be followed.
“This is the first demonstration of [the use of] a tailored DF aperture to greatly increase sensitivity to specific structural modulations and, most importantly, the first demonstration of time-resolved, real-space imaging of a structural phase transition,” points out Ropers.
The approach reveals – over a timescale of hundreds of femtoseconds to a few nanoseconds – a series of DF micrographs of the changing phase domains in the sample after being bombarded with an ultrashort laser pulse. Above its transition temperature, tantalum disulfide takes up a superstructure aligned with the underlying hexagonal lattice, while below it switches to a different phase.
“Initially, the sample is in a homogeneous CDW state at room temperature … After optical excitation, we witness the emergence and evolution of spatially localized domains of a second CDW phase,” explains Danz.
The researchers believe that their demonstration of ultrafast DF imaging of tantalum disulfide shows the potential of the approach to enhance image contrast and sensitivity to different phases in complex materials. Other materials could be studied in the same way simply by adapting the design of the DF aperture array.
“Many materials and devices exhibit nanoscale dynamic heterogeneities, associated with structural evolution varying in space and time, often on nanometer length-scales and picosecond time-scales. This work presents a new way of probing such phenomena using time domain dark field electron microscopy,” comments Aaron Lindenberg of Stanford University.
The researchers are now working to improve and generalize their approach, for example imaging multiple phases in a material simultaneously or changing the sample environment to image optically triggered currents. “We think our approach will have greatest utility wherever insights into subtle structural changes determining the behavior of nanoscale devices are required. In order to control [the electrical and magnetic] functions, a thorough understanding of the behavior of the crystal structure is important,” says Ropers.
This article was originally published in Nano Today 37 (2021) 101113.